Java Beginner Tutorial

Preface

Javabeginner.com is a beginner Java tutorial site that attempts to teach basics of Java programming Language in plain English using huge number of java source code examples spread across various topics. This site is for absolute starters to learn java who do not require any prerequisite Java knowledge. This site can also be used by advanced java developers. This site has links to some of my other sites that will help you in learning advanced Java topics and other technical subjects.

Java has evolved to be the most predominant and popular general purpose programming language of the current age. Java is a simple, portable, distributed, robust, secure, dynamic, architecture neutral, object oriented programming language. It was developed by Sun Microsystems. This technology allows the software designed and developed once for an idealized ‘virtual machine’ and run on various computing platforms.

Java plays a significant role in the corporate world. Companies of all sizes are using Java as the main programming language to develop various applications/projects world wide. It has found its use in various sectors including banking, insurance, retail, media, education, manufacturing and so on. E-commerce, Gaming, Mobile, Embedded, Media and many more types of applications are being developed using Java. Organizations are developing mission critical applications using Java technologies. This necessitates the corporations to hire highly skilled java developers. On the other hand it opens the doors for many opportunities for java developers. There is significant demand for java developers with sound technical skills. Now Colleges and Universities all over the world are using it in their introduction courses as well as their junior and senior software engineering courses.

This site was developed considering the importance of this language and the benefits that it provides to corporations, developers, students, education institutions and more. Javabeginner.com is free online Java tutorial site with a huge number of java programs for java development. This site is evolving with more content, features, links and useful information. If you have any suggestions, feedback, concerns, corrections or questions please communicate through Feedback section.

Table of Contents

Javabeginner TutorialTable of Contents

Introduction to Java

About JavaPlatform Independence Java Virtual Machine Object Oriented ProgrammingJava Features Java Applications

Getting Started with JavaJava ArchitectureCompiling and Running an ApplicationJava Development KitjavadocJAR FilesPATH and CLASSPATHIntroduction to Java 1.5

Basic Language Elements KeywordsCommentsVariable, Identifiers and Data TypesClassesObjectsInterfaceInstance MembersStatic MembersArrays

Java OperatorsJava OperatorsAssignment operatorsArithmetic operatorsRelational operatorsLogical operatorsBitwise operatorsCompound operators

Conditional operatorsOperator Precedence

Java Control StatementsIntroduction to Control StatementsSelection StatementsIteration StatementsTransfer Statements

Java Access ModifiersIntroduction to Java Access Modifierspublic access modifierprivate access modifierprotected access modifierdefault access modifier

Classes and ObjectsClass Variables – Static FieldsClass Methods – Static MethodsInstance VariablesFinal Variable, Methods and ClassesIntroduction to Java ObjectsMethod Overloading

Java ConstructorsOverloaded ConstructorsConstructor Chaining

Object SerializationIntroduction to Object SerializationTransient Fields and SerializationInput and Output Object Streams

Java Class InheritanceJava Class Inheritancethis and super keywords

Java Object CastingObject Reference Type Castinginstanceof Operator

Abstract class and InterfaceAbstract Class in javaJava Interface

Polymorphism

Java Method Overriding

Java String ClassString ClassCreation of StringsString EqualityString Functions

Java toString() MethodJava toString() Method

Java String ComparisonCompare String objects to determine Equality

Java StringBuffer StringBuffer ClassCreation of StringBuffer'sStringBuffer Functions

Java Exception HandlingExceptions in JavaException ClassesException Statement SyntaxRules for try, catch and finally Blockstry, catch and finallyDefining new Exceptionsthrow, throws statementHandling Multiple Exceptions

Java Singleton Design PatternSingletonImplementing the Singleton Pattern

Java Threads TutorialIntroduction to ThreadsThread CreationThread SynchronizationSynchronized MethodsSynchronized BlocksThread States

Thread PriorityThread SchedulerYieldingSleeping and Waking UpWaiting and NotifyingJoiningDeadlock

Java Collections FrameworkCore Collection InterfacesConcrete ClassesStandard utility methods and algorithmsHow are Collections UsedJava ArrayListJava LinkedListJava TreeSetJava HashMapJava VectorJava HashTableJava HashSet

Java Date UtilJava Date APIJava Date Source Code

Introduction to Java Programming

Introduction to Computer Science using Java

Java is a simple and yet powerful object oriented programming language and it is in many respects similar to C++. Java originated at Sun Microsystems, Inc. in 1991. It was conceived by James Gosling, Patrick Naughton, Chris Warth, Ed Frank, and Mike Sheridan at Sun Microsystems, Inc. It was developed to provide a platform-independent programming language. This site gives you an Introduction to Java Programming accompanied with many java examples. Its a complete course in java programming for beginners to advanced java.

Platform independent

Unlike many other programming languages including C and C++ when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.

Java Virtual Machine

What is the Java Virtual Machine? What is its role?

Java was designed with a concept of ‘write once and run everywhere’. Java Virtual Machine plays the central role in this concept. The JVM is the environment in which Java programs execute. It is a software that is implemented on top of real hardware and operating system. When the source code (.java files) is compiled, it is translated into byte codes and then placed into (.class) files. The JVM executes these bytecodes. So Java byte codes can be thought of as the machine language of the JVM. A JVM can either interpret the bytecode one instruction at a time or the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler. The JVM must be implemented on a particular platform before compiled programs can run on that platform.

Object Oriented Programming

Since Java is an object oriented programming language it has following features:

• Reusability of Code• Emphasis on data rather than procedure• Data is hidden and cannot be accessed by external functions• Objects can communicate with each other through functions• New data and functions can be easily addedJava has powerful features. The following

are some of them:-

SimpleReusablePortable (Platform Independent)DistributedRobustSecureHigh PerformanceDynamicThreadedInterpreted

Object Oriented Programming is a method of implementation in which programs are organized as cooperative collection of objects, each of which represents an instance of a class, and whose classes are all members of a hierarchy of classes united via inheritance relationships.

OOP Concepts

Four principles of Object Oriented Programming are:AbstractionEncapsulationInheritancePolymorphism

Abstraction

Abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.

Encapsulation

Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior ; encapsulation serves to separate the contractual interface of an abstraction and its implementation.

Encapsulation

* Hides the implementation details of a class.* Forces the user to use an interface to access data* Makes the code more maintainable.

Inheritance

Inheritance is the process by which one object acquires the properties of another object.

Polymorphism

Polymorphism is the existence of the classes or methods in different forms or single name denoting differentimplementations.

Java is Distributed

With extensive set of routines to handle TCP/IP protocols like HTTP and FTP java can open and access the objects across net via URLs.

Java is Multithreaded

One of the powerful aspects of the Java language is that it allows multiple threads of execution to run concurrently within the same program A single Java program can have many different threads executing independently and continuously. Multiple Java applets can run on the browser at the same time sharing the CPU time.

Java is Secure

Java was designed to allow secure execution of code across network. To make Java secure many of the features of C and C++ were eliminated. Java does not use Pointers. Java programs cannot access arbitrary addresses in memory.

Garbage collection

Automatic garbage collection is another great feature of Java with which it prevents inadvertent corruption of memory. Similar to C++, Java has a new operator to allocate memory on the heap for a new object. But it does not use delete operator to free the memory as it is done in C++ to free the memory if the object is no longer needed. It is done automatically with garbage collector.

Java Applications

Java has evolved from a simple language providing interactive dynamic content for web pages to a predominant enterprise-enabled programming language suitable for developing significant and critical applications. Today, It is used for many types of applications including Web based applications, Financial applications, Gaming applications, embedded systems, Distributed enterprise applications, mobile applications, Image processors, desktop applications and many more. This site outlines the building blocks of java by stating few java examples along with some java tutorials.

Getting Started with Java

Getting Java Basics quickly has become easy with Javabeginner.com. This site is for the java programmers who want to use the Java programming language to create applications using the Java basics. This site is for absolute beginners to advanced java programmers who do not require any prerequisite Java knowledge. For getting started with Java you’ll have to have some basic understanding of the concepts of programming.

After going through all the tutorials in this site, you would have learnt the essential concepts and features of the Java Programming Language which include exceptions, Swing GUI programming, Collections framework etc. A lot of code examples are used through out tutorial to make you understand the language better.All the listings and programs in the website are compiled and run using the JDK 1.5.Download : JDK and JRE 1.5

Java Architecture

The Java environment is composed of a number of system components. You use these components at compile time to create the Java program and at run time to execute the program. Java achieves its independence by creating programs designed to run on the Java Virtual Machine rather than any specific computer system.

• After you write a Java program, you use a compiler that reads the statements in the program and translates them into a machine independent format called bytecode.

• Bytecode files, which are very compact, are easily transported through a distributed system like the Internet.

• The compiled Java code (resulting byte code) will be executed at run time.

Java programs can be written and executed in two ways:

• Stand-alone application (A Java Swing Application)• Applet which runs on a web browser (Example: Internet Explorer)

Java source code

A Java program is a collection of one or more java classes. A Java source file can contain more than one class definition and has a .java extension. Each class definition in a source file is compiled into a separate class file. The name of this compiled file is comprised of the name of the class with .class as an extension. Before we proceed further in this section, I would recommend you to go through the ‘Basic Language Elements’.

Below is a java sample code for the traditional Hello World program. Basically, the idea behind this Hello World program is to learn how to create a program, compile and run it. To create your java source code you can use any editor( Text pad/Edit plus are my favorites) or you can use an IDE like Eclipse.

public class HelloWorld {public static void main(String[] args) {

System.out.println("Hello World");}//End of main

}//End of HelloWorld Class

OutputHello World

ABOUT THE PROGRAM

I created a class named “HelloWorld” containing a simple main function within it. The keyword class specifies that we are defining a class. The name of a public class is spelled exactly as the name of the file (Case Sensitive). All java programs begin execution with the method named main(). main method that gets executed has the following signature : public static void main(String args[]).Declaring this method as public means that it is accessible from outside the class so that the JVM can find it when it looks for the program to start it. It is necessary that the method is declared with return type void (i.e. no arguments are returned from the method). The main method contains a String argument array that can contain the command line arguments. The brackets { and } mark the beginning and ending of the class. The program contains a line ‘System.out.println(“Hello World”);’ that tells the computer to print out on one line of text namely ‘Hello World’. The semi-colon ‘;’ ends the line of code. The double slashes ‘//’ are used for comments that can be used to describe what a source code is doing. Everything to the right of the slashes on the same line does not get compiled, as they are simply the comments in a program.

Java Main method Declarations

class MainExample1 {public static void main(String[] args) {}}class MainExample2 {public static void main(String []args) {}}class MainExample3 {public static void main(String args[]) {}}

All the 3 valid main method’s shown above accepts a single String array argument.

Compiling and Running an Application

To compile and run the program you need the JDK distributed by Sun Microsystems. The JDK contains documentation, examples, installation instructions, class

libraries and packages, and tools. Download an editor like Textpad/EditPlus to type your code. You must save your source code with a .java extension. The name of the file must be the name of the public class contained in the file.

Steps for Saving, compiling and Running a Java

Step 1:Save the program With .java Extension.Step 2:Compile the file from DOS prompt by typing javac <filename>.Step 3:Successful Compilation, results in creation of .class containing byte codeStep 4:Execute the file by typing java <filename without extension>

Java Development Kit

The Java Developer’s Kit is distributed by Sun Microsystems. The JDK contains documentation, examples, installation instructions, class libraries and packages, and tools

javadoc

The javadoc tool provided by Sun is used to produce documentation for an application or program,

Jar Files

A jar file is used to group together related class files into a single file for more compact storage, distribution, and transmission.

PATH and CLASSPATH

The following are the general programming errors, which I think every beginning java programmer would come across. Here is a solution on how to solve the problems when running on a Microsoft Windows Machine.

1. ‘javac’ is not recognized as an internal or external command, operable program or batch file

When you get this error, you should conclude that your operating system cannot find the compiler (javac). To solve this error you need to set the PATH variable.

How to set the PATH Variable?

Firstly the PATH variable is set so that we can compile and execute programs from any directory without having to type the full path of the command. To set the PATH of jdk

on your system (Windows XP), add the full path of the jdk<version>\bin directory to the PATH variable. Set the PATH as follows on a Windows machine:

a. Click Start > Right Click “My Computer” and click on “Properties”b. Click Advanced > Environment Variables.c. Add the location of bin folder of JDK installation for PATH in User Variables and System Variables. A typical value for PATH is:

C:\jdk<version>\bin (jdk<version is nothing but the name of the directory where jdk is installed)

If there are already some entries in the PATH variable then you must add a semicolon and then add the above value (Version being replaced with the version of JDK). The new path takes effect in each new command prompt window you open after setting the PATH variable.

2. Exception in thread “main” java.lang.NoClassDefFoundError: HelloWorld

If you receive this error, java cannot find your compiled byte code file, HelloWorld.class.If both your class files and source code are in the same working directory and if you try running your program from the current working directory than, your program must get executed without any problems as, java tries to find your .class file is your current directory. If your class files are present in some other directory other than that of the java files we must set the CLASSPATH pointing to the directory that contain your compiled class files.CLASSPATH can be set as follows on a Windows machine:

a. Click Start > Right Click “My Computer” and click on “Properties”b. Click Advanced > Environment Variables.

Add the location of classes’ folder containing all your java classes in User Variables.

If there are already some entries in the CLASSPATH variable then you must add a semicolon and then add the new value . The new class path takes effect in each new command prompt window you open after setting the CLASSPATH variable.

Java 1.5

The Java 1.5 released in September 2004.

Goals

Less code complexityBetter readabilityMore compile-time type safetySome new functionality (generics, scanner)

New Features

Enhanced for loopEnumerated typesAutoboxing & unboxingGeneric typesScannerVariable number of arguments (varargs)Static importsAnnotations

Basic Language Elements

(Identifiers, keywords, literals, white spaces and comments)

This part of the java tutorial teaches you the basic language elements and syntax for the java programming language. Once you get these basic language concepts you can continue with the other object oriented programming language concepts.

Keywords

There are certain words with a specific meaning in java which tell (help) the compiler what the program is supposed to do. These Keywords cannot be used as variable names, class names, or method names. Keywords in java are case sensitive, all characters being lower case.

Keywords are reserved words that are predefined in the language; see the table below (Taken from Sun Java Site). All the keywords are in lowercase.

abstract default if private thisboolean do implements protected throwbreak double import public throwsbyte else instanceof return transientcase extends int short trycatch final interface static voidchar finally long strictfp volatileclass float native super whileconst for new switchcontinue goto package synchronized

Keywords are marked in yellow as shown in the sample code below

/** This class is a Hello World Program used to introducethe Java Language*/public class HelloWorld {public static void main(String[] args) {System.out.println(“Hello World”); //Prints output to console

}

For more information on different Keywords – Java Keywords

Some Tricky Observations: The words virtual, ifdef, typedef, friend, struct and union are all words related to the C programming language. const and goto are Java keywords. The word finalize is the name of a method of the Object class and hence not a keyword. enum and label are not keywords.

Comments

Comments are descriptions that are added to a program to make code easier to understand. The compiler ignores comments and hence its only for documentation of the program.

Java supports three comment styles.

Block style comments begin with /* and terminate with */ that spans multiple lines.

Line style comments begin with // and terminate at the end of the line. (Shown in the above program)

Documentation style comments begin with /** and terminate with */ that spans multiple lines. They are generally created using the automatic documentation generation tool, such as javadoc. (Shown in the above program). Name of this compiled file is comprised of the name of the class with .class as an extension.

Variable, Identifiers and Data Types

Variables are used for data that change during program execution. All variables have a name, a type, and a scope. The programmer assigns the names to variables, known as identifiers. An Identifier must be unique within a scope of the Java program. Variables have a data type, that indicates the kind of value they can store. Variables declared inside of a block or method are called local variables; They are not automatically initialized. The compiler will generate an error as a result of the attempt to access the local variables before a value has been assigned.

public class localVariableEx { public static int a; public static void main(String[] args) { int b; System.out.println("a : "+a); System.out.println("b : "+b); //Compilation error}}

Note in the above example, a compilation error results in where the variable is tried to be accessed and not at the place where its declared without any value.

The data type indicates the attributes of the variable, such as the range of values that can be stored and the operators that can be used to manipulate the variable. Java has four main primitive data types built into the language. You can also create your own composite data types.

Java has four main primitive data types built into the language. We can also create our own data types.

Integer: byte, short, int, and long.

Floating Point: float and double

Character: char

Boolean: variable with a value of true or false.

The following chart (Taken from Sun Java Site) summarizes the default values for the java built in data types. Since I thought Mentioning the size was not important as part of learning Java, I have not mentioned it in the below table. The size for each Java type can be obtained by a simple Google search.

Data Type Default Value (for fields) Range

byte 0 -127 to +128

short 0 -32768 to +32767

int 0

long 0L

float 0.0f

double 0.0d

char ‘\u0000′ 0 to 65535

String (object) null

boolean false

When we declare a variable we assign it an identifier and a data type.

For Example

String message = “hello world”

In the above statement, String is the data type for the identifier message. If you don’t specify a value when the variable is declared, it will be assigned the default value for its data type.

Identifier Naming Rules

• Can consist of upper and lower case letters, digits, dollar sign ($) and the underscore ( _ ) character.

• Must begin with a letter, dollar sign, or an underscore• Are case sensitive• Keywords cannot be used as identifiers• Within a given section of your program or scope, each user defined item must have a

unique identifier• Can be of any length.

Classes

A class is nothing but a blueprint for creating different objects which defines its properties and behaviors. An object exhibits the properties and behaviors defined by its class. A class can contain fields and methods to describe the behavior of an object. Methods are nothing but members of a class that provide a service for an object or perform some business logic.

Objects

An object is an instance of a class created using a new operator. The new operator returns a reference to a new instance of a class. This reference can be assigned to a reference variable of the class. The process of creating objects from a class is called instantiation. An object reference provides a handle to an object that is created and stored in memory. In Java, objects can only be manipulated via references, which can be stored in variables.

Interface

An Interface is a contract in the form of collection of method and constant declarations. When a class implements an interface, it promises to implement all of the methods declared in that interface.

Instance Members

Each object created will have its own copies of the fields defined in its class called instance variables which represent an object’s state. The methods of an object define its behaviour called instance methods. Instance variables and instance methods, which belong to objects, are collectively called instance members. The dot ‘.’ notation with a object reference is used to access Instance Members.

Static Members

Static members are those that belong to a class as a whole and not to a particular instance (object). A static variable is initialized when the class is loaded. Similarly, a class can have static methods. Static variables and static methods are collectively known as static members, and are declared with a keyword static. Static members in the class can be accessed either by using the class name or by using the object reference, but instance members can only be accessed via object references.

Below is a program showing the various parts of the basic language syntax that were discussed above.

/** Comment * Displays "Hello World!" to the standard output.

*/public class HelloWorld { String output = ""; static HelloWorld helloObj; //Line 1

public HelloWorld(){ output = "Hello World"; }

public String printMessage(){ return output; }

public static void main (String args[]) { helloObj = new HelloWorld(); //Line 2 System.out.println(helloObj.printMessage()); }

}

Class Name: HelloWorldObject Reference: helloObj (in Line 1)Object Created: helloObj (In Line 2)Member Function: printMessageField: output (String)Static Member: helloObjInstance Member : output (String)

Java Operators Tutorial

Java Operators

They are used to manipulate primitive data types. Java operators can be classified as unary, binary, or ternary—meaning taking one, two, or three arguments, respectively. A unary operator may appearbefore (prefix) its argument or after (postfix) its argument. A binary or ternary operator appears between its arguments.

Operators in java fall into 8 different categories:

Java operators fall into eight different categories: assignment, arithmetic, relational, logical, bitwise, compound assignment, conditional, and type.

Assignment Operators = Arithmetic Operators - + * / % ++ -- Relational Operators > < >= <= == != Logical Operators && || & | ! ^ Bit wise Operator & | ^ >> >>> Compound Assignment Operators += -= *= /= %= <<= >>= >>>= Conditional Operator ?:

Java has eight different operator types: assignment, arithmetic, relational, logical, bitwise, compound assignment, conditional, and type.

Assignment operators

The java assignment operator statement has the following syntax:

<variable> = <expression>

If the value already exists in the variable it is overwritten by the assignment operator (=).

public class AssignmentOperatorsDemo {

public AssignmentOperatorsDemo() {// Assigning Primitive Valuesint j, k;j = 10; // j gets the value 10.j = 5; // j gets the value 5. Previous value is overwritten.k = j; // k gets the value 5.System.out.println("j is : " + j);System.out.println("k is : " + k);// Assigning ReferencesInteger i1 = new Integer("1");Integer i2 = new Integer("2");System.out.println("i1 is : " + i1);System.out.println("i2 is : " + i2);i1 = i2;System.out.println("i1 is : " + i1);System.out.println("i2 is : " + i2);// Multiple Assignmentsk = j = 10; // (k = (j = 10))System.out.println("j is : " + j);System.out.println("k is : " + k);

}public static void main(String args[]) {

new AssignmentOperatorsDemo();}

}Download AssignmentOperatorsDemoSource code

Arithmetic operators

Java provides eight Arithmetic operators. They are for addition, subtraction, multiplication, division, modulo (or remainder), increment (or add 1), decrement (or subtract 1), and negation. An example program is shown below that demonstrates the different arithmetic operators in java.

The binary operator + is overloaded in the sense that the operation performed is determined by the type of the operands. When one of the operands is a String object, the other operand is implicitly converted to its string representation and string concatenation is performed.

String message = 100 + "Messages"; //"100 Messages"

public class ArithmeticOperatorsDemo {

public ArithmeticOperatorsDemo() {int x, y = 10, z = 5;x = y + z;System.out.println("+ operator resulted in " + x);x = y - z;System.out.println("- operator resulted in " + x);x = y * z;System.out.println("* operator resulted in " + x);x = y / z;System.out.println("/ operator resulted in " + x);x = y % z;System.out.println("% operator resulted in " + x);x = y++;System.out.println("Postfix ++ operator resulted in " + x);x = ++z;System.out.println("Prefix ++ operator resulted in " + x);x = -y;System.out.println("Unary operator resulted in " + x);// Some examples of special Casesint tooBig = Integer.MAX_VALUE + 1; // -2147483648 which is// Integer.MIN_VALUE.int tooSmall = Integer.MIN_VALUE - 1; // 2147483647 which is// Integer.MAX_VALUE.System.out.println("tooBig becomes " + tooBig);System.out.println("tooSmall becomes " + tooSmall);System.out.println(4.0 / 0.0); // Prints: InfinitySystem.out.println(-4.0 / 0.0); // Prints: -InfinitySystem.out.println(0.0 / 0.0); // Prints: NaNdouble d1 = 12 / 8; // result: 1 by integer division. d1 gets the value// 1.0.double d2 = 12.0F / 8; // result: 1.5System.out.println("d1 is " + d1);System.out.println("d2 iss " + d2);

}

public static void main(String args[]) {new ArithmeticOperatorsDemo();

}}Download ArithmeticOperatorsDemo Source code

Relational operators

Relational operators in Java are used to compare 2 or more objects. Java provides six relational operators:

greater than (>), less than (<), greater than or equal (>=), less than or equal (<=), equal (==), and not equal (!=).

All relational operators are binary operators, and their operands are numeric expressions.

Binary numeric promotion is applied to the operands of these operators. The evaluation results in a boolean value. Relational operators have precedence lower than arithmetic operators, but higher than that of the assignment operators. An example program is shown below that demonstrates the different relational operators in java.

public class RelationalOperatorsDemo { public RelationalOperatorsDemo( ) {

int x = 10, y = 5; System.out.println("x > y : "+(x > y)); System.out.println("x < y : "+(x < y)); System.out.println("x >= y : "+(x >= y)); System.out.println("x <= y : "+(x <= y)); System.out.println("x == y : "+(x == y)); System.out.println("x != y : "+(x != y)); }

public static void main(String args[]){ new RelationalOperatorsDemo(); } }Download RelationalOperatorsDemo Source code

Logical operators

Logical operators return a true or false value based on the state of the Variables. There are six logical, or boolean, operators. They are AND, conditional AND, OR, conditional OR, exclusive OR, and NOT. Each argument to a logical operator must be a boolean data type, and the result is always a boolean data type. An example program is shown below that demonstrates the different Logical operators in java.

public class LogicalOperatorsDemo {

public LogicalOperatorsDemo() {boolean x = true;boolean y = false;System.out.println("x & y : " + (x & y));System.out.println("x && y : " + (x && y));System.out.println("x | y : " + (x | y));System.out.println("x || y: " + (x || y));System.out.println("x ^ y : " + (x ^ y));System.out.println("!x : " + (!x));

}public static void main(String args[]) {

new LogicalOperatorsDemo();}

}Download LogicalOperatorsDemo Source code

Given that x and y represent boolean expressions, the boolean logical operators are defined in the Table below.

x y !x x & y

x && y

x | y

x || y

x ^ y

true true false true true false

true false false false true true

false true true false true true

false false true false false false

Bitwise operators

Java provides Bit wise operators to manipulate the contents of variables at the bit level.

These variables must be of numeric data type ( char, short, int, or long). Java provides seven bitwise

operators. They are AND, OR, Exclusive-OR, Complement, Left-shift, Signed Right-shift, and Unsigned Right-shift. An example program is shown below that demonstrates the different Bit wise operators in java.

public class BitwiseOperatorsDemo { public BitwiseOperatorsDemo() { int x = 0xFAEF; //1 1 1 1 1 0 1 0 1 1 1 0 1 1 1 1 int y = 0xF8E9; //1 1 1 1 1 0 0 0 1 1 1 0 1 0 0 1 int z; System.out.println("x & y : " + (x & y)); System.out.println("x | y : " + (x | y)); System.out.println("x ^ y : " + (x ^ y)); System.out.println("~x : " + (~x)); System.out.println("x << y : " + (x << y)); System.out.println("x >> y : " + (x >> y)); System.out.println("x >>> y : " + (x >>> y)); //There is no unsigned left shift operator }

public static void main(String args[]) new BitwiseOperatorsDemo();

}Download BitwiseOperatorsDemo Source code

The result of applying bitwise operators between two corresponding bits in the operands is shown in the Table below.

A B ~A A & B A | B A ^ B

1 1 0 1 1 0

1 0 0 0 1 1

0 1 1 0 1 1

0 0 1 0 0 0

Output

3,0,3

/* * The below program demonstrates bitwise operators keeping in mind operator precedence * Operator Precedence starting with the highest is -> |, ^, & */

public class BitwisePrecedenceEx {

public static void main(String[] args) {int a = 1 | 2 ^ 3 & 5;int b = ((1 | 2) ^ 3) & 5;int c = 1 | (2 ^ (3 & 5));System.out.print(a + "," + b + "," + c);

}}

Downloadad BitwiseOperatorsDemo2 Source code

Compound operators

The compound operators perform shortcuts in common programming operations. Java has eleven compound assignment operators.

Syntax:

argument1 operator = argument2.

The above statement is the same as, argument1 = argument1 operator argument2. An example program is shown below that demonstrates the different Compound operators in java.

public class CompoundOperatorsDemo {

public CompoundOperatorsDemo() {int x = 0, y = 5;x += 3;System.out.println("x : " + x);y *= x;System.out.println("y : " + y);/*Similarly other operators can be applied as shortcuts. Other

compound assignment operators include boolean logical

, bitwiseand shift operators*/}public static void main(String args[]) {

new CompoundOperatorsDemo();}

}

Download CompoundOperatorsDemo Source code

Conditional operators

The Conditional operator is the only ternary (operator takes three arguments) operator in Java. The operator evaluates the first argument and, if true, evaluates the second argument. If the first argument evaluates to false, then the third argument is evaluated. The conditional operator is the expression equivalent of the if-else statement. The conditional expression can be nested and the conditional operator associates from right to left: (a?b?c?d:e:f:g) evaluates as (a?(b?(c?d:e):f):g)

An example program is shown below that demonstrates the Ternary operator in java.

public class TernaryOperatorsDemo {

public TernaryOperatorsDemo() {int x = 10, y = 12, z = 0;z = x > y ? x : y;System.out.println("z : " + z);

}

public static void main(String args[]) {new TernaryOperatorsDemo();

}}Download TernaryOperatorsDemo Source code

/* * The following programs shows that when no explicit parenthesis is used then the conditional operator * evaluation is from right to left */

public class BooleanEx1 {

static String m1(boolean b) {return b ? "T" : "F";

}public static void main(String[] args) {

boolean t1 = false ? false : true ? false : true ? false : true;boolean t2 = false ? false

: (true ? false : (true ? false : true));boolean t3 = ((false ? false : true) ? false : true) ? false

: true;System.out.println(m1(t1) + m1(t2) + m1(t3));

}}

Output

FFT

Download TernaryOperatorsDemo2 Source code

Type conversion allows a value to be changed from one primitive data type to another. Conversion can occur explicitly, as specified in

The program, or implicitly, by Java itself. Java allows both type widening and type narrowing conversions.

In java Conversions can occur by the following ways:

• Using a cast operator (explicit promotion)• Using an arithmetic operator is used with arguments of different data types (arithmetic

promotion)

• A value of one type is assigned to a variable of a different type (assignment promotion)

Operator Precedence

The order in which operators are applied is known as precedence. Operators with a higher precedence are applied before operators with a lower precedence. The operator precedence order of Java is shown below. Operators at the top of the table are applied before operators lower down in the table. If two operators have the same precedence, they are applied in the order they appear in a statement.

That is, from left to right. You can use parentheses to override the default precedence.

postfix [] . () expr++ expr--unary ++expr --expr +expr -expr ! ~

creation/caste new (type)exprmultiplicative * / %

additive + -shift >> >>>

relational < <= > >= instanceofequality == !=

bitwise AND &bitwise exclusive OR ^bitwise inclusive OR |

logical AND &&logical OR ||

ternary ?:assignment = "op="

Java Control Flow Statements

Java Control statements control the order of execution in a java program, based on data values and conditional logic. There are three main categories of control flow statements;

· Selection statements: if, if-else and switch.

· Loop statements: while, do-while and for.

· Transfer statements: break, continue, return, try-catch-finally and assert.

We use control statements when we want to change the default sequential order of execution

Selection Statements

The If Statement

The if statement executes a block of code only if the specified expression is true. If the value is false, then the if block is skipped and execution continues with the rest of the program. You can either have a single statement or a block of code within an if statement. Note that the conditional expression must be a Boolean expression.

The simple if statement has the following syntax:

if (<conditional expression>)<statement action>

Below is an example that demonstrates conditional execution based on if statement condition.

public class IfStatementDemo {

public static void main(String[] args) {int a = 10, b = 20;if (a > b)

System.out.println("a > b");if (a < b)

System.out.println("b > a");}

}

The If-else Statement

The if/else statement is an extension of the if statement. If the statements in the if statement fails, the statements in the else block are executed. You can either have a single statement or a block of code within if-else blocks. Note that the conditional expression must be a Boolean expression.

The if-else statement has the following syntax:

if (<conditional expression>)<statement action>else<statement action>

Below is an example that demonstrates conditional execution based on if else statement condition.

public class IfElseStatementDemo {

public static void main(String[] args) {int a = 10, b = 20;if (a > b) {

System.out.println("a > b");} else {

System.out.println("b > a");}

}}

Switch Case Statement

The switch case statement, also called a case statement is a multi-way branch with several choices. A switch is easier to implement than a series of if/else statements. The switch statement begins with a keyword, followed by an expression that equates to a no long integral value. Following the controlling expression is a code block that contains zero or more labeled cases. Each label must equate to an integer constant and each must be unique. When the switch statement executes, it compares the value of the controlling expression to the values of each case label. The program will select the value of the case label that equals the value of the controlling expression and branch down that path to the end of the code block. If none of the case label values match, then none of the codes within the switch statement code block will be executed. Java includes a default label to use in cases where there are no matches. We can have a nested switch within a case block of an outer switch.

Its general form is as follows:

switch (<non-long integral expression>) {case label1: <statement1>case label2: <statement2>…case labeln: <statementn>default: <statement>} // end switch

When executing a switch statement, the program falls through to the next case. Therefore, if you want to exit in the middle of the switch statement code block, you must insert a break statement, which causes the program to continue executing after the current code block.

Below is a java example that demonstrates conditional execution based on nested if else statement condition to find the greatest of 3 numbers.

public class SwitchCaseStatementDemo {

public static void main(String[] args) {int a = 10, b = 20, c = 30;int status = -1;if (a > b && a > c) {

status = 1;} else if (b > c) {

status = 2;} else {

status = 3;}switch (status) {case 1:

System.out.println("a is the greatest");break;

case 2:System.out.println("b is the greatest");break;

case 3:System.out.println("c is the greatest");break;

default:System.out.println("Cannot be determined");

}

}}

Iteration Statements

While Statement

The while statement is a looping construct control statement that executes a block of code while a condition is true. You can either have a single statement or a block of code within the while loop. The loop will never be executed if the testing expression evaluates to false. The loop condition must be a boolean expression.

The syntax of the while loop is

while (<loop condition>)<statements>

Below is an example that demonstrates the looping construct namely while loop used to print numbers from 1 to 10.

public class WhileLoopDemo {

public static void main(String[] args) {int count = 1;System.out.println("Printing Numbers from 1 to 10");while (count <= 10) {

System.out.println(count++);}

}}

Output

Printing Numbers from 1 to 10

Download WhileLoopDemo.java

Do-while Loop Statement

The do-while loop is similar to the while loop, except that the test is performed at the end of the loop instead of at the beginning. This ensures that the loop will be executed at least once. A do-while loop begins with the keyword do, followed by the statements that make up the body of the loop. Finally, the keyword while and the test expression completes the do-while loop. When the loop condition becomes false, the loop is terminated and execution continues with the statement immediately following the loop. You can either have a single statement or a block of code within the do-while loop.

The syntax of the do-while loop is

do<loop body>while (<loop condition>);

Below is an example that demonstrates the looping construct namely do-while loop used to print numbers from 1 to 10.

public class DoWhileLoopDemo {

public static void main(String[] args) {int count = 1;System.out.println("Printing Numbers from 1 to 10");do {

System.out.println(count++);} while (count <= 10);

}}

Output

Printing Numbers from 1 to 10

Download DoWhileLoopDemo.java

Below is an example that creates A Fibonacci sequence controlled by a do-while loop

public class Fibonacci {

public static void main(String args[]) {System.out.println("Printing Limited set of Fibonacci Sequence");double fib1 = 0;double fib2 = 1;double temp = 0;System.out.println(fib1);System.out.println(fib2);do {

temp = fib1 + fib2;System.out.println(temp);fib1 = fib2; //Replace 2nd with first numberfib2 = temp; //Replace temp number with 2nd number

} while (fib2 < 5000);}

}

Output:Printing Limited set of Fibonacci Sequence (Download Fibonacci.java)

0.01.01.02.03.05.08.013.021.034.055.089.0144.0233.0377.0610.0987.01597.02584.04181.06765.0

For Loops

The for loop is a looping construct which can execute a set of instructions a specified number of times. It’s a counter controlled loop.

The syntax of the loop is as follows:

for (<initialization>; <loop condition>; <increment expression>)<loop body>

The first part of a for statement is a starting initialization, which executes once before the loop begins. The <initialization> section can also be a comma-separated list of expression statements. The second part of a for statement is a test expression. As long as the expression is true, the loop will continue. If this expression is evaluated as false the first time, the loop will never be executed. The third part of the for statement is the body of the loop. These are the instructions that are repeated each time the program executes the loop. The final part of the for statement is an increment expression that automatically executes after each repetition of the loop body. Typically, this statement changes the value of the counter, which is then tested to see if the loop should continue.All the sections in the for-header are optional. Any one of them can be left empty, but the two semicolons are mandatory. In particular, leaving out the <loop condition> signifies that the loop condition is true. The (;;) form of for loop is commonly used to construct an infinite loop.

Below is an example that demonstrates the looping construct namely for loop used to print numbers from 1 to 10.

public class ForLoopDemo {

public static void main(String[] args) {System.out.println("Printing Numbers from 1 to 10");for (int count = 1; count <= 10; count++) {

System.out.println(count);}

}}

Output

Printing Numbers from 1 to 10

Transfer Statements

Continue Statement

A continue statement stops the iteration of a loop (while, do or for) and causes execution to resume at the top of the nearest enclosing loop. You use a continue statement when you do not want to execute the remaining statements in the loop, but you do not want to exit the loop itself.

The syntax of the continue statement is

continue; // the unlabeled formcontinue <label>; // the labeled form

You can also provide a loop with a label and then use the label in your continue statement. The label name is optional, and is usually only used when you wish to return to the outermost loop in a series of nested loops.

Below is a program to demonstrate the use of continue statement to print Odd Numbers between 1 to 10.

public class ContinueExample {

public static void main(String[] args) {System.out.println("Odd Numbers");for (int i = 1; i <= 10; ++i) {

if (i % 2 == 0)continue;

// Rest of loop body skipped when i is evenSystem.out.println(i + "\t");

}}

}

Output

Odd Numbers13579

Download ContinueExample.java

Break Statement

The break statement transfers control out of the enclosing loop ( for, while, do or switch statement). You use a break statement when you want to jump immediately to the statement following the enclosing control structure. You can also provide a loop with a label, and then use the label in your break statement. The label name is optional, and is usually only used when you wish to terminate the outermost loop in a series of nested loops.

The Syntax for break statement is as shown below;

break; // the unlabeled formbreak <label>; // the labeled form

Below is a program to demonstrate the use of break statement to print numbers Numbers 1 to 10.

public class BreakExample {

public static void main(String[] args) {System.out.println("Numbers 1 - 10");for (int i = 1;; ++i) {

if (i == 11)break;

// Rest of loop body skipped when i is evenSystem.out.println(i + "\t");

}}

}

Output

Numbers 1 – 1012345678910

Introduction to Java Access Modifiers

Java Access Specifiers

The access to classes, constructors, methods and fields are regulated using access modifiers i.e. a class can control what information or data can be accessible by other classes. To take advantage of encapsulation, you should minimize access whenever possible.

Java provides a number of access modifiers to help you set the level of access you want for classes as well as the fields, methods and constructors in your classes. A member has package or default accessibility when no accessibility modifier is specified.

Access Modifiers

1. private2. protected3. default4. public

public access modifier

Fields, methods and constructors declared public (least restrictive) within a public class are visible to any class in the Java program, whether these classes are in the same package or in another package.

private access modifier

The private (most restrictive) fields or methods cannot be used for classes and Interfaces. It also cannot be used for fields and methods within an interface. Fields, methods or constructors declared private are strictly controlled, which means they cannot be accesses by anywhere outside the enclosing class. A standard design strategy is to make all fields private and provide public getter methods for them.

protected access modifier

The protected fields or methods cannot be used for classes and Interfaces. It also cannot be used for fields and methods within an interface. Fields, methods and constructors declared protected in a superclass can be accessed only by subclasses in other packages. Classes in the same package can also access protected fields, methods and constructors as well, even if they are not a subclass of the protected member’s class.

default access modifier

Java provides a default specifier which is used when no access modifier is present. Any class, field, method or constructor that has no declared access modifier is accessible only by classes in the same package. The default modifier is not used for fields and methods within an interface.

Below is a program to demonstrate the use of public, private, protected and default access modifiers while accessing fields and methods. The output of each of these java files depict the Java access specifiers.

The first class is SubclassInSamePackage.java which is present in pckage1 package. This java file contains the Base class and a subclass within the enclosing class that belongs to the same class as shown below.

package pckage1;

class BaseClass {

public int x = 10;private int y = 10;protected int z = 10;int a = 10; //Implicit Default Access Modifierpublic int getX() {

return x;}public void setX(int x) {

this.x = x;}private int getY() {

return y;}private void setY(int y) {

this.y = y;}protected int getZ() {

return z;}protected void setZ(int z) {

this.z = z;}int getA() {

return a;}void setA(int a) {

this.a = a;}

}

public class SubclassInSamePackage extends BaseClass {

public static void main(String args[]) {BaseClass rr = new BaseClass();rr.z = 0;SubclassInSamePackage subClassObj = new SubclassInSamePackage();//Access Modifiers - PublicSystem.out.println("Value of x is : " + subClassObj.x);subClassObj.setX(20);System.out.println("Value of x is : " + subClassObj.x);//Access Modifiers - Public// If we remove the comments it would result in a compilaton// error as the fields and methods being accessed are private/* System.out.println("Value of y is : "+subClassObj.y);

subClassObj.setY(20);

System.out.println("Value of y is : "+subClassObj.y);*///Access Modifiers - ProtectedSystem.out.println("Value of z is : " + subClassObj.z);subClassObj.setZ(30);System.out.println("Value of z is : " + subClassObj.z);//Access Modifiers - DefaultSystem.out.println("Value of x is : " + subClassObj.a);subClassObj.setA(20);System.out.println("Value of x is : " + subClassObj.a);

}}

Output

Value of x is : 10 Value of x is : 20 Value of z is : 10Value of z is : 30 Value of x is : 10 Value of x is : 20

The second class is SubClassInDifferentPackage.java which is present in a different package then the first one. This java class extends First class (SubclassInSamePackage.java).

import pckage1.*;

public class SubClassInDifferentPackage extends SubclassInSamePackage {

public int getZZZ() {return z;

}

public static void main(String args[]) {SubClassInDifferentPackage subClassDiffObj = new SubClassInDifferentPackage();SubclassInSamePackage subClassObj = new SubclassInSamePackage();//Access specifiers - PublicSystem.out.println("Value of x is : " + subClassObj.x);subClassObj.setX(30);System.out.println("Value of x is : " + subClassObj.x);//Access specifiers - Private// if we remove the comments it would result in a compilaton// error as the fields and methods being accessed are private/* System.out.println("Value of y is : "+subClassObj.y);

subClassObj.setY(20);

System.out.println("Value of y is : "+subClassObj.y);*///Access specifiers - Protected// If we remove the comments it would result in a compilaton// error as the fields and methods being accessed are protected./* System.out.println("Value of z is : "+subClassObj.z);

subClassObj.setZ(30);

System.out.println("Value of z is : "+subClassObj.z);*/System.out.println("Value of z is : " + subClassDiffObj.getZZZ());//Access Modifiers - Default// If we remove the comments it would result in a compilaton// error as the fields and methods being accessed are default./*

System.out.println("Value of a is : "+subClassObj.a);

subClassObj.setA(20);

System.out.println("Value of a is : "+subClassObj.a);*/}

}

Output

Value of x is : 10 Value of x is : 30 Value of z is : 10

The third class is ClassInDifferentPackage.java which is present in a different package then the first one.

import pckage1.*;

public class ClassInDifferentPackage {

public static void main(String args[]) {SubclassInSamePackage subClassObj = new SubclassInSamePackage();//Access Modifiers - PublicSystem.out.println("Value of x is : " + subClassObj.x);subClassObj.setX(30);System.out.println("Value of x is : " + subClassObj.x);//Access Modifiers - Private// If we remove the comments it would result in a compilaton// error as the fields and methods being accessed are private/* System.out.println("Value of y is : "+subClassObj.y);

subClassObj.setY(20);

System.out.println("Value of y is : "+subClassObj.y);*///Access Modifiers - Protected// If we remove the comments it would result in a compilaton// error as the fields and methods being accessed are protected./* System.out.println("Value of z is : "+subClassObj.z);

subClassObj.setZ(30);

System.out.println("Value of z is : "+subClassObj.z);*///Access Modifiers - Default// If we remove the comments it would result in a compilaton// error as the fields and methods being accessed are default./* System.out.println("Value of a is : "+subClassObj.a);

subClassObj.setA(20);

System.out.println("Value of a is : "+subClassObj.a);*/}

}

Output: Value of x is : 10 Value of x is : 30

Java Classes and Objects

Introduction to Java Classes

A class is nothing but a blueprint or a template for creating different objects which defines its properties and behaviors. Java class objects exhibit the properties and behaviors defined by its class. A class can contain fields and methods to describe the behavior of an object.

Methods are nothing but members of a class that provide a service for an object or perform some business logic. Java fields and member functions names are case sensitive. Current states of a class’s corresponding object are stored in the object’s instance variables. Methods define the operations that can be performed in java programming.

A class has the following general syntax:

<class modifiers>class<class name><extends clause> <implements clause>{

// Dealing with Classes (Class body)<field declarations (Static and Non-Static)><method declarations (Static and Non-Static)><Inner class declarations><nested interface declarations><constructor declarations><Static initializer blocks>}

Below is an example showing the Objects and Classes of the Cube class that defines 3 fields namely length, breadth and height. Also the class contains a member function getVolume().

public class Cube {

int length;int breadth;int height;public int getVolume() {

return (length * breadth * height);}

}

How do you reference a data member/function?

This is accomplished by stating the name of the object reference, followed by a period (dot), followed by the name of the member inside the object.( objectReference.member ). You call a method for an object by naming the object followed by a period (dot), followed by the name of the method and its argument list, like this: objectName.methodName(arg1, arg2, arg3).

For example:

cubeObject.length = 4;cubeObject.breadth = 4;cubeObject.height = 4;cubeObject.getvolume()

Class Variables – Static Fields

We use class variables also know as Static fields when we want to share characteristics across all objects within a class. When you declare a field to be static, only a single instance of the associated variable is created common to all the objects of that class. Hence when one object changes the value of a class variable, it affects all objects of the class. We can access a class variable by using the name of the class, and not necessarily using a reference to an individual object within the class. Static variables can be accessed even though no objects of that class exist. It is declared using static keyword.

Class Methods – Static Methods

Class methods, similar to Class variables can be invoked without having an instance of the class. Class methods are often used to provide global functions for Java programs. For example, methods in the java.lang.Math package are class methods. You cannot call non-static methods from inside a static method.

Instance Variables

Instance variables stores the state of the object. Each class would have its own copy of the variable. Every object has a state that is determined by the values stored in the object. An object is said to have changed its state when one or more data values stored in the object have been modified. When an object responds to a message, it will usually perform an action, change its state etc. An object that has the ability to store values is often said to have persistence.

Consider this simple Java program showing the use of static fields and static methods

// Class and Object initialization showing the Object Oriented concepts in Javaclass Cube {

int length = 10;int breadth = 10;int height = 10;public static int numOfCubes = 0; // static variablepublic static int getNoOfCubes() { //static method

return numOfCubes;}public Cube() {

numOfCubes++; //}

}

public class CubeStaticTest {

public static void main(String args[]) {System.out.println("Number of Cube objects = " + Cube.numOfCubes);System.out.println("Number of Cube objects = "

+ Cube.getNoOfCubes());}

}

Download CubeStaticTest.java

Output

Number of Cube objects = 0Number of Cube objects = 0

Final Variable, Methods and Classes

In Java we can mark fields, methods and classes as final. Once marked as final, these items cannot be changed.

Variables defined in an interface are implicitly final. You can’t change value of a final variable (is a constant). A final class can’t be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can’t be overridden when its class is inherited. Any attempt to override or hide a final method will result in a compiler error.

Introduction to Java Objects

The Object Class is the super class for all classes in Java.

Some of the object class methods are:

equalstoString()wait()notify()notifyAll()hashcode()clone()

An object is an instance of a class created using a new operator. The new operator returns a reference to a new instance of a class. This reference can be assigned to a reference variable of the class. The process of creating objects from a class is called instantiation. An object encapsulates state and behavior.

An object reference provides a handle to an object that is created and stored in memory. In Java, objects can only be manipulated via references, which can be stored in variables.

Creating variables of your class type is similar to creating variables of primitive data types, such as integer or float. Each time you create an object, a new set of instance variables comes into existence which defines the characteristics of that object. If you want to create an object of the class and have the reference variable associated with this object, you must also allocate memory for the object by using the new operator. This process is called instantiating an object or creating an object instance.

When you create a new object, you use the new operator to instantiate the object. The new operator returns the location of the object which you assign o a reference type.

Below is an example showing the creation of Cube objects by using the new operator.

public class Cube {

int length = 10;int breadth = 10;int height = 10;public int getVolume() {

return (length * breadth * height);}public static void main(String[] args) {

Cube cubeObj; // Creates a Cube ReferencecubeObj = new Cube(); // Creates an Object of CubeSystem.out.println("Volume of Cube is : " + cubeObj.getVolume());

}}

Download Cube.java

Method Overloading

Method overloading results when two or more methods in the same class have the same name but different parameters. Methods with the same name must differ in their types or number of parameters. This allows the compiler to match parameters and choose the correct method when a number of choices exist. Changing just the return type is not enough to overload a method, and will be a compile-time error. They must have a different signature. When no method matching the input parameters is found, the compiler attempts to convert the input parameters to types of greater precision. A match may then be found without error. At compile time, the right implementation is chosen based on the signature of the method call.

Below is an example of a class demonstrating Method Overloading

public class MethodOverloadDemo {

void sumOfParams() { // First VersionSystem.out.println("No parameters");

}void sumOfParams(int a) { // Second Version

System.out.println("One parameter: " + a);}int sumOfParams(int a, int b) { // Third Version

System.out.println("Two parameters: " + a + " , " + b);return a + b;

}double sumOfParams(double a, double b) { // Fourth Version

System.out.println("Two double parameters: " + a + " , " + b);return a + b;

}

public static void main(String args[]) {MethodOverloadDemo moDemo = new MethodOverloadDemo();int intResult;double doubleResult;moDemo.sumOfParams();System.out.println();moDemo.sumOfParams(2);System.out.println();intResult = moDemo.sumOfParams(10, 20);System.out.println("Sum is " + intResult);System.out.println();doubleResult = moDemo.sumOfParams(1.1, 2.2);System.out.println("Sum is " + doubleResult);System.out.println();

}}

Download MethodOverloadDemo.java

Output: No parameters One parameter: 2 Two parameters: 10 , 20Sum is 30

Two double parameters: 1.1 , 2.2Sum is 3.3000000000000003

Below is a code snippet to shows the interfaces that a Class Implements:

Class cls = java.lang.String.class;Class[] intfs = cls.getInterfaces();// [java.lang.Comparable, java.lang.CharSequence, java.io.Serializable]// The interfaces for a primitive type is an empty arraycls = int.class;

intfs = cls.getInterfaces(); // []Below is a code snippet to show whether a Class Object Represents a Class or Interface:

Class cls = java.lang.String.class;boolean isClass = !cls.isInterface(); // truecls = java.lang.Cloneable.class;isClass = !cls.isInterface(); // false

Java Constructors

A java constructor has the same name as the name of the class to which it belongs. Constructor’s syntax does not include a return type, since constructors never return a value.

Constructors may include parameters of various types. When the constructor is invoked using the new operator, the types must match those that are specified in the constructor definition.

Java provides a default constructor which takes no arguments and performs no special actions or initializations, when no explicit constructors are provided.

The only action taken by the implicit default constructor is to call the superclass constructor using the super() call. Constructor arguments provide you with a way to provide parameters for the initialization of an object.

Below is an example of a cube class containing 2 constructors. (one default and one parameterized constructor).

public class Cube1 {

int length;int breadth;int height;public int getVolume() {

return (length * breadth * height);}Cube1() {

length = 10;breadth = 10;height = 10;

}Cube1(int l, int b, int h) {

length = l;breadth = b;

height = h;}public static void main(String[] args) {

Cube1 cubeObj1, cubeObj2;cubeObj1 = new Cube1();cubeObj2 = new Cube1(10, 20, 30);System.out.println("Volume of Cube1 is : " + cubeObj1.getVolume());System.out.println("Volume of Cube1 is : " + cubeObj2.getVolume());

}}

Download Cube1.java

Note: If a class defines an explicit constructor, it no longer has a default constructor to set the state of the objects.If such a class requires a default constructor, its implementation must be provided. Any attempt to call the default constructor will be a compile time error if an explicit default constructor is not provided in such a case.

Java Overloaded Constructors

Like methods, constructors can also be overloaded. Since the constructors in a class all have the same name as the class, />their signatures are differentiated by their parameter lists. The above example shows that the Cube1 constructor is overloaded one being the default constructor and the other being a parameterized constructor.

It is possible to use this() construct, to implement local chaining of constructors in a class. The this() call in a constructorinvokes the an other constructor with the corresponding parameter list within the same class. Calling the default constructor to create a Cube object results in the second and third parameterized constructors being called as well. Java requires that any this() call must occur as the first statement in a constructor.

Below is an example of a cube class containing 3 constructors which demostrates the this() method in Constructors context

public class Cube2 {

int length;int breadth;int height;public int getVolume() {

return (length * breadth * height);}Cube2() {

this(10, 10);

System.out.println("Finished with Default Constructor");}Cube2(int l, int b) {

this(l, b, 10);System.out.println("Finished with Parameterized Constructor having 2 params");

}Cube2(int l, int b, int h) {

length = l;breadth = b;height = h;System.out.println("Finished with Parameterized Constructor having 3 params");

}public static void main(String[] args) {

Cube2 cubeObj1, cubeObj2;cubeObj1 = new Cube2();cubeObj2 = new Cube2(10, 20, 30);System.out.println("Volume of Cube1 is : " + cubeObj1.getVolume());System.out.println("Volume of Cube2 is : " + cubeObj2.getVolume());

}}

public class Cube2 {

int length;int breadth;int height;public int getVolume() {

return (length * breadth * height);}Cube2() {

this(10, 10);System.out.println("Finished with Default Constructor");

}Cube2(int l, int b) {

this(l, b, 10);System.out.println("Finished with Parameterized Constructor having 2 params");

}Cube2(int l, int b, int h) {

length = l;breadth = b;height = h;System.out.println("Finished with Parameterized Constructor having 3 params");

}public static void main(String[] args) {

Cube2 cubeObj1, cubeObj2;cubeObj1 = new Cube2();cubeObj2 = new Cube2(10, 20, 30);System.out.println("Volume of Cube1 is : " + cubeObj1.getVolume());System.out.println("Volume of Cube2 is : " + cubeObj2.getVolume());

}}

Output (Download Cube2.java)

Finished with Parameterized Constructor having 3 paramsFinished with Parameterized Constructor having 2 paramsFinished with Default ConstructorFinished with Parameterized Constructor having 3 paramsVolume of Cube1 is : 1000Volume of Cube2 is : 6000

Constructor Chaining

Every constructor calls its superclass constructor. An implied super() is therefore included in each constructor which does not include either the this() function or an explicit super() call as its first statement. The super() statement invokes a constructor of the super class.The implicit super() can be replaced by an explicit super(). The super statement must be the first statement of the constructor.

The explicit super allows parameter values to be passed to the constructor of its superclass and must have matching parameter types A super() call in the constructor of a subclass will result in the call of the relevant constructor from the superclass, based on the signature of the call. This is called constructor chaining.

Below is an example of a class demonstrating constructor chaining using super() method.

class Cube {

int length;int breadth;int height;public int getVolume() {

return (length * breadth * height);}Cube() {

this(10, 10);System.out.println("Finished with Default Constructor of Cube");

}Cube(int l, int b) {

this(l, b, 10);System.out.println("Finished with Parameterized Constructor having

2 params of Cube");}Cube(int l, int b, int h) {

length = l;breadth = b;height = h;

System.out.println("Finished with Parameterized Constructor having3 params of Cube");

}}

public class SpecialCube extends Cube {

int weight;SpecialCube() {

super();weight = 10;

}

SpecialCube(int l, int b) {this(l, b, 10);System.out.println("Finished with Parameterized Constructor having

2 params of SpecialCube");}SpecialCube(int l, int b, int h) {

super(l, b, h);weight = 20;System.out.println("Finished with Parameterized Constructor having

3 params of SpecialCube");

}public static void main(String[] args) {

SpecialCube specialObj1 = new SpecialCube();SpecialCube specialObj2 = new SpecialCube(10, 20);System.out.println("Volume of SpecialCube1 is : "

+ specialObj1.getVolume());System.out.println("Weight of SpecialCube1 is : "

+ specialObj1.weight);System.out.println("Volume of SpecialCube2 is : "

+ specialObj2.getVolume());System.out.println("Weight of SpecialCube2 is : "

+ specialObj2.weight);}

}

Download SpecialCube.java

Output

Finished with Parameterized Constructor having 3 params of SpecialCubeFinished with Parameterized Constructor having 2 params of SpecialCubeVolume of SpecialCube1 is : 1000Weight of SpecialCube1 is : 10Volume of SpecialCube2 is : 2000Weight of SpecialCube2 is : 20

The super() construct as with this() construct: if used, must occur as the first statement in a constructor, and it can only be used in a constructor declaration. This implies that this() and super() calls cannot both occur in the same constructor. Just as the this() construct leads to chaining of constructors in the same class, the super() construct leads to chaining of subclass constructors to superclass constructors.if a constructor has neither a this() nor a super() construct as its first statement, then a super() call to the default constructor in the superclass is inserted.

Note: If a class only defines non-default constructors, then its subclasses will not include an implicit super() call. This will be flagged as a compile-time error. The subclasses must then explicitly call a superclass constructor, using the super() construct with the right arguments to match the appropriate constructor of the superclass.

Below is an example of a class demonstrating constructor chaining using explicit super() call.

class Cube {

int length;int breadth;int height;public int getVolume() {

return (length * breadth * height);}Cube(int l, int b, int h) {

length = l;breadth = b;height = h;System.out.println("Finished with Parameterized Constructor having

3 params of Cube");}

}

public class SpecialCube1 extends Cube {

int weight;SpecialCube1() {

super(10, 20, 30); //Will Give a Compilation Error without this lineweight = 10;

}public static void main(String[] args) {

SpecialCube1 specialObj1 = new SpecialCube1();System.out.println("Volume of SpecialCube1 is : "+ specialObj1.getVolume());

}}

Output

Finished with Parameterized Constructor having 3 params of CubeVolume of SpecialCube1 is : 6000

Java Serialization

Introduction to Object Serialization

Java object serialization is used to persist Java objects to a file, database, network, process or any other system. Serialization flattens objects into an ordered, or serialized stream of bytes. The ordered stream of bytes can then be read at a later time, or in another environment, to recreate the original objects.

Java serialization does not cannot occur for transient or static fields. Marking the field transient prevents the state from being written to the stream and from being restored during deserialization. Java provides classes to support writing objects to streams and restoring objects from streams. Only objects that support the java.io.Serializable interface or the java.io.Externalizable interface can be written to streams.public interface Serializable

• The Serializable interface has no methods or fields. (Marker Interface)• Only objects of classes that implement java.io.Serializable interface can be serialized or

deserialized

Transient Fields and Java Serialization

The transient keyword is a modifier applied to instance variables in a class. It specifies that the variable is not part of the persistent state of the object and thus never saved during serialization.

You can use the transient keyword to describe temporary variables, or variables that contain local information,such as a process ID or a time lapse.

Input and Output Object Streams

ObjectOutputStream is the primary output stream class that implements the ObjectOutput interface for serializing objects. ObjectInputStream is the primary input stream class that implements the ObjectInput interface for deserializing objects.

These high-level streams are each chained to a low-level stream, such as FileInputStream or FileOutputStream.The low-level streams handle the bytes of data. The writeObject method saves the state of the class by writing the individual fields to the ObjectOutputStream. The readObject method is used to deserialize the object fromthe object input stream.

Case 1: Below is an example that demonstrates object Serialization into a File

PersonDetails is the bean class that implements the Serializable interface

import java.io.Serializable;public class PersonDetails implements Serializable {

private String name;private int age;private String sex;public PersonDetails(String name, int age, String sex) {

this.name = name;this.age = age;this.sex = sex;

}public int getAge() {

return age;}public void setAge(int age) {

this.age = age;}public String getName() {

return name;}public void setName(String name) {

this.name = name;}public String getSex() {

return sex;}public void setSex(String sex) {

this.sex = sex;}

}

GetPersonDetails is the class that is used to Deserialize object from the File (person.txt).

import java.io.FileInputStream;import java.io.IOException;import java.io.ObjectInputStream;import java.util.ArrayList;import java.util.List;

public class GetPersonDetails {

public static void main(String[] args) {String filename = "person.txt";List pDetails = null;FileInputStream fis = null;ObjectInputStream in = null;

try {fis = new FileInputStream(filename);in = new ObjectInputStream(fis);pDetails = (ArrayList) in.readObject();in.close();

} catch (IOException ex) {ex.printStackTrace();

} catch (ClassNotFoundException ex) {ex.printStackTrace();

}// print out the sizeSystem.out.println("Person Details Size: " + pDetails.size());System.out.println();

}}

PersonPersist is the class that is used to serialize object into the File (person.txt).

public class PersonPersist {

public static void main(String[] args) {String filename = "person.txt";PersonDetails person1 = new PersonDetails("hemanth", 10, "Male");PersonDetails person2 = new PersonDetails("bob", 12, "Male");PersonDetails person3 = new PersonDetails("Richa", 10, "Female");List list = new ArrayList();list.add(person1);list.add(person2);list.add(person3);FileOutputStream fos = null;ObjectOutputStream out = null;try {

fos = new FileOutputStream(filename);out = new ObjectOutputStream(fos);out.writeObject(list);out.close();System.out.println("Object Persisted");

} catch (IOException ex) {ex.printStackTrace();

}}

}

——————————————————————————–

Case 2: Below is an example that demonstrates object Serialization into the database

PersonDetails remains the same as shown above

GetPersonDetails remains the same as shown above

Create SerialTest Table

create table SerialTest(name BLOB,viewname VARCHAR2(30));

PersonPersist is the class that is used to serialize object into the into the Database Table SerialTest.

import java.io.ByteArrayInputStream;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.sql.Connection;import java.sql.DriverManager;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;import java.sql.Statement;public class PersonPersist {

static String userid = "scott", password = "tiger";static String url = "jdbc:odbc:bob";static int count = 0;static Connection con = null;public static void main(String[] args) {

Connection con = getOracleJDBCConnection();PersonDetails person1 = new PersonDetails("hemanth", 10, "Male");PersonDetails person2 = new PersonDetails("bob", 12, "Male");PersonDetails person3 = new PersonDetails("Richa", 10, "Female");PreparedStatement ps;try {

ps = con.prepareStatement("INSERT INTO SerialTest VALUES (?, ?)");

write(person1, ps);ps.execute();write(person2, ps);ps.execute();write(person3, ps);ps.execute();ps.close();Statement st = con.createStatement();ResultSet rs = st.executeQuery("SELECT * FROM SerialTest");while (rs.next()) {

Object obj = read(rs, "Name");PersonDetails p = (PersonDetails) obj;System.out.println(p.getName() + "\t" + p.getAge() + "\t"

+ p.getSex());}rs.close();st.close();

} catch (Exception e) {}

}public static void write(Object obj, PreparedStatement ps)

throws SQLException, IOException {ByteArrayOutputStream baos = new ByteArrayOutputStream();ObjectOutputStream oout = new ObjectOutputStream(baos);oout.writeObject(obj);oout.close();ps.setBytes(1, baos.toByteArray());ps.setInt(2, ++count);

}public static Object read(ResultSet rs, String column)

throws SQLException, IOException, ClassNotFoundException {byte[] buf = rs.getBytes(column);if (buf != null) {

ObjectInputStream objectIn = new ObjectInputStream(new ByteArrayInputStream(buf));

return objectIn.readObject();}return null;

}public static Connection getOracleJDBCConnection() {

try {Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

} catch (java.lang.ClassNotFoundException e) {System.err.print("ClassNotFoundException: ");System.err.println(e.getMessage());

}try {

con = DriverManager.getConnection(url, userid, password);} catch (SQLException ex) {

System.err.println("SQLException: " + ex.getMessage());

}return con;

}}

——————————————————————————–

Case 3: Below is an example that demonstrates object Serialization into the database using Base 64 Encoder

PersonDetails remains the same as shown above

GetPersonDetails remains the same as shown above

Create SerialTest Table

create table SerialTest(name BLOB,viewname VARCHAR2(30));

PersonPersist is the class that is used to serialize object into the Database Table SerialTest

import java.io.ByteArrayInputStream;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.sql.Connection;import java.sql.DriverManager;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;import java.sql.Statement;public class PersonPersist {

static String userid = "scott", password = "tiger";static String url = "jdbc:odbc:bob";static int count = 0;static Connection con = null;static String s;public static void main(String[] args) {

Connection con = getOracleJDBCConnection();PersonDetails person1 = new PersonDetails("hemanth", 10, "Male");PersonDetails person2 = new PersonDetails("bob", 12, "Male");PersonDetails person3 = new PersonDetails("Richa", 10, "Female");PreparedStatement ps;try {

ps = con.prepareStatement("INSERT INTO SerialTest VALUES (?, ?)");

write(person1, ps);ps.execute();write(person2, ps);ps.execute();write(person3, ps);ps.execute();ps.close();Statement st = con.createStatement();ResultSet rs = st.executeQuery("SELECT * FROM SerialTest");while (rs.next()) {

Object obj = read(rs, "Name");PersonDetails p = (PersonDetails) obj;System.out.println(p.getName() + "\t" + p.getAge() + "\t"

+ p.getSex());}rs.close();st.close();

} catch (Exception e) {}

}public static void write(Object obj, PreparedStatement ps)

throws SQLException, IOException {ByteArrayOutputStream baos = new ByteArrayOutputStream();ObjectOutputStream oout = new ObjectOutputStream(baos);oout.writeObject(obj);oout.close();byte[] buf = baos.toByteArray();s = new sun.misc.BASE64Encoder().encode(buf);ps.setString(1, s);// ps.setBytes(1, Base64.byteArrayToBase64(baos.toByteArray()));ps.setBytes(1, baos.toByteArray());ps.setInt(2, ++count);

}public static Object read(ResultSet rs, String column)

throws SQLException, IOException, ClassNotFoundException {byte[] buf = new sun.misc.BASE64Decoder().decodeBuffer(s);// byte[] buf = Base64.base64ToByteArray(new// String(rs.getBytes(column)));if (buf != null) {

ObjectInputStream objectIn = new ObjectInputStream(new ByteArrayInputStream(buf));

Object obj = objectIn.readObject(); // Contains the objectPersonDetails p = (PersonDetails) obj;System.out.println(p.getName() + "\t" + p.getAge() + "\t"

+ p.getSex());}return null;

}public static Connection getOracleJDBCConnection() {

try {

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");} catch (java.lang.ClassNotFoundException e) {

System.err.print("ClassNotFoundException: ");System.err.println(e.getMessage());

}try {

con = DriverManager.getConnection(url, userid, password);} catch (SQLException ex) {

System.err.println("SQLException: " + ex.getMessage());}return con;

}}

Below is a program that shows the serialization of a JButton object to a file and a Byte Array Stream. As before theobject to be serialized must implement the Serializable interface. PersonDetails is the bean class that implements the Serializable interface

import java.io.ByteArrayOutputStream;import java.io.FileOutputStream;import java.io.ObjectOutput;import java.io.ObjectOutputStream;

public class ObjectSerializationExample {

public static void main(String args[]) {try {

Object object = new javax.swing.JButton("Submit");// Serialize to a file namely "filename.dat"ObjectOutput out = new ObjectOutputStream(

new FileOutputStream("filename.dat"));out.writeObject(object);out.close();// Serialize to a byte arrayByteArrayOutputStream bos = new ByteArrayOutputStream();out = new ObjectOutputStream(bos);out.writeObject(object);out.close();// Get the bytes of the serialized objectbyte[] buf = bos.toByteArray();

} catch (Exception e) {e.printStackTrace();

}}

}

Download Object Serialization Source code

Java Inheritance

Java Inheritance defines an is-a relationship between a superclass and its subclasses. This means that an object of a subclass can be used wherever an object of the superclass can be used. Class Inheritance in java mechanism is used to build new classes from existing classes. The inheritance relationship is transitive: if class x extends class y, then a class z, which extends class x, will also inherit from class y.

For example a car class can inherit some properties from a General vehicle class. Here we find that the base class is the vehicle class and the subclass is the more specific car class. A subclass must use the extends clause to derive from a super class which must be written in the header of the subclass definition. The subclass inherits members of the superclass and hence promotes code reuse. The subclass itself can add its own new behavior and properties. The java.lang.Object class is always at the top of any Class inheritance hierarchy.

class Box {

double width;double height;double depth;Box() {}Box(double w, double h, double d) {

width = w;height = h;depth = d;

}void getVolume() {

System.out.println("Volume is : " + width * height * depth);}

}

public class MatchBox extends Box {

double weight;MatchBox() {}MatchBox(double w, double h, double d, double m) {

super(w, h, d);weight = m;

}public static void main(String args[]) {

MatchBox mb1 = new MatchBox(10, 10, 10, 10);mb1.getVolume();System.out.println("width of MatchBox 1 is " + mb1.width);

System.out.println("height of MatchBox 1 is " + mb1.height);System.out.println("depth of MatchBox 1 is " + mb1.depth);System.out.println("weight of MatchBox 1 is " + mb1.weight);

}}

Output

Volume is : 1000.0width of MatchBox 1 is 10.0height of MatchBox 1 is 10.0depth of MatchBox 1 is 10.0weight of MatchBox 1 is 10.0

Download MatchBox.java

What is not possible using java class Inheritance?

1. Private members of the superclass are not inherited by the subclass and can only be indirectly accessed.2. Members that have default accessibility in the superclass are also not inherited by subclasses in other packages, as these members are only accessible by their simple names in subclasses within the same package as the superclass.3. Since constructors and initializer blocks are not members of a class, they are not inherited by a subclass.4. A subclass can extend only one superclass

class Vehicle {

// Instance fieldsint noOfTyres; // no of tyresprivate boolean accessories; // check if accessorees present or notprotected String brand; // Brand of the car// Static fieldsprivate static int counter; // No of Vehicle objects created// ConstructorVehicle() {

System.out.println("Constructor of the Super class called");noOfTyres = 5;accessories = true;brand = "X";counter++;

}// Instance methodspublic void switchOn() {

accessories = true;}

public void switchOff() {accessories = false;

}public boolean isPresent() {

return accessories;}private void getBrand() {

System.out.println("Vehicle Brand: " + brand);}// Static methodspublic static void getNoOfVehicles() {

System.out.println("Number of Vehicles: " + counter);}

}

class Car extends Vehicle {

private int carNo = 10;public void printCarInfo() {

System.out.println("Car number: " + carNo);System.out.println("No of Tyres: " + noOfTyres); // Inherited.// System.out.println("accessories: " + accessories); // Not Inherited.System.out.println("accessories: " + isPresent()); // Inherited.// System.out.println("Brand: " + getBrand()); // Not Inherited.System.out.println("Brand: " + brand); // Inherited.// System.out.println("Counter: " + counter); // Not Inherited.getNoOfVehicles(); // Inherited.

}}

public class VehicleDetails { // (3)

public static void main(String[] args) {new Car().printCarInfo();

}}

Output

Constructor of the Super class calledCar number: 10No of Tyres: 5accessories: trueBrand: XNumber of Vehicles: 1

Download VehicleDetails.java

this and super keywords

The two keywords, this and super to help you explicitly name the field or method that you want. Using this and super you have full control on whether to call a method or field present in the same class or to call from the immediate superclass. This keyword is used as a reference to the current object which is an instance of the current class. The keyword super also references the current object, but as an instance of the current class’s super class.

The this reference to the current object is useful in situations where a local variable hides, or shadows, a field with the same name. If a method needs to pass the current object to another method, it can do so using the this reference. Note that the this reference cannot be used in a static context, as static code is not executed in the context of any object.

class Counter {

int i = 0;Counter increment() {

i++;return this;

}void print() {

System.out.println("i = " + i);}

}

public class CounterDemo extends Counter {

public static void main(String[] args) {Counter x = new Counter();x.increment().increment().increment().print();

}}

Output

Volume is : 1000.0width of MatchBox 1 is 10.0height of MatchBox 1 is 10.0depth of MatchBox 1 is 10.0weight of MatchBox 1 is 10.0

Download CounterDemo.java

Java Object Typecasting

Object Reference Type Casting

In java object typecasting one object reference can be type cast into another object reference. The cast can be to its own class type or to one of its subclass or superclass types or interfaces. There are compile-time rules and runtime rules for casting in java.

How to Typecast Objects with a dynamically loaded Class ? – The casting of object references depends on the relationship of the classes involved in the same hierarchy. Any object reference can be assigned to a reference variable of the type Object, because the Object class is a superclass of every Java class.There can be 2 casting java scenarios

· Upcasting· Downcasting

When we cast a reference along the class hierarchy in a direction from the root class towards the children or subclasses, it is a downcast. When we cast a reference along the class hierarchy in a direction from the sub classes towards the root, it is an upcast. We need not use a cast operator in this case.

The compile-time rules are there to catch attempted casts in cases that are simply not possible. This happens when we try to attempt casts on objects that are totally unrelated (that is not subclass super class relationship or a class-interface relationship) At runtime a ClassCastException is thrown if the object being cast is not compatible with the new type it is being cast to.

Below is an example showing when a ClassCastException can occur during object casting

//X is a supper class of Y and Z which are sibblings.public class RunTimeCastDemo {

public static void main(String args[]) {X x = new X();Y y = new Y();Z z = new Z();X xy = new Y(); // compiles ok (up the hierarchy)X xz = new Z(); // compiles ok (up the hierarchy)// Y yz = new Z(); incompatible type (siblings)// Y y1 = new X(); X is not a Y

// Z z1 = new X(); X is not a ZX x1 = y; // compiles ok (y is subclass of X)X x2 = z; // compiles ok (z is subclass of X)Y y1 = (Y) x; // compiles ok but produces runtime errorZ z1 = (Z) x; // compiles ok but produces runtime errorY y2 = (Y) x1; // compiles and runs ok (x1 is type Y)Z z2 = (Z) x2; // compiles and runs ok (x2 is type Z)// Y y3 = (Y) z; inconvertible types (siblings)// Z z3 = (Z) y; inconvertible types (siblings)Object o = z;Object o1 = (Y) o; // compiles ok but produces runtime error

}}

Download ClassCastException example Source Code

Casting Object References: Implicit Casting using a Compiler

In general an implicit cast is done when an Object reference is assigned (cast) to:

* A reference variable whose type is the same as the class from which the object was instantiated.An Object as Object is a super class of every Class.* A reference variable whose type is a super class of the class from which the object was instantiated.* A reference variable whose type is an interface that is implemented by the class from which the object was instantiated.* A reference variable whose type is an interface that is implemented by a super class of the class from which the object was instantiated.

Consider an interface Vehicle, a super class Car and its subclass Ford. The following example shows the automatic conversion of object references handled by the compiler

interface Vehicle {}class Car implements Vehicle {}

class Ford extends Car {}

Let c be a variable of type Car class and f be of class Ford and v be an vehicle interface reference. We can assign the Ford reference to the Car variable:I.e. we can do the following

Example 1c = f; //Ok Compiles fine

Where c = new Car();And, f = new Ford();The compiler automatically handles the conversion (assignment) since the types are compatible (sub class – super class relationship), i.e., the type Car can hold the type Ford since a Ford is a Car.

Example 2v = c; //Ok Compiles finec = v; // illegal conversion from interface type to class type results in compilation error

Where c = new Car();And v is a Vehicle interface reference (Vehicle v)

The compiler automatically handles the conversion (assignment) since the types are compatible (class – interface relationship), i.e., the type Car can be cast to Vehicle interface type since Car implements Vehicle Interface. (Car is a Vehicle).

Casting Object References: Explicit Casting

Sometimes we do an explicit cast in java when implicit casts don’t work or are not helpful for a particular scenario. The explicit cast is nothing but the name of the new “type” inside a pair of matched parentheses. As before, we consider the same Car and Ford Class

class Car {void carMethod(){}}

class Ford extends Car {void fordMethod () {}}

We also have a breakingSystem() function which takes Car reference (Superclass reference) as an input parameter.The method will invoke carMethod() regardless of the type of object (Car or Ford

Reference) and if it is a Ford object, it will also invoke fordMethod(). We use the instanceof operator to determine the type of object at run time.

public void breakingSystem (Car obj) {obj.carMethod();if (obj instanceof Ford)

((Ford)obj).fordMethod ();}

To invoke the fordMethod(), the operation (Ford)obj tells the compiler to treat the Car object referenced by obj as if it is a Ford object. Without the cast, the compiler will give an error message indicating that fordMethod() cannot be found in the Car definition.

The following program shown illustrates the use of the cast operator with references.

Note: Classes Honda and Ford are Siblings in the class Hierarchy. Both these classes are subclasses of Class Car. Both Car and HeavyVehicle Class extend Object Class. Any class that does not explicitly extend some other class will automatically extends the Object by default. This code instantiates an object of the class Ford and assigns the object’s reference to a reference variable of type Car. This assignment is allowed as Car is a superclass of Ford. In order to use a reference of a class type to invoke a method, the method must be defined at or above that class in the class hierarchy. Hence an object of Class Car cannot invoke a method present in Class Ford, since the method fordMethod is not present in Class Car or any of its superclasses. Hence this problem can be colved by a simple downcast by casting the Car object reference to the Ford Class Object reference as done in the program. Also an attempt to cast an object reference to its Sibling Object reference produces a ClassCastException at runtime, although compilation happens without any error.

class Car extends Object {

void carMethod() {}

}

class HeavyVehicle extends Object {}

class Ford extends Car {

void fordMethod() {

System.out.println("I am fordMethod defined in Class Ford");}

}

class Honda extends Car {

void fordMethod() {System.out.println("I am fordMethod defined in Class Ford");

}}

public class ObjectCastingEx {

public static void main(String[] args) {Car obj = new Ford();// Following will result in compilation error// obj.fordMethod(); //As the method fordMethod is undefined for the Car Type// Following will result in compilation error// ((HeavyVehicle)obj).fordMethod();

//fordMethod is undefined in the HeavyVehicle Type// Following will result in compilation error((Ford) obj).fordMethod();//Following will compile and run// Honda hondaObj = (Ford)obj; Cannot convert as they are sibblings

}}

Download Object Reference Casting Source Code

One common casting that is performed when dealing with collections is, you can cast an object reference into a String.

import java.util.Vector;

public class StringCastDemo {

public static void main(String args[]) {String username = "asdf";String password = "qwer";Vector v = new Vector();v.add(username);v.add(password);// String u = v.elementAt(0); Cannot convert from object to StringObject u = v.elementAt(0); //Cast not doneSystem.out.println("Username : " + u);

String uname = (String) v.elementAt(0); // cast allowedString pass = (String) v.elementAt(1); // cast allowedSystem.out.println();System.out.println("Username : " + uname);System.out.println("Password : " + pass);

}}

Download Object String Casting Source Code

Output: Username : asdf Username : asdf Password : qwer

Instanceof Operator

The instanceof operator is called the type comparison operator, lets you determine if an object belongs to a specific class, or implements a specific interface. It returns true if an object is an instance of the class or if the object implements the interface, otherwise it returns false.

Below is an example showing the use of instanceof operator

class Vehicle {

String name;Vehicle() {

name = "Vehicle";}

}

class HeavyVehicle extends Vehicle {

HeavyVehicle() {name = "HeavyVehicle";

}}

class Truck extends HeavyVehicle {

Truck() {name = "Truck";

}}

class LightVehicle extends Vehicle {

LightVehicle() {name = "LightVehicle";

}}

public class InstanceOfExample {

static boolean result;static HeavyVehicle hV = new HeavyVehicle();static Truck T = new Truck();static HeavyVehicle hv2 = null;public static void main(String[] args) {

result = hV instanceof HeavyVehicle;System.out.print("hV is an HeavyVehicle: " + result + "\n");result = T instanceof HeavyVehicle;System.out.print("T is an HeavyVehicle: " + result + "\n");result = hV instanceof Truck;System.out.print("hV is a Truck: " + result + "\n");result = hv2 instanceof HeavyVehicle;System.out.print("hv2 is an HeavyVehicle: " + result + "\n");hV = T; //Sucessful Cast form child to parentT = (Truck) hV; //Sucessful Explicit Cast form parent to child

}}

Download instanceof operator Source Code

Output

hV is an HeavyVehicle: trueT is an HeavyVehicle: truehV is a Truck: falsehv2 is an HeavyVehicle: false

Note: hv2 does not yet reference an HeavyVehicle object, instanceof returns false. Also we can’t use instanceof operator with siblings

Java Abstract class and Interface

Abstract Class in java

Java Abstract classes are used to declare common characteristics of subclasses. An abstract class cannot be instantiated. It can only be used as a superclass for other classes that extend the abstract class. Abstract classes are declared with the abstract keyword. Abstract classes are used to provide a template or design for concrete subclasses down the inheritance tree.

Like any other class, an abstract class can contain fields that describe the characteristics and methods that describe the actions that a class can perform. An abstract class can include methods that contain no implementation. These are called abstract methods. The abstract method declaration must then end with a semicolon rather than a block. If a class has any abstract methods, whether declared or inherited, the entire class must be declared abstract. Abstract methods are used to provide a template for the classes that inherit the abstract methods.

Abstract classes cannot be instantiated; they must be subclassed, and actual implementations must be provided for the abstract methods. Any implementation specified can, of course, be overridden by additional subclasses. An object must have an implementation for all of its methods. You need to create a subclass that provides an implementation for the abstract method.

A class abstract Vehicle might be specified as abstract to represent the general abstraction of a vehicle, as creating instances of the class would not be meaningful.

abstract class Vehicle {

int numofGears;String color;abstract boolean hasDiskBrake();abstract int getNoofGears();

}

Example of a shape class as an abstract class

abstract class Shape {

public String color;public Shape() {}public void setColor(String c) {

color = c;}public String getColor() {

return color;}abstract public double area();

}

We can also implement the generic shapes class as an abstract class so that we can draw lines, circles, triangles etc. All shapes have some common fields and methods, but each can, of course, add more fields and methods. The abstract class guarantees that each shape will have the same set of basic properties. We declare this class abstract because there is no such thing as a generic shape. There can only be concrete shapes such as squares, circles, triangles etc.

public class Point extends Shape {

static int x, y;public Point() {

x = 0;y = 0;

}public double area() {

return 0;}public double perimeter() {

return 0;}public static void print() {

System.out.println("point: " + x + "," + y);}public static void main(String args[]) {

Point p = new Point();p.print();

}}

Output

point: 0, 0

Notice that, in order to create a Point object, its class cannot be abstract. This means that all of the abstract methods of the Shape class must be implemented by the Point class.

The subclass must define an implementation for every abstract method of the abstract superclass, or the subclass itself will also be abstract. Similarly other shape objects can be created using the generic Shape Abstract class.

A big Disadvantage of using abstract classes is not able to use multiple inheritance. In the sense, when a class extends an abstract class, it can’t extend any other class.

Java Interface

In Java, this multiple inheritance problem is solved with a powerful construct called interfaces. Interface can be used to define a generic template and then one or more abstract classes to define partial implementations of the interface. Interfaces just specify the method declaration (implicitly public and abstract) and can only contain fields (which are implicitly public static final). Interface definition begins with a keyword interface. An interface like that of an abstract class cannot be instantiated.

Multiple Inheritance is allowed when extending interfaces i.e. one interface can extend none, one or more interfaces. Java does not support multiple inheritance, but it allows you to extend one class and implement many interfaces.

If a class that implements an interface does not define all the methods of the interface, then it must be declared abstract and the method definitions must be provided by the subclass that extends the abstract class.

Example 1: Below is an example of a Shape interface

interface Shape {

public double area();public double volume();

}

Below is a Point class that implements the Shape interface.

public class Point implements Shape {

static int x, y;public Point() {

x = 0;y = 0;

}

public double area() {return 0;

}public double volume() {

return 0;}public static void print() {

System.out.println("point: " + x + "," + y);}public static void main(String args[]) {

Point p = new Point();p.print();

}}

Similarly, other shape objects can be created by interface programming by implementing generic Shape Interface.

Example 2: Below is a java interfaces program showing the power of interface programming in java

Listing below shows 2 interfaces and 4 classes one being an abstract class.Note: The method toString in class A1 is an overridden version of the method defined in the class named Object. The classes B1 and C1 satisfy the interface contract. But since the class D1 does not define all the methods of the implemented interface I2, the class D1 is declared abstract.Also,i1.methodI2() produces a compilation error as the method is not declared in I1 or any of its super interfaces if present. Hence a downcast of interface reference I1 solves the problem as shown in the program. The same problem applies to i1.methodA1(), which is again resolved by a downcast.

When we invoke the toString() method which is a method of an Object, there does not seem to be any problem as every interface or class extends Object and any class can override the default toString() to suit your application needs. ((C1)o1).methodI1() compiles successfully, but produces a ClassCastException at runtime. This is because B1 does not have any relationship with C1 except they are “siblings”. You can’t cast siblings into one another.

When a given interface method is invoked on a given reference, the behavior that results will be appropriate to the class from which that particular object was instantiated. This is runtime polymorphism based on interfaces and overridden methods.

interface I1 {

void methodI1(); // public static by default}

interface I2 extends I1 {

void methodI2(); // public static by default}

class A1 {

public String methodA1() {String strA1 = "I am in methodC1 of class A1";return strA1;

}public String toString() {

return "toString() method of class A1";}

}

class B1 extends A1 implements I2 {

public void methodI1() {System.out.println("I am in methodI1 of class B1");

}public void methodI2() {

System.out.println("I am in methodI2 of class B1");}

}

class C1 implements I2 {

public void methodI1() {System.out.println("I am in methodI1 of class C1");

}public void methodI2() {

System.out.println("I am in methodI2 of class C1");}

}

// Note that the class is declared as abstract as it does not// satisfy the interface contractabstract class D1 implements I2 {

public void methodI1() {}// This class does not implement methodI2() hence declared abstract.

}

public class InterFaceEx {

public static void main(String[] args) {I1 i1 = new B1();i1.methodI1(); // OK as methodI1 is present in B1// i1.methodI2(); Compilation error as methodI2 not present in I1// Casting to convert the type of the reference from type I1 to type I2((I2) i1).methodI2();I2 i2 = new B1();i2.methodI1(); // OKi2.methodI2(); // OK// Does not Compile as methodA1() not present in interface reference I1// String var = i1.methodA1();// Hence I1 requires a cast to invoke methodA1String var2 = ((A1) i1).methodA1();System.out.println("var2 : " + var2);String var3 = ((B1) i1).methodA1();System.out.println("var3 : " + var3);String var4 = i1.toString();System.out.println("var4 : " + var4);String var5 = i2.toString();System.out.println("var5 : " + var5);I1 i3 = new C1();String var6 = i3.toString();System.out.println("var6 : " + var6); // It prints the Object toString() methodObject o1 = new B1();// o1.methodI1(); does not compile as Object class does not define// methodI1()// To solve the probelm we need to downcast o1 reference. We can do it// in the following 4 ways((I1) o1).methodI1(); // 1((I2) o1).methodI1(); // 2((B1) o1).methodI1(); // 3/* * * B1 does not have any relationship with C1 except they are "siblings". * * Well, you can't cast siblings into one another. * */// ((C1)o1).methodI1(); Produces a ClassCastException

}}

Output

I am in methodI1 of class B1I am in methodI2 of class B1I am in methodI1 of class B1I am in methodI2 of class B1var2 : I am in methodC1 of class A1var3 : I am in methodC1 of class A1var4 : toString() method of class A1var5 : toString() method of class A1var6 : C1@190d11I am in methodI1 of class B1I am in methodI1 of class B1I am in methodI1 of class B1

Download Java Interface Program Example

Polymorphism

Polymorphism means one name, many forms. There are 3 distinct forms of Java Polymorphism;

• Method overloading (Compile time polymorphism)• Method overriding through inheritance (Run time polymorphism)• Method overriding through the Java interface (Run time polymorphism)

Polymorphism allows a reference to denote objects of different types at different times during execution. A super type reference exhibits polymorphic behavior, since it can denote objects of its subtypes.

interface Shape {

public double area();public double volume();

}

class Cube implements Shape {

int x = 10;public double area( ) {

return (6 * x * x);}

public double volume() {return (x * x * x);

}

}

class Circle implements Shape {

int radius = 10;public double area() {

return (Math.PI * radius * radius);}public double volume() {

return 0;}

}

public class PolymorphismTest {

public static void main(String args[]) {Shape[] s = { new Cube(), new Circle() };for (int i = 0; i < s.length; i++) {

System.out.println("The area and volume of " + s[i].getClass()+ " is " + s[i].area() + " , " + s[i].volume());

}}

}

Output

The area and volume of class Cube is 600.0 , 1000.0The area and volume of class Circle is 314.1592653589793 , 0.0

Download Java Polymorphism Program Example

The methods area() and volume() are overridden in the implementing classes. The invocation of the both methods area and volume is determined based on run time polymorphism of the current object as shown in the output.

Interface vs Abstract Class

Interface vs Abstract Class

Taken from http://interview-questions-java.com/abstract-class-interface.htm

1. Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. An abstract class can contain no abstract methods also i.e. abstract class may contain concrete methods. A Java Interface can contain only method declarations and public static final constants and doesn’t contain their implementation. The classes which implement the Interface must provide the method definition for all the methods present.

2. Abstract class definition begins with the keyword “abstract” keyword followed by Class definition. An Interface definition begins with the keyword “interface”.

3. Abstract classes are useful in a situation when some general methods should be implemented and specialization behavior should be implemented by subclasses. Interfaces are useful in a situation when all its properties need to be implemented by subclasses

4. All variables in an Interface are by default – public static final while an abstract class can have instance variables.

5. An interface is also used in situations when a class needs to extend an other class apart from the abstract class. In such situations its not possible to have multiple inheritance of classes. An interface on the other hand can be used when it is required to implement one or more interfaces. Abstract class does not support Multiple Inheritance whereas an Interface supports multiple Inheritance.

6. An Interface can only have public members whereas an abstract class can contain private as well as protected members.

7. A class implementing an interface must implement all of the methods defined in the interface, while a class extending an abstract class need not implement any of the methods defined in the abstract class.

8. The problem with an interface is, if you want to add a new feature (method) in its contract, then you MUST implement those method in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass

9. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast

10.Interfaces are often used to describe the peripheral abilities of a class, and not its central identity, E.g. an Automobile class mightimplement the Recyclable interface, which could apply to many otherwise totally unrelated objects.

Note: There is no difference between a fully abstract class (all methods declared as abstract and all fields are public static final) and an interface.

Note: If the various objects are all of-a-kind, and share a common state and behavior, then tend towards a common base class. If all theyshare is a set of method signatures, then tend towards an interface.

Java Method Overriding

Method Overriding is achieved when a subclass overrides non-static methods defined in the superclass, following which the new method implementation in the subclass that is executed.

The new method definition must have the same method signature (i.e., method name and parameters) and return type. Only parameter types and return type are chosen as criteria for matching method signature. So if a subclass has its method parameters as final it doesn’t really matter for method overriding scenarios as it still holds true. The new method definition cannot narrow the accessibility of the method, but it can widen it. The new method definition can only specify all or none, or a subset of the exception classes (including their subclasses) specified in the throws clause of the overridden method in the super class

A program to explain the different concepts of Java Method Overridding

class CustomException extends Exception {

}

class SuperClassWithDifferentMethods {

protected int field1 = 10;protected static int field2 = 20;public void method1() {

System.out.println("SuperClassWithDifferentMethods.method1()");}public final void method2() {

System.out.println("SuperClassWithDifferentMethods.method2()");}private void method3() {

System.out.println("SuperClassWithDifferentMethods.method3()");}private final void method4() {

System.out.println("SuperClassWithDifferentMethods.method4()");}public static void method5() {

System.out.println("SuperClassWithDifferentMethods.method5()");}

public void method6() throws Exception {System.out.println("SuperClassWithDifferentMethods.method6()");

}private void method7() {

System.out.println("SuperClassWithDifferentMethods.method7()");}

private void method8(int x) {System.out.println("SuperClassWithDifferentMethods.method8()");

}public static void method9() {

System.out.println("SuperClassWithDifferentMethods.method9()");}

}

class OverridingClass extends SuperClassWithDifferentMethods {

public int field1 = 30;public static int field2 = 40;public void method1() {

System.out.println("OverridingClass.method1()");}//We can't override a public final method/* public final void method2(){

System.out.println("OverridingClass.method2()");

}*/private void method3() {

System.out.println("OverridingClass.method3()");}private final void method4() {

System.out.println("OverridingClass.method4()");}public static void method5() {

System.out.println("OverridingClass.method5()");}public void method6() throws CustomException {

System.out.println("OverridingClass.method6()");}public void method7() {

System.out.println("OverridingClass.method7()");}public void method8(final int x) {

System.out.println("OverridingClass.method8()");}//A static method cannot be overridden to be non-static instance method/*public void method9() {

System.out.println("OverridingClass.method9()");

}*/}

public class MethodOverridingDemo {

public static void main(String[] args) {OverridingClass oc1 = new OverridingClass();SuperClassWithDifferentMethods sc3 = new OverridingClass();oc1.method1();oc1.method2();// Since its private, the below 2 methods are not visible/* oc1.method3();

oc1.method4();*/oc1.method5();try {

oc1.method6();} catch (CustomException e) {

e.printStackTrace();}oc1.method7();oc1.method8(100);System.out.println("oc1.field1 : " + oc1.field1);System.out.println("oc1.field2 : " + oc1.field2);System.out.println("sc3.field1 : " + sc3.field1);System.out.println("sc3.field2 : " + sc3.field2);sc3.method5();OverridingClass overClass = new OverridingClass();

SuperClassWithDifferentMethods supClass = (SuperClassWithDifferentMethods) overClass;supClass.method5();supClass.method1();

}}

Output

OverridingClass.method1()SuperClassWithDifferentMethods.method2()OverridingClass.method5()OverridingClass.method6()OverridingClass.method7()OverridingClass.method8()oc1.field1 : 30oc1.field2 : 40sc3.field1 : 10sc3.field2 : 20SuperClassWithDifferentMethods.method5()

SuperClassWithDifferentMethods.method5()OverridingClass.method1()

Download MethodOverridingDemo.java

The new method definitions in the subclass OverridingClass have the same signature and the same return type as the methods in the superclass SuperClassWithDifferentMethods. The new overridden method6 definition specifies a subset of the exceptions (CustomException). The new overridden method7 definition also widens the accessibility to public from private. The overriding method8 also declares the parameter to be final, which is not a part of the method signature and Method Overriding holds good. A static method cannot be overridden to be non-static instance method as shown in the overridden method declaration of method9. A static method is class-specific and not part of any object, while overriding methods are invoked on behalf of objects of the subclass. There are no such restrictions on the fields, as for fields only the field names matter. A final method cannot be overridden, an attempt to which will result in a compile-time error. A private method is not accessible outside the class in which it is defined; therefore, a subclass cannot override it.

A subclass must use the ‘super’ keyword in order to invoke an overridden method in the superclass. A subclass cannot override fields of the superclass, but it can hide them. Code in the subclass can use the keyword super to access members, including hidden fields.

The following distinction between invoking instance methods on an object and accessing fields of an object must be noted. When an instance method is invoked on an object using a reference, it is the class of the current object denoted by the reference, not the type of the reference, that determines which method implementation will be executed. When a field of an object is accessed using a reference, it is the type of the reference, not the class of the current object denoted by the reference, that determines which field will actually be accessed. This is demonstrated in the above program

Java toString Method

Implementing toString method in java is done by overriding the Object’s toString method. The java toString() method is used when we need a string representation of an object. It is defined in Object class. This method can be overridden to customize the String representation of the Object. Below is a program showing the use of the Object’s Default toString java method.

class PointCoordinates {

private int x, y;public PointCoordinates(int x, int y) {

this.x = x;this.y = y;

}public int getX() {

return x;}public int getY() {

return y;}

}

public class ToStringDemo {

public static void main(String args[]) {PointCoordinates point = new PointCoordinates(10, 10);// using the Default Object.toString() MethodSystem.out.println("Object toString() method : " + point);// implicitly call toString() on object as part of string concatenationString s = point + " testing";System.out.println(s);

}}

Download ToStringDemo.java

When you run the ToStringDemo program, the output is:Object toString() method : PointCoordinates@119c082PointCoordinates@119c082 testing

In the above example when we try printing PointCoordinates object, it internally calls the Object’s toString() method as we have not overridden the java toString() method. Since out example has no toString method, the default one in Object is used. The format of the default toString method of the Object is as shown below.

Class Name, “@”, and the hex version of the object’s hashcode concatenated into a string.The default hashCode method in Object is typically implemented by converting the memory address of the object into an integer.

Below is an example shown of the same program by Overriding the default Object toString() method. The toString() method must be descriptive and should generally cover all the contents of the object.

class PointCoordinates {

private int x, y;public PointCoordinates(int x, int y) {

this.x = x;this.y = y;

}public int getX() {

return x;}public int getY() {

return y;}// Custom toString() Method.public String toString() {

return "X=" + x + " " + "Y=" + y;}

}

public class ToStringDemo2 {

public static void main(String args[]) {PointCoordinates point = new PointCoordinates(10, 10);System.out.println(point);String s = point + " testing";System.out.println(s);

}

}

Download ToStringDemo2.java

When you run the ToStringDemo2 program, the output is:

X=10 Y=10X=10 Y=10 testing

Java String Class

Strings in java

Java String Class is immutable, i.e. Strings in java, once created and initialized, cannot be changed on the same reference. A java.lang.String class is final which implies no class and extend it. The java.lang.String class differs from other classes, one difference being that the String objects can be used with the += and + operators for concatenation.

Two useful methods for String objects are equals( ) and substring( ). The equals( ) method is used for testing whether two Strings contain the same value. The substring( ) method is used to obtain a selected portion of a String.

Java.lang.String class creation

A simple String can be created using a string literal enclosed inside double quotes as shown;

String str1 = “My name is bob”;

Since a string literal is a reference, it can be manipulated like any other String reference. The reference value of a string literal can be assigned to another String reference.

If 2 or more Strings have the same set of characters in the same sequence then they share the same reference in memory. Below illustrates this phenomenon.

String str1 = “My name is bob”;String str2 = “My name is bob”;String str3 = “My name ”+ “is bob”; //Compile time expressionString name = “bob”;

String str4 = “My name is” + name;String str5 = new String(“My name is bob”);

In the above code all the String references str1, str2 and str3 denote the same String object, initialized with the character string: “My name is bob”. But the Strings str4 and str5 denote new String objects.

Constructing String objects can also be done from arrays of bytes, arrays of characters, or string buffers. A simple way to convert any primitive value to its string representation is by concatenating it with the empty string (“”), using the string concatenation operator (+).

public class StringsDemo {

public static void main(String[] args) {

byte[] bytes = {2, 4, 6, 8};

char[] characters = {'a', 'b', 'C', 'D'};

StringBuffer strBuffer = new StringBuffer("abcde");

// Examples of Creation of Strings

String byteStr = new String(bytes);

String charStr = new String(characters);

String buffStr = new String(strBuffer);

System.out.println("byteStr : "+byteStr);

System.out.println("charStr : "+charStr);

System.out.println("buffStr : "+buffStr);

}

}

Download StringDemo1.java

Output

byteStr : charStr : abCDbuffStr : abcde

String Equalitypublic class StringsDemo2 {

public static void main(String[] args) {String str1 = "My name is bob";String str2 = "My name is bob";String str3 = "My name " + "is bob"; //Compile time expressionString name = "bob";String str4 = "My name is " + name;String str5 = new String("My name is bob");System.out.println("str1 == str2 : " + (str1 == str2));System.out.println("str2 == str3 : " + (str2 == str3));System.out.println("str3 == str1 : " + (str3 == str1));System.out.println("str4 == str5 : " + (str4 == str5));System.out.println("str1 == str4 : " + (str1 == str4));System.out.println("str1 == str5 : " + (str1 == str5));System.out.println("str1.equals(str2) : " + str1.equals(str2));System.out.println("str2.equals(str3) : " + str2.equals(str3));System.out.println("str3.equals(str1) : " + str3.equals(str1));System.out.println("str4.equals(str5) : " + str4.equals(str5));System.out.println("str1.equals(str4) : " + str1.equals(str4));System.out.println("str1.equals(str5) : " + str1.equals(str5));

}}

Download StringDemo2.java

Output

str1 == str2 : truestr2 == str3 : truestr3 == str1 : truestr4 == str5 : falsestr1 == str4 : false

str1 == str5 : falsestr1.equals(str2) : truestr2.equals(str3) : truestr3.equals(str1) : truestr4.equals(str5) : truestr1.equals(str4) : truestr1.equals(str5) : true

Java String Functions

The following program explains the usage of the some of the basic String methods like ;

1. compareTo(String anotherString)Compares two strings lexicographically.

2. charAt(int index)Returns the character at the specified index.

3. getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)Copies characters from this string into the destination character array.

4. length()Returns the length of this string.

5. equals(Object anObject)Compares this string to the specified object.

6. equalsIgnoreCase(String anotherString)Compares this String to another String, ignoring case considerations.

7. toUpperCase()Converts all of the characters in this String to upper case using the rules of the default locale.

7. toLowerCase()Converts all of the characters in this String to upper case using the rules of the default locale.

9. concat(String str)Concatenates the specified string to the end of this string.

10. indexOf(int ch)

Returns the index within this string of the first occurrence of the specified character.

11. indexOf(int ch, int fromIndex)

Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

12. indexOf(String str)

Returns the index within this string of the first occurrence of the specified substring.

13. indexOf(String str, int fromIndex)

Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.

14. lastIndexOf(int ch)

Returns the index within this string of the last occurrence of the specified character.

15. lastIndexOf(int ch, int fromIndex)

Returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index.

16. lastIndexOf(String str)

Returns the index within this string of the rightmost occurrence of the specified substring.

17. lastIndexOf(String str, int fromIndex)

Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.

18. substring(int beginIndex)

Returns a new string that is a substring of this string.

19. substring(int beginIndex, int endIndex)

Returns a new string that is a substring of this string.

20. replace(char oldChar, char newChar)

Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.

21. trim()

Returns a copy of the string, with leading and trailing whitespace omitted.

22. toString()

This object (which is already a string!) is itself returned.

public class StringsDemo3 {

public static void main(String[] args) {String str1 = "My name is bob";char str2[] = new char[str1.length()];String str3 = "bob";String str4 = "cob";String str5 = "BoB";String str6 = "bob";System.out.println("Length of the String str1 : " + str1.length());System.out.println("Character at position 3 is : "

+ str1.charAt(3));str1.getChars(0, str1.length(), str2, 0);System.out.print("The String str2 is : ");for (int i = 0; i < str2.length; i++) {

System.out.print(str2[i]);}System.out.println();System.out.print("Comparision Test : ");if (str3.compareTo(str4) < 0) {

System.out.print(str3 + " < " + str4);} else if (str3.compareTo(str4) > 0) {

System.out.print(str3 + " > " + str4);} else {

System.out.print(str3 + " equals " + str4);}System.out.println();System.out.print("Equals Test");System.out.println("str3.equalsIgnoreCase(5) : "

+ str3.equalsIgnoreCase(str5));System.out.println("str3.equals(6) : " + str3.equals(str6));System.out.println("str1.equals(3) : " + str1.equals(str3));str5.toUpperCase(); //Strings are immutableSystem.out.println("str5 : " + str5);String temp = str5.toUpperCase();System.out.println("str5 Uppercase: " + temp);

temp = str1.toLowerCase();System.out.println("str1 Lowercase: " + str1);System.out.println("str1.concat(str4): " + str1.concat(str4));String str7temp = " \t\n Now for some Search and Replace Examples ";String str7 = str7temp.trim();System.out.println("str7 : " + str7);String newStr = str7.replace('s', 'T');System.out.println("newStr : " + newStr);System.out.println("indexof Operations on Strings");System.out.println("Index of p in " + str7 + " : "

+ str7.indexOf('p'));System.out.println("Index of for in " + str7 + " : "

+ str7.indexOf("for"));System.out.println("str7.indexOf(for, 30) : "

+ str7.indexOf("for", 30));System.out.println("str7.indexOf('p', 30) : "

+ str7.indexOf('p', 30));System.out.println("str7.lastIndexOf('p') : "

+ str7.lastIndexOf('p'));System.out.println("str7.lastIndexOf('p', 4) : "

+ str7.lastIndexOf('p', 4));System.out.print("SubString Operations on Strings");String str8 = "SubString Example";String sub5 = str8.substring(5); // "ring Example"String sub3_6 = str8.substring(3, 6); // "Str"System.out.println("str8 : " + str8);System.out.println("str8.substring(5) : " + sub5);System.out.println("str8.substring(3,6) : " + sub3_6);

}}

Download StringDemo3.java

Output

Length of the String str1 : 14Character at position 3 is : nThe String str2 is : My name is bobComparision Test : bob < cobEquals Teststr3.equalsIgnoreCase(5) : truestr3.equals(6) : truestr1.equals(3) : falsestr5 : BoBstr5 Uppercase: BOBstr1 Lowercase: My name is bobstr1.concat(str4): My name is bobcobstr7 : Now for some Search and Replace ExamplesnewStr : Now for Tome Search and Replace ExampleT

Indexof Operations on StringsIndex of p in Now for some Search and Replace Examples : 26Index of for in Now for some Search and Replace Examples : 4str7.indexOf(for, 30) : -1str7.indexOf(‘p’, 30) : 36str7.lastIndexOf(‘p’) : 36str7.lastIndexOf(‘p’, 4) : -1SubString Operations on Stringsstr8 : SubString Examplestr8.substring(5) : ring Examplestr8.substring(3,6) : Str

=======================================

Below is a program to check for Alpha Numeric character’s in the string.

public class TestAlphaNumericCharacters {

private void isAlphaNumeric(final String input) {boolean isCharFlag = false;boolean isNumberFlag = false;final char[] chars = input.toCharArray();for (int x = 0; x < chars.length; x++) {

char c = chars[x];// lowercase && uppercase alphabetif ((c >= 'a') && (c <= 'z') || (c >= 'A') && (c <= 'Z')) {

isCharFlag = true;continue;

}if ((c >= '0') && (c <= '9')) { // numeric

isNumberFlag = true;continue;

}}System.out.println("characters are present:" + isCharFlag);System.out.println("Numbers are present :" + isNumberFlag);

}public static void main(String[] args) {

TestAlphaNumericCharacters tANC = new TestAlphaNumericCharacters();tANC.isAlphaNumeric("beginn3ers");

}}Download TestAlphaNumericCharacters.java

Output

characters are present:trueNumbers are present :true

Below is a java program to Reverse a string (Reverse by words / characters)

import java.util.*;

public class StringReverse {

public static void main(String[] args) {String input = "beginner java tutorial";Stack stack = new Stack(); //A Stack is a Last In First Out Data StructureStringTokenizer stringTokenizer = new StringTokenizer(input);while (stringTokenizer.hasMoreTokens()) {

stack.push(stringTokenizer.nextElement());}System.out.println("Original String: " + input);System.out.print("Reversed String by Words: ");while (!stack.empty()) {

System.out.print(stack.pop());System.out.print(" ");

}System.out.println();System.out.print("Reversed String by characters: ");StringBuffer rev = new StringBuffer(input).reverse();System.out.print(rev);

}}

Download StringReverse.java

Output

Original String: beginner java tutorialReversed String by Words: tutorial java beginnerReversed String by characters: lairotut avaj rennigeb

Java String Comparison

Java String compare to determine Equality

java string compare can be done in many ways as shown below. Depending on the type of java string compare you need, each of them is used.

* == Operator* equals method* compareTo method

Comparing using the == Operator

The == operator is used when we have to compare the String object references. If two String variables point to the same object in memory, the comparison returns true. Otherwise, the comparison returns false. Note that the ‘==’ operator does not compare the content of the text present in the String objects. It only compares the references the 2 Strings are pointing to. The following Program would print “The strings are unequal” In the first case and “The strings are equal” in the second case.

public class StringComparision1 {

public static void main(String[] args) {String name1 = "Bob";String name2 = new String("Bob");String name3 = "Bob";// 1st caseif (name1 == name2) {

System.out.println("The strings are equal.");} else {

System.out.println("The strings are unequal.");}// 2nd caseif (name1 == name3) {

System.out.println("The strings are equal.");} else {

System.out.println("The strings are unequal.");}

}}

Download StringComparision1.java

Comparing using the equals Method

The equals method is used when we need to compare the content of the text present in the String objects. This method returns true when two String objects hold the same content (i.e. the same values). The following Program would print “The strings are unequal” In the first case and “The strings are equal” in the second case.

public class StringComparision2 {

public static void main(String[] args) {String name1 = "Bob";String name2 = new String("Bob1");String name3 = "Bob";// 1st caseif (name1.equals(name2)) {

System.out.println("The strings are equal.");} else {

System.out.println("The strings are unequal.");}// 2nd caseif (name1.equals(name3)) {

System.out.println("The strings are equal.");} else {

System.out.println("The strings are unequal.");}

}}

Download StringComparision2.java

Comparing using the compareTo Method

The compareTo method is used when we need to determine the order of Strings lexicographically. It compares char values similar to the equals method. The compareTo

method returns a negative integer if the first String object precedes the second string. It returns zero if the 2 strings being compared are equal. It returns a positive integer if the first String object follows the second string. The following Program would print “name2 follows name1” In the first case and “name1 follows name3” in the second case.

public class StringComparision3 {

public static void main(String[] args) {String name1 = "bob";String name2 = new String("cob");String name3 = "Bob";// 1st caseif (name1.compareTo(name2) == 0) {

System.out.println("The strings are equal.");} else if (name1.compareTo(name2) < 0) {

System.out.println("name2 follows name1");} else {

System.out.println("name1 follows name2");}// 2nd case. Comparing Ascii Uppercase will be smaller then Lower Caseif (name1.compareTo(name3) == 0) {

System.out.println("The strings are equal.");} else if (name1.compareTo(name3) < 0) {

System.out.println("name3 follows name1");} else {

System.out.println("name1 follows name3");}

}}

Download StringComparision3.java

Java StringBuffer

StringBuffer Class

StringBuffer class is a mutable class unlike the String class which is immutable. Both the capacity and character string of a StringBuffer Class. StringBuffer can be changed dynamically. String buffers are preferred when heavy modification of character strings is involved (appending, inserting, deleting, modifying etc).

Strings can be obtained from string buffers. Since the StringBuffer class does not override the equals() method from the Object class, contents of string buffers should be converted to String objects for string comparison.A StringIndexOutOfBoundsException is thrown if an index is not valid when using wrong index in String Buffer manipulations

Creation of StringBuffers

StringBuffer Constructors

public class StringBufferDemo {

public static void main(String[] args) {// Examples of Creation of StringsStringBuffer strBuf1 = new StringBuffer("Bob");StringBuffer strBuf2 = new StringBuffer(100); //With capacity 100StringBuffer strBuf3 = new StringBuffer(); //Default Capacity 16System.out.println("strBuf1 : " + strBuf1);System.out.println("strBuf2 capacity : " + strBuf2.capacity());System.out.println("strBuf3 capacity : " + strBuf3.capacity());

}

}

Download StringBufferDemo.java

Output

strBuf1 : BobstrBuf2 capacity : 100strBuf3 capacity : 16

StringBuffer Functions

The following program explains the usage of the some of the basic StringBuffer methods like ;

1. capacity()Returns the current capacity of the String buffer.

2. length()Returns the length (character count) of this string buffer.

3. charAt(int index)The specified character of the sequence currently represented by the string buffer, as indicated by the index argument, is returned.

4. setCharAt(int index, char ch)The character at the specified index of this string buffer is set to ch

5. toString()Converts to a string representing the data in this string buffer

6. insert(int offset, char c)Inserts the string representation of the char argument into this string buffer.Note that the StringBuffer class has got many overloaded ‘insert’ methods which can be used based on the application need.

7. delete(int start, int end)Removes the characters in a substring of this StringBuffer

8. replace(int start, int end, String str)Replaces the characters in a substring of this StringBuffer with characters in the specified String.

9. reverse()The character sequence contained in this string buffer is replaced by the reverse of the sequence.

10. append(String str)Appends the string to this string buffer.Note that the StringBuffer class has got many overloaded ‘append’ methods which can be used based on the application need.

11. setLength(int newLength)Sets the length of this String buffer.

public class StringBufferFunctionsDemo {

public static void main(String[] args) {// Examples of Creation of StringsStringBuffer strBuf1 = new StringBuffer("Bobby");StringBuffer strBuf2 = new StringBuffer(100); //With capacity 100StringBuffer strBuf3 = new StringBuffer(); //Default Capacity 16System.out.println("strBuf1 : " + strBuf1);System.out.println("strBuf1 capacity : " + strBuf1.capacity());System.out.println("strBuf2 capacity : " + strBuf2.capacity());System.out.println("strBuf3 capacity : " + strBuf3.capacity());System.out.println("strBuf1 length : " + strBuf1.length());System.out.println("strBuf1 charAt 2 : " + strBuf1.charAt(2));// A StringIndexOutOfBoundsException is thrown if the index is not valid.strBuf1.setCharAt(1, 't');System.out.println("strBuf1 after setCharAt 1 to t is : "

+ strBuf1);System.out

.println("strBuf1 toString() is : " + strBuf1.toString());strBuf3.append("beginner-java-tutorial");System.out.println("strBuf3 when appended with a String : "

+ strBuf3.toString());strBuf3.insert(1, 'c');System.out.println("strBuf3 when c is inserted at 1 : "

+ strBuf3.toString());strBuf3.delete(1, 'c');System.out.println("strBuf3 when c is deleted at 1 : "

+ strBuf3.toString());strBuf3.reverse();System.out.println("Reversed strBuf3 : " + strBuf3);strBuf2.setLength(5);strBuf2.append("jdbc-tutorial");System.out.println("strBuf2 : " + strBuf2);

// We can clear a StringBuffer using the following linestrBuf2.setLength(0);System.out.println("strBuf2 when cleared using setLength(0): "

+ strBuf2);}

}

Download StringBufferFunctionsDemo.java

Output

strBuf1 : BobbystrBuf1 capacity : 21strBuf2 capacity : 100strBuf3 capacity : 16strBuf1 length : 5strBuf1 charAt 2 : bstrBuf1 after setCharAt 1 to t is : BtbbystrBuf1 toString() is : BtbbystrBuf3 when appended with a String : beginner-java-tutorialstrBuf3 when c is inserted at 1 : bceginner-java-tutorialstrBuf3 when c is deleted at 1 : bReversed strBuf3 : bstrBuf2 :

Understanding Java Exceptions

Exceptions in java are any abnormal, unexpected events or extraordinary conditions that may occur at runtime. They could be file not found exception, unable to get connection exception and so on. On such conditions java throws an exception object. Java Exceptions are basically Java objects. No Project can never escape a java error exception.

Java exception handling is used to handle error conditions in a program systematically by taking the necessary action. Exception handlers can be written to catch a specific exception such as Number Format exception, or an entire group of exceptions by using a generic exception handlers. Any exceptions not specifically handled within a Java program are caught by the Java run time environment

An exception is a subclass of the Exception/Error class, both of which are subclasses of the Throwable class. Java exceptions are raised with the throw keyword and handled within a catch block.

A Program Showing How the JVM throws an Exception at runtime

public class DivideException {

public static void main(String[] args) { division(100,4); // Line 1 division(100,0); // Line 2 System.out.println("Exit main()."); }

public static void division(int totalSum, int totalNumber) { System.out.println("Computing Division."); int average = totalSum/totalNumber; System.out.println("Average : "+ average); }}

Download DivideException.java

An ArithmeticException is thrown at runtime when Line 11 is executed because integer division by 0 is an illegal operation. The “Exit main()” message is never reached in the main method

Output

Computing Division.java.lang.ArithmeticException: / by zeroAverage : 25Computing Division.at DivideException.division(DivideException.java:11)at DivideException.main(DivideException.java:5)Exception in thread “main”

Exceptions in Java

Throwable Class

The Throwable class provides a String variable that can be set by the subclasses to provide a detail message that provides more information of the exception occurred. All classes of throwables define a one-parameter constructor that takes a string as the detail message.

The class Throwable provides getMessage() function to retrieve an exception. It has a printStackTrace() method to print the stack trace to the standard error stream. Lastly It also has a toString() method to print a short description of the exception. For more information on what is printed when the following messages are invoked, please refer the java docs.

Syntax

String getMessage()

void printStackTrace()

String toString()

Class Exception

The class Exception represents exceptions that a program faces due to abnormal or special conditions during execution. Exceptions can be of 2 types: Checked (Compile time Exceptions)/ Unchecked (Run time Exceptions).

Class RuntimeException

Runtime exceptions represent programming errors that manifest at runtime. For example ArrayIndexOutOfBounds, NullPointerException and so on are all subclasses of the java.lang.RuntimeException class, which is a subclass of the Exception class. These are basically business logic programming errors.

Class Error

Errors are irrecoverable condtions that can never be caught. Example: Memory leak, LinkageError etc. Errors are direct subclass of Throwable class.

Checked and Unchecked Exceptions

Checked exceptions are subclass’s of Exception excluding class RuntimeException and its subclasses. Checked Exceptions forces programmers to deal with the exception that may be thrown. Example: Arithmetic exception. When a checked exception occurs in a method, the method must either catch the exception and take the appropriate action, or pass the exception on to its caller

Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. Unchecked exceptions , however, the compiler doesn’t force the programmers to either catch the exception or declare it in a throws clause. In fact, the programmers may not even know that the exception could be thrown. Example: ArrayIndexOutOfBounds Exception. They are either irrecoverable (Errors) and the program should not attempt to deal with them, or they are logical programming errors. (Runtime

Exceptions). Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Exception Statement Syntax

Exceptions are handled using a try-catch-finally construct, which has the Syntax

try {<code>} catch (<exception type1> <parameter1>) { // 0 or more<statements>}} finally { // finally block<statements>}

try BlockThe java code that you think may produce an exception is placed within a try block for asuitable catch block to handle the error.

If no exception occurs the execution proceeds with the finally block else it will look for thematching catch block to handle the error. Again if the matching catch handler is not found executionproceeds with the finally block and the default exception handler throws an exception.. If an exception isgenerated within the try block, the remaining statements in the try block are not executed.

catch BlockExceptions thrown during execution of the try block can be caught and handled in a catch block. On exit from a catch block, normal execution continues and the finally block is executed(Though the catch block throws an exception).

finally BlockA finally block is always executed, regardless of the cause of exit from the try block, or whether any catch block was executed. Generally finally block is used for freeing resources, cleaning up, closing connections etc. If the finally clock executes a control transfer statement such as a return or a break statement, then this controlstatement determines how the execution will proceed regardless of any return or control statement present in the try or catch.

The following program illustrates the scenario.

try { <code>} catch (<exception type1> <parameter1>) { // 0 or more <statements>

}} finally { // finally block <statements>}

Download DivideException2.java

Output

Computing Division.Exception : / by zeroFinally Block Executes. Exception Occurredresult : -1

Below is a program showing the Normal Execution of the Program.

Please note that no NullPointerException is generated as was expected by most people

public class DivideException2 {

public static void main(String[] args) { int result = division(100,0); // Line 2 System.out.println("result : "+result); }

public static int division(int totalSum, int totalNumber) { int quotient = -1; System.out.println("Computing Division."); try{ quotient = totalSum/totalNumber;

} catch(Exception e){ System.out.println("Exception : "+ e.getMessage());

} finally{ if(quotient != -1){ System.out.println("Finally Block Executes"); System.out.println("Result : "+ quotient); }else{ System.out.println("Finally Block Executes. Exception Occurred"); return quotient; }

} return quotient; }}

Output: null (And not NullPointerException)

Rules for try, catch and finally Blocks

1. For each try block there can be zero or more catch blocks, but only one finally block.

2. The catch blocks and finally block must always appear in conjunction with a try block.

3. A try block must be followed by either at least one catch block or one finally block.

4. The order exception handlers in the catch block must be from the most specific exception

Java exception handling mechanism enables you to catch exceptions in java using try, catch, finally block. be An exception consists of a block of code called a try block, a block of code called a catch block, and the finally block. Let’s examine each of these in detail.

public class DivideException1 {

public static void main(String[] args) { division(100,0); // Line 2 System.out.println("Main Program Terminating"); }

public static void division(int totalSum, int totalNumber) { int quotient = -1; System.out.println("Computing Division."); try{ quotient = totalSum/totalNumber;

System.out.println("Result is : "+quotient); } catch(Exception e){ System.out.println("Exception : "+ e.getMessage()); } finally{ if(quotient != -1){ System.out.println("Finally Block Executes"); System.out.println("Result : "+ quotient); }else{ System.out.println("Finally Block Executes. Exception Occurred"); }

} }}

Download DivideException1.javaOutput

Output

Computing Division.Exception : / by zeroFinally Block Executes. Exception OccurredMain Program Terminating

As shown above when the divide by zero calculation is attempted, an ArithmeticException is thrown. and program execution is transferred to the catch statement. Because the exception is thrown from the try block, the remaining statements of the try blockare skipped. The finally block executes.

Defining new EXCEPTIONS!!We can have our own custom Exception handler to deal with special exception conditions instead of using existing exception classes. Custom exceptions usually extend the Exception class directly or any subclass of Exception (making it checked).The super() call can be used to set a detail message in the throwable. Below is an example that shows the use of Custom exception’s along with how the throw and throws clause are used.

class BadTemperature extends Exception{BadTemperature( String reason ){

super ( reason ); }

}

class TooHot extends BadTemperature{

TooHot(){super ("Default messaeg : Hot");

}

TooHot(String message){super (message);

}}

class TooCold extends BadTemperature{

TooCold(){super ("Default messaeg : Cold");

}

TooCold(String message){super (message);

}}

class TempertureObject{

int temperature;

TempertureObject( int temp ) { temperature = temp; }

void test() throws TooHot, TooCold { if ( temperature < 70 ) throw new TooCold("Very Cold"); if ( temperature > 80 ) throw new TooHot("Very Hot"); }}

public class ExceptionExample1{

private static void temperatureReport( TempertureObject batch ){try{ batch.test();

System.out.println( "Perfect Temperature" );

} catch ( BadTemperature bt ){ System.out.println( bt.getMessage( ) ); } }

public static void main( String[] args ){ temperatureReport( new TempertureObject( 100 ) ); temperatureReport( new TempertureObject( 50 ) ); temperatureReport( new TempertureObject( 75 ) ); }}

Download ExceptionExample.javaOutput

Output : Very Hot Very Cold Perfect Temperature

throw, throws statement

A program can explicitly throw an exception using the throw statement besides the implicit exception thrown.

The general format of the throw statement is as follows:

throw <exception reference>;

The Exception reference must be of type Throwable class or one of its subclasses. A detail message can be passed to the constructor when the exception object is created.

throw new TemperatureException(“Too hot”);

A throws clause can be used in the method prototype.

Method() throws <ExceptionType1>,…, <ExceptionTypen> {

}

Each <ExceptionTypei> can be a checked or unchecked or sometimes even a custom Exception. The exception type specified in the throws clause in the method prototype can be a super class type of the actual exceptions thrown. Also an overriding method cannot allow more checked exceptions in its throws clause than the inherited method does.

When an exception is thrown, normal execution is suspended. The runtime system proceeds to find a matching catch block that can handle the exception. Any associated finally block of a try block encountered along the search path is executed. If no handler is found, then the exception is dealt with by the default exception handler at the top level. If a handler is found, execution resumes with the code in its catch block. Below is an example to show the use of a throws and a throw statement.

public class DivideException3 {

public static void main(String[] args) { try{ int result = division(100,10); result = division(100,0); System.out.println("result : "+result); } catch(ArithmeticException e){ System.out.println("Exception : "+ e.getMessage()); } }

public static int division(int totalSum, int totalNumber) throws ArithmeticException { int quotient = -1; System.out.println("Computing Division."); try{ if(totalNumber == 0){ throw new ArithmeticException("Division attempt by 0"); } quotient = totalSum/totalNumber;

} finally{ if(quotient != -1){ System.out.println("Finally Block Executes"); System.out.println("Result : "+ quotient); }else{ System.out.println("Finally Block Executes. Exception Occurred"); }

} return quotient; }}

Download DivideException3.javaOutput

Output

Computing Division.Finally Block ExecutesResult : 10Computing Division.Finally Block Executes. Exception OccurredException : Division attempt by 0

Using break and return with Exceptions

This example demonstrates the use of the break, continue and return statements with exceptions. Note that the finally block is executed always except when the return statement is executed.

public class ExceptionExample6 {

public static void main(String[] args) {

int x = 10, y = 2;int counter = 0;boolean flag = true;while (flag) {start:

try {if ( y > 1 )

break start;if ( y < 0 )

return;x = x / y;System.out.println ( "x : " + x + " y : "+y );

}catch ( Exception e ) {

System.out.println ( e.getMessage() );}

finally {++counter;System.out.println ( "Counter : " + counter );

}--y;

}}

}

Download ExceptionExample6.javaOutput

Output

Counter : 1 x : 10 y : 1 Counter : 2/ by zero Counter : 3 Counter : 4

Handling Multiple Exceptions

It should be known by now that we can have multiple catch blocks for a particular try block to handle many different kind of exceptions that can be generated. Below is a program to demonstrate the use of multiple catch blocks.

import java.io.DataInputStream;import java.io.IOException;

import javax.swing.JOptionPane;public class ExceptionExample7{ static int numerator, denominator;

public ExceptionExample7( int t, int b ){numerator = t;denominator = b;

}

public int divide( ) throws ArithmeticException{ return numerator/denominator; }

public static void main( String args[] ){

String num, denom;

num = JOptionPane.showInputDialog(null, "Enter the Numerator");

denom = JOptionPane.showInputDialog(null, "Enter the Denominator");

try{ numerator = Integer.parseInt( num ); denominator = Integer.parseInt( denom );}catch ( NumberFormatException nfe ){ System.out.println( "One of the inputs is not an integer" ); return;}

catch ( Exception e ){ System.out.println( "Exception: " + e.getMessage( ) ); return;

}

ExceptionExample7 d = new ExceptionExample7( numerator, denominator );try{ double result = d.divide( ); JOptionPane.showMessageDialog(null, "Result : " + result);}catch ( ArithmeticException ae ){

System.out.println( "You can't divide by zero" );}finally{

System.out.println( "Finally Block is always Executed" ); }

}}

Download ExceptionExample7.java

Java Singleton Design Pattern

Java has several design patterns Singleton Pattern being the most commonly used. Java Singleton pattern belongs to the family of design patterns, that govern the instantiation process. This design pattern proposes that at any time there can only be one instance of a singleton (object) created by the JVM.

The class’s default constructor is made private, which prevents the direct instantiation of the object by others (Other Classes). A static modifier is applied to the instance method that returns the object as it then makes this method a class level method that can be accessed without creating an object.

One such scenario where it might prove useful is when we develop the help Module in a project. Java Help is an extensible, platform-independent help system that enables authors and developers to incorporate online help into applications.

Singletons can be used to create a Connection Pool. If programmers create a new connection object in every class that requires it, then its clear waste of resources. In this scenario by using a singleton connection class we can maintain a single connection object which can be used throughout the application.

Implementing Singleton Pattern

To implement this design pattern we need to consider the following 4 steps:

Step 1: Provide a default Private constructorpublic class SingletonObjectDemo {

// Note that the constructor is privateprivate SingletonObjectDemo() {

// Optional Code}

}

Step 2: Create a Method for getting the reference to the Singleton Object

public class SingletonObjectDemo {

private static SingletonObject singletonObject;// Note that the constructor is privateprivate SingletonObjectDemo() {

// Optional Code}public static SingletonObjectDemo getSingletonObject() {

if (singletonObject == null) {singletonObject = new SingletonObjectDemo();

}return singletonObject;

}}

We write a public static getter or access method to get the instance of the Singleton Object at runtime. First time the object is created inside this method as it is null. Subsequent calls to this method returns the same object created as the object is globally declared (private) and the hence the same referenced object is returned.

Step 3: Make the Access method Synchronized to prevent Thread Problems.

public static synchronized SingletonObjectDemo getSingletonObject()

It could happen that the access method may be called twice from 2 different classes at the same time and hence more than one object being created. This could violate the design patter principle. In order to prevent the simultaneous invocation of the getter method by 2 threads or classes simultaneously we add the synchronized keyword to the method declaration

Step 4: Override the Object clone method to prevent cloning

We can still be able to create a copy of the Object by cloning it using the Object’s clone method. This can be done as shown below

SingletonObjectDemo clonedObject = (SingletonObjectDemo) obj.clone();

This again violates the Singleton Design Pattern’s objective. So to deal with this we need to override the Object’s clone method which throws a CloneNotSupportedException exception.

public Object clone() throws CloneNotSupportedException {throw new CloneNotSupportedException();}

The below program shows the final Implementation of Singleton Design Pattern in java, by using all the 4 steps mentioned above.

class SingletonClass {

private static SingletonClass singletonObject;/** A private Constructor prevents any other class from instantiating. */private SingletonClass() {

// Optional Code}public static synchronized SingletonClass getSingletonObject() {

if (singletonObject == null) {singletonObject = new SingletonClass();

}return singletonObject;

}public Object clone() throws CloneNotSupportedException {

throw new CloneNotSupportedException();}

}

public class SingletonObjectDemo {

public static void main(String args[]) {

// SingletonClass obj = new SingletonClass(); //Compilation error not allowed

SingletonClass obj = SingletonClass.getSingletonObject();// Your Business LogicSystem.out.println("Singleton object obtained");

}}

Download SingletonObjectDemo.java

Another approach

We don’t need to do a lazy initialization of the instance object or to check for null in the get method. We can also make the singleton class final to avoid sub classing that may cause other problems.

public class SingletonClass {

private static SingletonClass ourInstance = new SingletonClass();

public static SingletonClass getInstance() {return singletonObj;

}private SingletonClass() {}

}

In Summary, the job of the Singleton class is to enforce the existence of a maximum of one object of the same type at any given time. Depending on your implementation, your class and all of its data might be garbage collected. Hence we must ensure that at any point there must be a live reference to the class when the application is running.

Java Threads Tutorial

Introduction to Threads

Multithreading refers to two or more tasks executing concurrently within a single program. A thread is an independent path of execution within a program. Many threads can run concurrently within a program. Every thread in Java is created and controlled by the java.lang.Thread class. A Java program can have many threads, and these threads can run concurrently, either asynchronously or synchronously.

Multithreading has several advantages over Multiprocessing such as;

• Threads are lightweight compared to processes• Threads share the same address space and therefore can share both data and code• Context switching between threads is usually less expensive than between processes• Cost of thread intercommunication is relatively low that that of process

intercommunication• Threads allow different tasks to be performed concurrently.

The following figure shows the methods that are members of the Object and Thread Class.

Thread CreationThere are two ways to create thread in java;

• Implement the Runnable interface (java.lang.Runnable)• By Extending the Thread class (java.lang.Thread)

Implementing the Runnable Interface

The Runnable Interface Signature

public interface Runnable {

void run();

}

One way to create a thread in java is to implement the Runnable Interface and then instantiate an object of the class. We need to override the run() method into our class which is the only method that needs to be implemented. The run() method contains the logic of the thread.

The procedure for creating threads based on the Runnable interface is as follows:

1. A class implements the Runnable interface, providing the run() method that will be executed by the thread. An object of this class is a Runnable object.

2. An object of Thread class is created by passing a Runnable object as argument to the Thread constructor. The Thread object now has a Runnable object that implements the run() method.

3. The start() method is invoked on the Thread object created in the previous step. The start() method returns immediately after a thread has been spawned.

4. The thread ends when the run() method ends, either by normal completion or by throwing an uncaught exception.

Below is a program that illustrates instantiation and running of threads using the runnable interface instead of extending the Thread class. To start the thread you need to invoke the start() method on your object.

class RunnableThread implements Runnable {

Thread runner;public RunnableThread() {}public RunnableThread(String threadName) {

runner = new Thread(this, threadName); // (1) Create a new thread.System.out.println(runner.getName());runner.start(); // (2) Start the thread.

}public void run() {

//Display info about this particular threadSystem.out.println(Thread.currentThread());

}}

public class RunnableExample {

public static void main(String[] args) {Thread thread1 = new Thread(new RunnableThread(), "thread1");Thread thread2 = new Thread(new RunnableThread(), "thread2");RunnableThread thread3 = new RunnableThread("thread3");//Start the threadsthread1.start();thread2.start();try {

//delay for one secondThread.currentThread().sleep(1000);

} catch (InterruptedException e) {}//Display info about the main threadSystem.out.println(Thread.currentThread());

}}

Output

thread3Thread[thread1,5,main]Thread[thread2,5,main]Thread[thread3,5,main]Thread[main,5,main]private

Download Runnable Thread Program Example

This approach of creating a thread by implementing the Runnable Interface must be used whenever the class being used to instantiate the thread object is required to extend some other class.

Extending Thread Class

The procedure for creating threads based on extending the Thread is as follows:

1. A class extending the Thread class overrides the run() method from the Thread class to define the code executed by the thread.

2. This subclass may call a Thread constructor explicitly in its constructors to initialize the thread, using the super() call.

3. The start() method inherited from the Thread class is invoked on the object of the class to make the thread eligible for running.

Below is a program that illustrates instantiation and running of threads by extending the Thread class instead of implementing the Runnable interface. To start the thread you need to invoke the start() method on your object.

class XThread extends Thread {

XThread() {}XThread(String threadName) {

super(threadName); // Initialize thread.System.out.println(this);start();

}public void run() {

//Display info about this particular threadSystem.out.println(Thread.currentThread().getName());

}}

public class ThreadExample {

public static void main(String[] args) {Thread thread1 = new Thread(new XThread(), "thread1");Thread thread2 = new Thread(new XThread(), "thread2");// The below 2 threads are assigned default namesThread thread3 = new XThread();Thread thread4 = new XThread();Thread thread5 = new XThread("thread5");//Start the threadsthread1.start();thread2.start();thread3.start();thread4.start();try {

//The sleep() method is invoked on the main thread to cause a one second delay.Thread.currentThread().sleep(1000);

} catch (InterruptedException e) {}//Display info about the main threadSystem.out.println(Thread.currentThread());

}}

Output

Thread[thread5,5,main]thread1thread5thread2Thread-3Thread-2Thread[main,5,main]

Download Java Thread Program Example

When creating threads, there are two reasons why implementing the Runnable interface may be preferable to extending the Thread class:

• Extending the Thread class means that the subclass cannot extend any other class, whereas a class implementing the Runnable interfacehas this option.

• A class might only be interested in being runnable, and therefore, inheriting the full overhead of the Thread class would be excessive.

An example of an anonymous class below shows how to create a thread and start it:

( new Thread() {

public void run() {

for(;;) System.out.println(“Stop the world!”);

}

}

).start();

Thread Synchronization

With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time.

In non synchronized multithreaded application, it is possible for one thread to modify a shared object whileanother thread is in the process of using or updating the object’s value. Synchronization prevents such typeof data corruption which may otherwise lead to dirty reads and significant errors.Generally critical sections of the code are usually marked with synchronized keyword.

Examples of using Thread Synchronization is in “The Producer/Consumer Model”.

Locks are used to synchronize access to a shared resource. A lock can be associated with a shared resource.Threads gain access to a shared resource by first acquiring the lock associated with the object/block of code.At any given time, at most only one thread can hold the lock and thereby have access to the shared resource.A lock thus implements mutual exclusion.

The object lock mechanism enforces the following rules of synchronization:

• A thread must acquire the object lock associated with a shared resource, before it can enter the sharedresource. The runtime system ensures that no other thread can enter a shared resource if another thread

already holds the object lock associated with the shared resource. If a thread cannot immediately acquirethe object lock, it is blocked, that is, it must wait for the lock to become available.

• When a thread exits a shared resource, the runtime system ensures that the object lock is also relinquished.If another thread is waiting for this object lock, it can proceed to acquire the lock in order to gain accessto the shared resource.

Classes also have a class-specific lock that is analogous to the object lock. Such a lock is actually alock on the java.lang.Class object associated with the class. Given a class A, the reference A.classdenotes this unique Class object. The class lock can be used in much the same way as an object lock toimplement mutual exclusion.

There can be 2 ways through which synchronized can be implemented in Java:

• synchronized methods• synchronized blocks

Synchronized statements are same as synchronized methods. A synchronized statement can only beexecuted after a thread has acquired the lock on the object/class referenced in the synchronized statement.

Synchronized Methods

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method’s object or class. .If the lock is already held by another thread, the calling thread waits. A thread relinquishes the lock simply by returning from the synchronized method, allowing the next thread waiting for this lock to proceed. Synchronized methods are useful in situations where methods can manipulate the state of an object in ways that can corrupt the state if executed concurrently. This is called a race condition. It occurs when two or more threads simultaneously update the same value, and as a consequence, leave the value in an undefined or inconsistent state. While a thread is inside a synchronized method of an object, all other threads that wish to execute this synchronized method or any other synchronized method of the object will have to wait until it gets the lock. This restriction does not apply to the thread that already has the lock and is executing a synchronized method of the object. Such a method can invoke other synchronized methods of the object

without being blocked. The non-synchronized methods of the object can of course be called at any time by any thread.

Below is an example shows how synchronized methods and object locks are used to coordinate access to a common object by multiple threads. If the ‘synchronized’ keyword is removed, the message is displayed in random fashion.

public class SyncMethodsExample extends Thread {

static String[] msg = { "Beginner", "java", "tutorial,", ".,", "com","is", "the", "best" };

public SyncMethodsExample(String id) {super(id);

}public static void main(String[] args) {

SyncMethodsExample thread1 = new SyncMethodsExample("thread1: ");SyncMethodsExample thread2 = new SyncMethodsExample("thread2: ");thread1.start();thread2.start();boolean t1IsAlive = true;boolean t2IsAlive = true;do {

if (t1IsAlive && !thread1.isAlive()) {t1IsAlive = false;System.out.println("t1 is dead.");

}if (t2IsAlive && !thread2.isAlive()) {

t2IsAlive = false;System.out.println("t2 is dead.");

}} while (t1IsAlive || t2IsAlive);

}void randomWait() {

try {Thread.currentThread().sleep((long) (3000 * Math.random()));

} catch (InterruptedException e) {System.out.println("Interrupted!");

}}public synchronized void run() {

SynchronizedOutput.displayList(getName(), msg);}

}

class SynchronizedOutput {

// if the 'synchronized' keyword is removed, the message// is displayed in random fashionpublic static synchronized void displayList(String name, String list[]) {

for (int i = 0; i < list.length; i++) {

SyncMethodsExample t = (SyncMethodsExample) Thread.currentThread();

t.randomWait();System.out.println(name + list[i]);

}}

}

Output

thread1: Beginnerthread1: javathread1: tutorial,thread1: .,thread1: comthread1: isthread1: thethread1: bestt1 is dead.thread2: Beginnerthread2: javathread2: tutorial,thread2: .,thread2: comthread2: isthread2: thethread2: bestt2 is dead.

Download Synchronized Methods Thread Program Example

Class Locks

Synchronized Blocks

Static methods synchronize on the class lock. Acquiring and relinquishing a class lock by a thread in order to execute a static synchronized method, proceeds analogous to that of an object lock for a synchronized instance method. A thread acquires the class lock before it can proceed with the execution of any static synchronized method in the class, blocking other threads wishing to execute any such methods in the same class. This, of course, does not apply to static, non-synchronized methods, which can be invoked at any time. Synchronization of static methods in a class is independent from the synchronization of instance methods on objects of the class. A subclass decides whether the new definition of an inherited synchronized method will remain synchronized in the subclass.The

synchronized block allows execution of arbitrary code to be synchronized on the lock of an arbitrary object.The general form of the synchronized block is as follows:

synchronized (<object reference expression>) {<code block>}

A compile-time error occurs if the expression produces a value of any primitive type. If execution of the block completes normally, then the lock is released. If execution of the block completes abruptly, then the lock is released.A thread can hold more than one lock at a time. Synchronized statements can be nested. Synchronized statements with identical expressions can be nested. The expression must evaluate to a non-null reference value, otherwise, a NullPointerException is thrown.

The code block is usually related to the object on which the synchronization is being done. This is the case with synchronized methods, where the execution of the method is synchronized on the lock of the current object:

public Object method() {synchronized (this) { // Synchronized block on current object// method block}}

Once a thread has entered the code block after acquiring the lock on the specified object, no other thread will be able to execute the code block, or any other code requiring the same object lock, until the lock is relinquished. This happens when the execution of the code block completes normally or an uncaught exception is thrown.

Object specification in the synchronized statement is mandatory. A class can choose to synchronize the execution of a part of a method, by using the this reference and putting the relevant part of the method in the synchronized block. The braces of the block cannot be left out, even if the code block has just one statement.

class SmartClient {BankAccount account;// …public void updateTransaction() {synchronized (account) { // (1) synchronized blockaccount.update(); // (2)}

}}

In the previous example, the code at (2) in the synchronized block at (1) is synchronized on the BankAccount object. If several threads were to concurrently execute the method updateTransaction() on an object of SmartClient, the statement at (2) would be executed by one thread at a time, only after synchronizing on the BankAccount object associated with this particular instance of SmartClient.

Inner classes can access data in their enclosing context. An inner object might need to synchronize on its associated outer object, in order to ensure integrity of data in the latter. This is illustrated in the following code where the synchronized block at (5) uses the special form of the this reference to synchronize on the outer object associated with an object of the inner class. This setup ensures that a thread executing the method setPi() in an inner object can only access the private double field myPi at (2) in the synchronized block at (5), by first acquiring the lock on the associated outer object. If another thread has the lock of the associated outer object, the thread in the inner object has to wait for the lock to be relinquished before it can proceed with the execution of the synchronized block at (5). However, synchronizing on an inner object and on its associated outer object are independent of each other, unless enforced explicitly, as in the following code:

class Outer { // (1) Top-level Classprivate double myPi; // (2)protected class Inner { // (3) Non-static member Classpublic void setPi() { // (4)synchronized(Outer.this) { // (5) Synchronized block on outer objectmyPi = Math.PI; // (6)}}}}

Below example shows how synchronized block and object locks are used to coordinate access to shared objects by multiple threads.

public class SyncBlockExample extends Thread {

static String[] msg = { "Beginner", "java", "tutorial,", ".,", "com","is", "the", "best" };

public SyncBlockExample(String id) {super(id);

}public static void main(String[] args) {

SyncBlockExample thread1 = new SyncBlockExample("thread1: ");SyncBlockExample thread2 = new SyncBlockExample("thread2: ");

thread1.start();thread2.start();boolean t1IsAlive = true;boolean t2IsAlive = true;do {

if (t1IsAlive && !thread1.isAlive()) {t1IsAlive = false;System.out.println("t1 is dead.");

}if (t2IsAlive && !thread2.isAlive()) {

t2IsAlive = false;System.out.println("t2 is dead.");

}} while (t1IsAlive || t2IsAlive);

}void randomWait() {

try {Thread.currentThread().sleep((long) (3000 * Math.random()));

} catch (InterruptedException e) {System.out.println("Interrupted!");

}}public void run() {

synchronized (System.out) {for (int i = 0; i < msg.length; i++) {

randomWait();System.out.println(getName() + msg[i]);

}}

}}

Output

thread1: Beginnerthread1: javathread1: tutorial,thread1: .,thread1: comthread1: isthread1: thethread1: bestt1 is dead.thread2: Beginnerthread2: javathread2: tutorial,thread2: .,thread2: com

thread2: isthread2: thethread2: bestt2 is dead.

Synchronized blocks can also be specified on a class lock:

synchronized (<class name>.class) { <code block> }

The block synchronizes on the lock of the object denoted by the reference <class name>.class. A static synchronized methodclassAction() in class A is equivalent to the following declaration:

static void classAction() {

synchronized (A.class) { // Synchronized block on class A

// …

}

}

In summary, a thread can hold a lock on an object

• by executing a synchronized instance method of the object• by executing the body of a synchronized block that synchronizes on the object• by executing a synchronized static method of a class

Thread States

A Java thread is always in one of several states which could be running, sleeping, dead, etc.

A thread can be in any of the following states:

• New Thread state (Ready-to-run state)• Runnable state (Running state)• Not Runnable state• Dead state

New Thread

A thread is in this state when the instantiation of a Thread object creates a new thread but does notstart it running. A thread starts life in the Ready-to-run state. You can call only the start() or stop()methods when the thread is in this state. Calling any method besides start() or stop() causes anIllegalThreadStateException.

Runnable

When the start() method is invoked on a New Thread() it gets to the runnable state or running state bycalling the run() method. A Runnable thread may actually be running, or may be awaiting its turn to run.

Not Runnable

A thread becomes Not Runnable when one of the following four events occurs:

• When sleep() method is invoked and it sleeps for a specified amount of time• When suspend() method is invoked• When the wait() method is invoked and the thread waits for notification of a free

resource or waits forthe completion of another thread or waits to acquire a lock of an object.

• The thread is blocking on I/O and waits for its completion

Example: Thread.currentThread().sleep(1000);

Note: Thread.currentThread() may return an output like Thread[threadA,5,main]

The output shown in bold describes

• the name of the thread,• the priority of the thread, and• the name of the group to which it belongs.

Here, the run() method put itself to sleep for one second and becomes Not Runnable during that period.A thread can be awakened abruptly by invoking the interrupt() method on the sleeping thread object or at the end of the period of time for sleep is over. Whether or not it will actually start running depends on its priority and the availability of the CPU.

Hence I hereby list the scenarios below to describe how a thread switches form a non runnable to a runnable state:

• If a thread has been put to sleep, then the specified number of milliseconds must elapse (or it must be interrupted).

• If a thread has been suspended, then its resume() method must be invoked• If a thread is waiting on a condition variable, whatever object owns the variable

must relinquish it by callingeither notify() or notifyAll().

• If a thread is blocked on I/O, then the I/O must complete.

Dead State

A thread enters this state when the run() method has finished executing or when the stop() method is invoked. Once in this state, the thread cannot ever run again.

Thread Priority

In Java we can specify the priority of each thread relative to other threads. Those threads having higherpriority get greater access to available resources then lower priority threads. A Java thread inherits its priorityfrom the thread that created it. Heavy reliance on thread priorities for the behavior of a program can make theprogram non portable across platforms, as thread scheduling is host platform–dependent.

You can modify a thread’s priority at any time after its creation using the setPriority() method and retrievethe thread priority value using getPriority() method.

The following static final integer constants are defined in the Thread class:

• MIN_PRIORITY (0) Lowest Priority• NORM_PRIORITY (5) Default Priority• MAX_PRIORITY (10) Highest Priority

The priority of an individual thread can be set to any integer value between and including the above defined constants.

When two or more threads are ready to be executed and system resource becomes available to execute a thread, the runtime system (the thread scheduler) chooses the Runnable thread with the highest priority for execution.

“If two threads of the same priority are waiting for the CPU, the thread scheduler chooses one of them to run in a > round-robin fashion. The chosen thread will run until one of the following conditions is true:

• a higher priority thread becomes Runnable. (Pre-emptive scheduling)• it yields, or its run() method exits• on systems that support time-slicing, its time allotment has expired

Thread Scheduler

Schedulers in JVM implementations usually employ one of the two following strategies:

Preemptive scheduling

If a thread with a higher priority than all other Runnable threads becomes Runnable, the scheduler willpreempt the running thread (is moved to the runnable state) and choose the new higher priority thread for execution.

Time-Slicing or Round-Robin scheduling

A running thread is allowed to execute for a fixed length of time (a time slot it’s assigned to), after which it moves to the Ready-to-run state (runnable) to await its turn to run again.

A thread scheduler is implementation and platform-dependent; therefore, how threads will be scheduled is unpredictable across different platforms.

Yielding

A call to the static method yield(), defined in the Thread class, will cause the current thread in the Running state to move to the Runnable state, thus relinquishing the CPU. The thread is then at the mercy of the thread scheduler as to when it will run again. If there are no threads waiting in the Ready-to-run state, this thread continues execution. If there are other threads in the Ready-to-run state, their priorities determine which thread gets to execute. The yield() method gives other threads of the same priority a chance to run. If there are no equal priority threads in the “Runnable” state, then the yield is ignored.

Sleeping and Waking Up

The thread class contains a static method named sleep() that causes the currently running thread to pause its execution and transit to the Sleeping state. The method does not relinquish any lock that the thread might have. The thread will sleep for at least the time specified in its argument, before entering the runnable state where it takes its turn to run again. If a thread is interrupted while sleeping, it will throw an InterruptedException when it awakes and gets to execute. The Thread class has several overloaded versions of the sleep() method.

Waiting and Notifying

Waiting and notifying provide means of thread inter-communication that synchronizes on the same object. The threads execute wait() and notify() (or notifyAll()) methods on the shared object for this purpose. The notifyAll(), notify() and wait() are methods of the Object class. These methods can be invoked only from within a synchronized context (synchronized method or synchronized block), otherwise, the call will result in an IllegalMonitorStateException. The notifyAll() method wakes up all the threads waiting on the resource. In this situation, the awakened threads compete for the resource. One thread gets the resource and the others go back to waiting.

wait() method signatures

final void wait(long timeout) throws InterruptedExceptionfinal void wait(long timeout, int nanos) throws InterruptedExceptionfinal void wait() throws InterruptedException

The wait() call can specify the time the thread should wait before being timed out. An another thread can invoke an interrupt() method on a waiting thread resulting in an InterruptedException. This is a checked exception and hence the code with the wait() method must be enclosed within a try catch block.

notify() method signatures

final void notify()final void notifyAll()

A thread usually calls the wait() method on the object whose lock it holds because a condition for its continued execution was not met. The thread leaves the Running state and transits to the Waiting-for-notification state. There it waits for this condition to occur. The thread relinquishes ownership of the object lock. The releasing of the lock of the shared object by the thread allows other threads to run and execute synchronized code on the same object after acquiring its lock.

The wait() method causes the current thread to wait until another thread notifies it of a condition change.

A thread in the Waiting-for-notification state can be awakened by the occurrence of any one of these three incidents:

1. Another thread invokes the notify() method on the object of the waiting thread, and the waiting thread is selected as the thread to be awakened.2. The waiting thread times out.3. Another thread interrupts the waiting thread.

Notify

Invoking the notify() method on an object wakes up a single thread that is waiting on the lock of this object.A call to the notify() method has no consequences if there are no threads in the wait set of the object.The notifyAll() method wakes up all threads in the wait set of the shared object.

Below program shows three threads, manipulating the same stack. Two of them are pushing elements on the stack, while the third one is popping elements off the stack. This example illustrates how a thread waiting as a result of calling the wait() method on an object, is notified by another thread calling the notify() method on the same object

class StackClass {

private Object[] stackArray;private volatile int topOfStack;StackClass(int capacity) {

stackArray = new Object[capacity];topOfStack = -1;

}public synchronized Object pop() {

System.out.println(Thread.currentThread() + ": popping");while (isEmpty()) {

try {System.out.println(Thread.currentThread()

+ ": waiting to pop");wait();

} catch (InterruptedException e) {e.printStackTrace();

}}Object obj = stackArray[topOfStack];stackArray[topOfStack--] = null;System.out.println(Thread.currentThread()

+ ": notifying after pop");

notify();return obj;

}public synchronized void push(Object element) {

System.out.println(Thread.currentThread() + ": pushing");while (isFull()) {

try {System.out.println(Thread.currentThread()

+ ": waiting to push");wait();

} catch (InterruptedException e) {e.printStackTrace();

}}stackArray[++topOfStack] = element;System.out.println(Thread.currentThread()

+ ": notifying after push");notify();

}public boolean isFull() {

return topOfStack >= stackArray.length - 1;}public boolean isEmpty() {

return topOfStack < 0;}

}

abstract class StackUser extends Thread {

protected StackClass stack;StackUser(String threadName, StackClass stack) {

super(threadName);this.stack = stack;System.out.println(this);setDaemon(true);start();

}}

class StackPopper extends StackUser { // Stack Popper

StackPopper(String threadName, StackClass stack) {super(threadName, stack);

}public void run() {

while (true) {stack.pop();

}}

}

class StackPusher extends StackUser { // Stack Pusher

StackPusher(String threadName, StackClass stack) {super(threadName, stack);

}public void run() {

while (true) {stack.push(new Integer(1));

}}

}

public class WaitAndNotifyExample {

public static void main(String[] args) {StackClass stack = new StackClass(5);new StackPusher("One", stack);new StackPusher("Two", stack);new StackPopper("Three", stack);System.out.println("Main Thread sleeping.");try {

Thread.sleep(500);} catch (InterruptedException e) {

e.printStackTrace();}System.out.println("Exit from Main Thread.");

}}

Download Wait Notify methods Thread Program Example

The field topOfStack in class StackClass is declared volatile, so that read and write operations on this variable will access the master value of this variable, and not any copies, during runtime.Since the threads manipulate the same stack object and the push() and pop() methods in the class StackClassare synchronized, it means that the threads synchronize on the same object.

How the program uses wait() and notify() for inter thread communication.

(1) The synchronized pop() method – When a thread executing this method on the StackClass object finds that the stack is empty, it invokes the wait() method in order to wait for some other thread to fill the stack by using the synchronized push. Once an other thread makes a push, it invokes the notify method.(2)The synchronized push() method – When a thread executing this method on the StackClass object finds that the stack is full, i t invokes the wait() method to await some other thread to remove an element to provide space for the newly to be pushed element.Once an other thread makes a pop, it invokes the notify method.

Java Date Utility

Java Date API

java.util.Class Date

java.lang.Objectextended by java.util.Date

All Implemented Interfaces:Cloneable, Comparable, Serializable

Direct Known Subclasses:Date, Time, Timestamp

public class Date extends Objectimplements Serializable, Cloneable, Comparable

The class Date represents a specific instant in time, with millisecond precision.

Java Date Source Codeimport java.text.DateFormat;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Calendar;import java.util.Date;

public class DateUtility {

/* Add Day/Month/Year to a Date add() is used to add values to a Calendar object. You specify which Calendar field is to be affected by the operation (Calendar.YEAR, Calendar.MONTH, Calendar.DATE). */

public static final String DATE_FORMAT = "dd-MM-yyyy";//See Java DOCS for different date formats// public static final String DATE_FORMAT = "yyyy-MM-dd";

public static void addToDate() {System.out.println("1. Add to a Date Operation\n");SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);//Gets a calendar using the default time zone and locale.Calendar c1 = Calendar.getInstance();Date d1 = new Date();// System.out.println("Todays date in Calendar Format : "+c1);System.out.println("c1.getTime() : " + c1.getTime());System.out.println("c1.get(Calendar.YEAR): "+ c1.get(Calendar.YEAR));System.out.println("Todays date in Date Format : " + d1);c1.set(1999, 0, 20); //(year,month,date)System.out.println("c1.set(1999,0 ,20) : " + c1.getTime());c1.add(Calendar.DATE, 20);System.out.println("Date + 20 days is : "+ sdf.format(c1.getTime()));System.out.println();System.out.println("-------------------------------------");

}

/*Substract Day/Month/Year to a Date

roll() is used to substract values to a Calendar object. You specify which Calendar field is to be affected by the operation (Calendar.YEAR, Calendar.MONTH, Calendar.DATE).

Note: To substract, simply use a negative argument. roll() does the same thing except you specify if you want to roll up (add 1) or roll down (substract 1) to the specified Calendar field. The operation only affects the specified field while add() adjusts other Calendar fields. See the following example, roll() makes january rolls to december in the same year while add() substract the YEAR field for the correct result. Hence add() is preferred even for subtraction by using a negative element.

*/

public static void subToDate() {

System.out.println("2. Subtract to a date Operation\n");SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);Calendar c1 = Calendar.getInstance();c1.set(1999, 0, 20);System.out.println("Date is : " + sdf.format(c1.getTime()));// roll down, substract 1 monthc1.roll(Calendar.MONTH, false);System.out.println("Date roll down 1 month : "+ sdf.format(c1.getTime()));c1.set(1999, 0, 20);System.out.println("Date is : " + sdf.format(c1.getTime()));c1.add(Calendar.MONTH, -1);// substract 1 monthSystem.out.println("Date minus 1 month : "+ sdf.format(c1.getTime()));System.out.println();System.out.println("-------------------------------------");

}

public static void daysBetween2Dates() {

System.out.println("3. No of Days between 2 dates\n");Calendar c1 = Calendar.getInstance(); //new GregorianCalendar();Calendar c2 = Calendar.getInstance(); //new GregorianCalendar();c1.set(1999, 0, 20);c2.set(1999, 0, 22);System.out.println("Days Between " + c1.getTime() + " and "

+ c2.getTime() + " is");System.out.println((c2.getTime().getTime() - c1.getTime()

.getTime()) / (24 * 3600 * 1000));System.out.println();System.out.println("-------------------------------------");

}

public static void daysInMonth() {

System.out.println("4. No of Days in a month for a given date\n");Calendar c1 = Calendar.getInstance(); // new GregorianCalendar();c1.set(1999, 6, 20);int year = c1.get(Calendar.YEAR);int month = c1.get(Calendar.MONTH);// int days = c1.get(Calendar.DATE);int[] daysInMonths = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30,31 };daysInMonths[1] += DateUtility.isLeapYear(year) ? 1 : 0;System.out.println("Days in " + month + "th month for year" + year

+ "is " + daysInMonths[c1.get(Calendar.MONTH)]);System.out.println();System.out.println("-------------------------------------");

}

public static void validateAGivenDate() {

System.out.println("5. Validate a given date\n");String dt = "20011223";String invalidDt = "20031315";String dateformat = "yyyyMMdd";Date dt1 = null, dt2 = null;try {

SimpleDateFormat sdf = new SimpleDateFormat(dateformat);sdf.setLenient(false);dt1 = sdf.parse(dt);dt2 = sdf.parse(invalidDt);System.out.println("Date is ok = " + dt1 + "(" + dt + ")");

} catch (ParseException e) {System.out.println(e.getMessage());

} catch (IllegalArgumentException e) {System.out.println("Invalid date");

}System.out.println();System.out.println("-------------------------------------");

}

public static void compare2Dates() {

System.out.println("6. Comparision of 2 dates\n");SimpleDateFormat fm = new SimpleDateFormat("dd-MM-yyyy");Calendar c1 = Calendar.getInstance();Calendar c2 = Calendar.getInstance();c1.set(2000, 02, 15);c2.set(2001, 02, 15);System.out.print(fm.format(c1.getTime()) + " is ");if (c1.before(c2)) {

System.out.println("less than " + fm.format(c2.getTime()));} else if (c1.after(c2)) {

System.out.println("greater than " + fm.format(c2.getTime()));} else if (c1.equals(c2)) {

System.out.println("is equal to " + fm.format(c2.getTime()));}System.out.println();System.out.println("-------------------------------------");

}

public static void getDayofTheDate() {

System.out.println("7. Get the day for a given date\n");Date d1 = new Date();String day = null;DateFormat f = new SimpleDateFormat("EEEE");try {

day = f.format(d1);} catch (Exception e) {

e.printStackTrace();

}System.out.println("The day for " + d1 + " is " + day);System.out.println();System.out.println("-------------------------------------");

}

//Utility Method to find whether an Year is a Leap year or Notpublic static boolean isLeapYear(int year) {

if ((year % 100 != 0) || (year % 400 == 0)) {return true;

}return false;

}

public static void main(String args[]) {addToDate(); //Add day, month or year to a date field.subToDate(); //Subtract day, month or year to a date field.daysBetween2Dates();//The "right" way would be to compute the Julian day number of//both dates and then do the subtraction.daysInMonth();//Find the number of days in a month for a datevalidateAGivenDate();//Check whether the date format is propercompare2Dates(); //Compare 2 datesgetDayofTheDate();

}}

Output

1. Add to a Date Operation

c1.getTime() : Sat Mar 31 10:47:54 IST 2007c1.get(Calendar.YEAR): 2007Todays date in Date Format : Sat Mar 31 10:47:54 IST 2007c1.set(1999,0 ,20) : Wed Jan 20 10:47:54 IST 1999Date + 20 days is : 09-02-1999

——————————————————-2. Subtract to a date Operation

Date is : 20-01-1999Date roll down 1 month : 20-12-1999Date is : 20-01-1999Date minus 1 month : 20-12-1998

——————————————————-3. No of Days between 2 dates

Days Between Wed Jan 20 10:47:54 IST 1999 and Fri Jan 22 10:47:54 IST 1999 is2

——————————————————-4. No of Days in a month for a given date

Days in 6th month for year 1999 is 31

——————————————————-5. Validate a given date

Unparseable date: “20031315″

——————————————————-6. Comparision of 2 dates

15-03-2000 is less than 15-03-2001

——————————————————-7. Get the day for a given date

The day for Sat Mar 31 10:47:54 IST 2007 is Saturday

——————————————————-

Download Date Utility Source Code

What is the GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars.

What is the SimpleTimeZone class?

The SimpleTimeZone class provides support for a Gregorian calendar.

Java Collections Framework

A collection represents a group of objects, known as its elements. This framework is provided in the java.util package. Objects can be stored, retrieved, and manipulated as elements of collections. Collection is a Java Interface. Collections can be used in various scenarios like Storing phone numbers, Employee names database etc. They are basically used to group multiple elements into a single unit. Some collections allow duplicate elements while others do not. Some collections are ordered and others are not.

A Collections Framework mainly contains the following 3 parts

A Collections Framework is defined by a set of interfaces, concrete class implementations for most of the interfaces and a set of standard utility methods and algorithms. In addition, the framework also provides several abstract implementations, which are designed to make it easier for you to create new and different implementations for handling collections of data.

Core Collection Interfaces

The core interfaces that define common functionality and allow collections to be manipulated independent of their implementation.The 6 core Interfaces used in the Collection framework are:

• Collection

• Set• List• Iterator (Not a part of the Collections Framework)• SortedSet• Map• SortedMap

Note: Collection and Map are the two top-level interfaces.

Collection Interface

Map Interface

Concrete Classes

The concrete classes that are specific implementations of the core interfaces, providing data structures that a java program can use.

Note: Concrete Classes for the Map is shown in the previous section.Standard utility methods and algorithms

Standard utility methods and algorithms

that can be used to perform various operations on collections, such as sorting, searching or creating customized collections.

How are Collections Used

• The collections stores object references, rather than objects themselves. Hence primitive values cannot be stored in a collection directly. They need to be encapsulated (using wrapper classes) into an Object prior to storing them into a Collection (such as HashSet, HashMap etc).

• The references are always stored as type Object. Thus, when you retrieve an element from a collection, you get an Object rather then the actual type of the collection stored in the database. Hence we need to downcast it to the Actual Type while retrieving an element from a collection.

• One of the capabilities of the Collection Framework is to create a new Collection object and populate it with the contents of an existing Collection object of a same or different actual type.

Below is an example program showing the storing and retrieving of a few Collection Types

import java.util.*;

public class CollectionsDemo {

public static void main(String[] args) {List a1 = new ArrayList();a1.add("Beginner");a1.add("Java");a1.add("tutorial");System.out.println(" ArrayList Elements");System.out.print("\t" + a1);List l1 = new LinkedList();l1.add("Beginner");l1.add("Java");l1.add("tutorial");System.out.println();System.out.println(" LinkedList Elements");System.out.print("\t" + l1);Set s1 = new HashSet(); // or new TreeSet() will order the elements;s1.add("Beginner");s1.add("Java");s1.add("Java");s1.add("tutorial");System.out.println();System.out.println(" Set Elements");System.out.print("\t" + s1);Map m1 = new HashMap(); // or new TreeMap() will order based on keys

m1.put("Windows", "98");m1.put("Win", "XP");m1.put("Beginner", "Java");m1.put("Tutorial", "Site");System.out.println();System.out.println(" Map Elements");System.out.print("\t" + m1);

}}

Output

ArrayList Elements[Beginner, Java, tutorial]LinkedList Elements[Beginner, Java, tutorial]Set Elements[tutorial, Beginner, Java]Map Elements{Tutorial=Site, Windows=98, Win=XP, Beginner=Java}

Download CollectionsDemo.java

Java Collections Source Code Examples

Java ArrayList

public class ArrayList<E>extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, Serializable

• Resizable-array implementation of the List interface. A java ArrayList is used to store an “ordered” group of elements where duplicates are allowed.

• Implements all optional list operations, and permits all elements, including null.• This class is similar to Vector, except that it is unsynchronized.• The size, isEmpty, get, set, iterator, and listIterator operations run in constant time.

ArrayList’s give great performance on get() and set() methods, but do not perform well on add() and remove() methods when compared to a LinkedList.

• An ArrayList capacity is the size of the array used to store the elements in the list. As elements are added to an ArrayList, its capacity grows automatically. It is an Array based implementation where elements of the List can be accessed directly through get() method.

To prevent unsynchronized access to the list: List list = Collections.synchronizedList(new ArrayList(…));

Below is an ArrayList Example showing how collections are manipulated using an ArrayList

import java.util.List;import java.util.ArrayList;import java.util.Iterator;import java.util.ListIterator;import java.util.Collections;import java.util.Random;

public class ArrayListExample {

public static void main(String[] args) {// ArrayList CreationList arraylistA = new ArrayList();List arraylistB = new ArrayList();// Adding elements to the ArrayListfor (int i = 0; i < 5; i++) {

arraylistA.add(new Integer(i));}arraylistB.add("beginner");arraylistB.add("java");arraylistB.add("tutorial");arraylistB.add(".");arraylistB.add("com");arraylistB.add("java");arraylistB.add("site");// Iterating through the ArrayList to display the Contents.Iterator i1 = arraylistA.iterator();System.out.print("ArrayList arraylistA --> ");while (i1.hasNext()) {

System.out.print(i1.next() + " , ");}System.out.println();System.out.print("ArrayList arraylistA --> ");for (int j = 0; j < arraylistA.size(); j++) {

System.out.print(arraylistA.get(j) + " , ");}System.out.println();Iterator i2 = arraylistB.iterator();System.out.println("ArrayList arraylistB --> ");while (i2.hasNext()) {

System.out.print(i2.next() + " , ");}System.out.println();System.out.println();System.out

.println("Using ListIterator to retrieve ArrayList Elements");

System.out.println();ListIterator li1 = arraylistA.listIterator();// next(), hasPrevious(), hasNext(), hasNext() nextIndex() can be used with a// ListIterator interface implementationSystem.out.println("ArrayList arraylistA --> ");while (li1.hasNext()) {

System.out.print(li1.next() + " , ");}System.out.println();// Searching for an element in the ArrayListint index = arraylistB.indexOf("java");System.out.println("'java' was found at : " + index);int lastIndex = arraylistB.lastIndexOf("java");System.out.println("'java' was found at : " + lastIndex

+ " from the last");System.out.println();// Getting the subList from the original ListList subList = arraylistA.subList(3, arraylistA.size());System.out.println("New Sub-List(arraylistA) from index 3 to "

+ arraylistA.size() + ": " + subList);System.out.println();// Sort an ArrayListSystem.out.print("Sorted ArrayList arraylistA --> ");Collections.sort(arraylistA);System.out.print(arraylistA);System.out.println();// Reversing an ArrayListSystem.out.print("Reversed ArrayList arraylistA --> ");Collections.reverse(arraylistA);System.out.println(arraylistA);System.out.println();// Checking emptyness of an ArrayListSystem.out.println("Is arraylistA empty? "

+ arraylistA.isEmpty());System.out.println();// Checking for Equality of ArrayListsArrayList arraylistC = new ArrayList(arraylistA);System.out.println("arraylistA.equals(arraylistC)? "

+ arraylistA.equals(arraylistC));System.out.println();// Shuffling the elements of an ArrayList in Random OrderCollections.shuffle(arraylistA, new Random());System.out

.print("ArrayList arraylistA after shuffling its elements--> ");i1 = arraylistA.iterator();while (i1.hasNext()) {

System.out.print(i1.next() + " , ");}System.out.println();System.out.println();// Converting an ArrayList to an ArrayObject[] array = arraylistA.toArray();

for (int i = 0; i < array.length; i++) {System.out.println("Array Element [" + i + "] = " + array[i]);

}System.out.println();// Clearing ArrayList ElementsarraylistA.clear();System.out.println("arraylistA after clearing : " + arraylistA);System.out.println();

}}

Output

ArrayList arraylistA –> 0 , 1 , 2 , 3 , 4 ,ArrayList arraylistA –> 0 , 1 , 2 , 3 , 4 ,ArrayList arraylistB –>beginner , java , tutorial , . , com , java , site ,

Using ListIterator to retrieve ArrayList Elements

ArrayList arraylistA –>0 , 1 , 2 , 3 , 4 ,‘java’ was found at : 1‘java’ was found at : 5 from the last

New Sub-List(arraylistA) from index 3 to 5: [3, 4]

Sorted ArrayList arraylistA –> [0, 1, 2, 3, 4]Reversed ArrayList arraylistA –> [4, 3, 2, 1, 0]

Is arraylistA empty? false

arraylistA.equals(arraylistC)? true

ArrayList arraylistA after shuffling its elements–> 3 , 2 , 1 , 0 , 4 ,

Array Element [0] = 3Array Element [1] = 2Array Element [2] = 1Array Element [3] = 0Array Element [4] = 4

arraylistA after clearing : []

Download ArrayListExample.java

JavaLinked List

• A LinkedList is used to store an “ordered” group of elements where duplicates are allowed.

• A LinkedList is based on a double linked list where elements of the List are typically accessed through add() and remove() methods.

Below is an LinkedList Example showing how collections are manipulated using an LinkedList

import java.util.List;import java.util.LinkedList;import java.util.Iterator;import java.util.ListIterator;import java.util.Collections;import java.util.Random;

public class LinkedListExample {

public static void main(String[] args) {// LinkedList CreationList linkedListA = new LinkedList();List linkedListB = new LinkedList();// Adding elements to the LinkedListfor (int i = 0; i < 5; i++) {

linkedListA.add(new Integer(i));}linkedListB.add("beginner");linkedListB.add("java");linkedListB.add("tutorial");linkedListB.add(".");linkedListB.add("com");linkedListB.add("java");linkedListB.add("site");// Iterating through the LinkedList to display the Contents.Iterator i1 = linkedListA.iterator();System.out.print("LinkedList linkedListA --> ");while (i1.hasNext()) {

System.out.print(i1.next() + " , ");}

System.out.println();System.out.print("LinkedList linkedListA --> ");for (int j = 0; j < linkedListA.size(); j++) {

System.out.print(linkedListA.get(j) + " , ");}System.out.println();Iterator i2 = linkedListB.iterator();System.out.println("LinkedList linkedListB --> ");while (i2.hasNext()) {

System.out.print(i2.next() + " , ");}System.out.println();System.out.println();System.out

.println("Using ListIterator to retrieve LinkedList Elements");System.out.println();ListIterator li1 = linkedListA.listIterator();// next(), hasPrevious(), hasNext(), hasNext() nextIndex() can be used with a// ListIterator interface implementationSystem.out.println("LinkedList linkedListA --> ");while (li1.hasNext()) {

System.out.print(li1.next() + " , ");}System.out.println();// Searching for an element in the LinkedListint index = linkedListB.indexOf("java");System.out.println("'java' was found at : " + index);int lastIndex = linkedListB.lastIndexOf("java");System.out.println("'java' was found at : " + lastIndex

+ " from the last");System.out.println();// Getting the subList from the original ListList subList = linkedListA.subList(3, linkedListA.size());System.out.println("New Sub-List(linkedListA) from index 3 to "

+ linkedListA.size() + ": " + subList);System.out.println();// Sort an LinkedListSystem.out.print("Sorted LinkedList linkedListA --> ");Collections.sort(linkedListA);System.out.print(linkedListA);System.out.println();// Reversing an LinkedListSystem.out.print("Reversed LinkedList linkedListA --> ");

Collections.reverse(linkedListA);System.out.println(linkedListA);System.out.println();// Checking emptyness of an LinkedListSystem.out.println("Is linkedListA empty? "

+ linkedListA.isEmpty());System.out.println();// Checking for Equality of LinkedListsLinkedList LinkedListC = new LinkedList(linkedListA);System.out.println("linkedListA.equals(LinkedListC)? "

+ linkedListA.equals(LinkedListC));System.out.println();// Shuffling the elements of an LinkedList in Random OrderCollections.shuffle(linkedListA, new Random());System.out

.print("LinkedList linkedListA after shuffling its elements--> ");i1 = linkedListA.iterator();while (i1.hasNext()) {

System.out.print(i1.next() + " , ");}System.out.println();System.out.println();// Converting an LinkedList to an ArrayObject[] array = linkedListA.toArray();for (int i = 0; i < array.length; i++) {

System.out.println("Array Element [" + i + "] = " + array[i]);}System.out.println();// Clearing LinkedList ElementslinkedListA.clear();System.out.println("linkedListA after clearing : " + linkedListA);System.out.println();

}}

Output

LinkedList linkedListA –> 0 , 1 , 2 , 3 , 4 ,LinkedList linkedListA –> 0 , 1 , 2 , 3 , 4 ,LinkedList linkedListB –>beginner , java , tutorial , . , com , java , site ,

Using ListIterator to retrieve LinkedList Elements

LinkedList linkedListA –>0 , 1 , 2 , 3 , 4 ,‘java’ was found at : 1‘java’ was found at : 5 from the last

New Sub-List(linkedListA) from index 3 to 5: [3, 4]

Sorted LinkedList linkedListA –> [0, 1, 2, 3, 4]Reversed LinkedList linkedListA –> [4, 3, 2, 1, 0]

Is linkedListA empty? false

linkedListA.equals(LinkedListC)? true

LinkedList linkedListA after shuffling its elements–> 3 , 2 , 4 , 0 , 1 ,

Array Element [0] = 3Array Element [1] = 2Array Element [2] = 4Array Element [3] = 0Array Element [4] = 1

linkedListA after clearing : []

Download LinkedListExample.java

Java TreeSet

public class TreeSet<E>extends AbstractSet<E>implements SortedSet<E>, Cloneable, Serializable

• This class implements the Set interface and guarantees that the sorted set will be in ascending element order, sorted according to the natural order of the elements or by the comparator provided at set creation time, depending on which constructor is used.

• This implementation not synchronized provides guaranteed log(n) time cost for the basic operations (add, remove and contains).

To prevent unsynchronized access to the Set.: SortedSet s = Collections.synchronizedSortedSet(new TreeSet(..));

Below is a TreeSet Example showing how collections are manipulated using a TreeSet

import java.util.Set;import java.util.TreeSet;import java.util.Iterator;

public class TreeSetExample {

public static void doTreeSetExample() {}public static void main(String[] args) {

Set treeSet = new TreeSet();// the treeset stores Integer objects into the TreeSetfor (int i = 0; i < 5; i++) {

treeSet.add(new Integer(i));}// Since its a Integer Object Set adding any other elements in the Same// set will produce a// ClassCastException exception at runtime.// treeSet.add("a string");System.out.print("The elements of the TreeSet are : ");Iterator i = treeSet.iterator();while (i.hasNext()) {

System.out.print(i.next() + "\t");}

}}

Output: The elements of the TreeSet are : 0 1 2 3 4 (Download TreeSetExample.java)

Java HashMap

• The java HashMap class does not guarantee that the order will remain constant over time.

• This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets.

• The HashMap implementation is not synchronized. If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally.

To prevent unsynchronized access to the Map: Map m = Collections.synchronizedMap(new HashMap(…));

Below is a HashMap Example used to store the number of words that begin with a given letter

/* * Using a HashMap to store the number of words that begin with a given letter. */import java.util.HashMap;

public class HashMapExample {

static String[] names = { "heman", "bob", "hhh", "shawn", "scot","shan", "keeth" };private static HashMap counter = new HashMap();private static Integer cnt = null;public static void main(String args[]) {

for (int i = 0; i < names.length; i++) {cnt = (Integer) (counter.get(new Character(names[i].charAt(0))));if (cnt == null) {

counter.put(new Character(names[i].charAt(0)),new Integer("1"));} else {

counter.put(new Character(names[i].charAt(0)),new Integer(cnt.intValue() + 1));

}}System.out.println("\nnumber of words beginning with each letter is shown below ");System.out.println(counter.toString());

}}

Output: number of words beginning with each letter is shown below{s=3, b=1, k=1, h=2}

Download HashMapExample.java

Java Vector

public class Vector<E>extends AbstractList<E>implements List<E>, RandomAccess, Cloneable, Serializable

• The Vector class implements a growable array of objects where the size of the vector can grow or shrink as needed dynamically.

• Like an array, it contains components that can be accessed using an integer index.• An application can increase the capacity of a vector before inserting a large number

of components; this reduces the amount of incremental reallocation.

Below is a Vector Example showing how collections are manipulated using a Vector

import java.awt.Container;import java.awt.FlowLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.event.WindowAdapter;import java.awt.event.WindowEvent;import java.util.Enumeration;import java.util.NoSuchElementException;import java.util.Vector;import javax.swing.JButton;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JOptionPane;import javax.swing.JTextField;

public class VectorDemo extends JFrame {

private JLabel jlbString = new JLabel("Enter a string");public VectorDemo() {

super("Vector class demo");// Made final as it can be accessed by inner classesfinal JLabel jlbStatus = new JLabel();Container contentPane = getContentPane();final Vector vector = new Vector(1);contentPane.setLayout(new FlowLayout());contentPane.add(jlbString);final JTextField jtfInput = new JTextField(10);contentPane.add(jtfInput);JButton jbnAdd = new JButton("Add");jbnAdd.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {vector.addElement(jtfInput.getText().trim());jlbStatus.setText("Appended to end: "

+ jtfInput.getText().trim());

jtfInput.setText("");}

});contentPane.add(jbnAdd);JButton jbnRemove = new JButton("Remove");jbnRemove.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {// Returns true if element in vectorif (vector.removeElement(jtfInput.getText().trim()))

jlbStatus.setText("Removed: " + jtfInput.getText());else

jlbStatus.setText(jtfInput.getText().trim()+ " not in vector");

}});contentPane.add(jbnRemove);JButton jbnFirst = new JButton("First");jbnFirst.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {try {

jlbStatus.setText("First element: "+ vector.firstElement());

} catch (NoSuchElementException exception) {jlbStatus.setText(exception.toString());

}}

});contentPane.add(jbnFirst);JButton jbnLast = new JButton("Last");jbnLast.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {try {

jlbStatus.setText("Last element: "+ vector.lastElement());

} catch (NoSuchElementException exception) {jlbStatus.setText(exception.toString());

}}

});contentPane.add(jbnLast);JButton jbnEmpty = new JButton("Is Empty?");jbnEmpty.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {jlbStatus.setText(vector.isEmpty() ? "Vector is empty"

: "Vector is not empty");}

});contentPane.add(jbnEmpty);

JButton jbnContains = new JButton("Contains");jbnContains.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {String searchKey = jtfInput.getText().trim();if (vector.contains(searchKey)) {

jlbStatus.setText("Vector contains " + searchKey);} else {

jlbStatus.setText("Vector does not contain "+ searchKey);

}}

});contentPane.add(jbnContains);JButton jbnFindElement = new JButton("Find");jbnFindElement.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {jlbStatus.setText("Element found at location "

+ vector.indexOf(jtfInput.getText().trim()));}

});contentPane.add(jbnFindElement);JButton jbnTrim = new JButton("Trim");jbnTrim.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {vector.trimToSize();jlbStatus.setText("Vector trimmed to size");

}});contentPane.add(jbnTrim);JButton jbnSize = new JButton("Size/Capacity");jbnSize.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {jlbStatus.setText("Size = " + vector.size()

+ "; Capacity = " + vector.capacity());}

});contentPane.add(jbnSize);JButton jbnDisplay = new JButton("Display");jbnDisplay.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {Enumeration enum1 = vector.elements();StringBuffer buf = new StringBuffer();while (enum1.hasMoreElements())

buf.append(enum1.nextElement()).append(" ");JOptionPane.showMessageDialog(null, buf.toString(),

"Contents of Vector", JOptionPane.PLAIN_MESSAGE);}

});contentPane.add(jbnDisplay);contentPane.add(jlbStatus);setSize(300, 200);setVisible(true);

}public static void main(String args[]) {

VectorDemo vectorDemo = new VectorDemo();vectorDemo.addWindowListener(new WindowAdapter() {

public void windowClosing(WindowEvent e) {System.exit(0);

}});

}}

Output

Download VectorExample.java

Java HashTable

• HashTable is synchronized.• Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn’t.• Hashtable doesn’t allow nulls

Below is a HashTable Example showing how collections are manipulated using a HashTable

Please Note that It must be Compiled in Java 1.4.

// Demonstrates the Hashtable class of the java.util package.

public class HashTableDemo extends JFrame {

public HashTableDemo() {super(" Hashtable Sourcecode Example");final JLabel jlbStatus = new JLabel();final Hashtable hashTable = new Hashtable();final JTextArea display = new JTextArea(4, 20);display.setEditable(false);JPanel jplNorth = new JPanel();jplNorth.setLayout(new BorderLayout());JPanel jplNorthSub = new JPanel();jplNorthSub.add(new JLabel("Name (Key)"));final JTextField jtfFirstName = new JTextField(8);jplNorthSub.add(jtfFirstName);jplNorthSub.add(new JLabel("Phone No"));final JTextField jtfPhone = new JTextField(8);jplNorthSub.add(jtfPhone);jplNorth.add(jplNorthSub, BorderLayout.NORTH);jplNorth.add(jlbStatus, BorderLayout.SOUTH);JPanel jplSouth = new JPanel();jplSouth.setLayout(new GridLayout(2, 5));JButton jbnAdd = new JButton("Add");jbnAdd.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {String strNum = jtfPhone.getText().trim();String strName = jtfFirstName.getText().trim();if ((strNum != null && strNum.equals(""))

|| (strName != null && strName.equals(""))) {JOptionPane.showMessageDialog(HashTableDemo.this,

"Please enter both Name and Phone No");return;

}int num = 0;try {

num = Integer.parseInt(strNum);

} catch (NumberFormatException ne) {ne.printStackTrace();

}EmployeeDetails emp = new EmployeeDetails(strName, num);Object val = hashTable.put(strName, emp);if (val == null)

jlbStatus.setText("Added: " + emp.toString());else

jlbStatus.setText("Added: " + emp.toString()+ "; Replaced: " + val.toString());

}});jplSouth.add(jbnAdd);JButton jbnGet = new JButton("Get");jbnGet.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {Object val = hashTable.get(jtfFirstName.getText().trim());if (val != null)

jlbStatus.setText("Get: " + val.toString());else

jlbStatus.setText("Get: " + jtfFirstName.getText()+" not in table");

}});jplSouth.add(jbnGet);JButton jbnRemove = new JButton("Remove Name");jbnRemove.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {Object val = hashTable.remove(jtfFirstName.getText()

.trim());if (val != null)

jlbStatus.setText("Remove: " + val.toString());else

jlbStatus.setText("Remove: " + jtfFirstName.getText()+" not in table");

}});jplSouth.add(jbnRemove);JButton jbnIsEmpty = new JButton("Empty ?");jbnIsEmpty.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {jlbStatus.setText("Empty: " + hashTable.isEmpty());

}});jplSouth.add(jbnIsEmpty);JButton jbnContains = new JButton("Contains key");jbnContains.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {

jlbStatus.setText("Contains key: "+hashTable.containsKey(jtfFirstName.getText().trim()));

}});jplSouth.add(jbnContains);JButton jbnClear = new JButton("Clear table");jbnClear.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {hashTable.clear();jlbStatus.setText("HashTable Emptied");

}});jplSouth.add(jbnClear);JButton jbnDisplay = new JButton("List objects");jbnDisplay.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {StringBuffer buf = new StringBuffer();for (Enumeration enum = hashTable.elements();

enum.hasMoreElements();)buf.append(enum.nextElement()).append('\n');

display.setText(buf.toString());}

});jplSouth.add(jbnDisplay);JButton jbnKeys = new JButton("List keys");jbnKeys.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {StringBuffer buf = new StringBuffer();for (Enumeration enum = hashTable.keys(); enum.hasMoreElements();)

buf.append(enum.nextElement()).append('\n');JOptionPane.showMessageDialog(null, buf.toString(),

"Display Keys of HashTable ",JOptionPane.PLAIN_MESSAGE);}

});jplSouth.add(jbnKeys);Container c = getContentPane();c.add(jplNorth, BorderLayout.NORTH);c.add(new JScrollPane(display), BorderLayout.CENTER);c.add(jplSouth, BorderLayout.SOUTH);setSize(540, 300);setVisible(true);

}

public static void main(String args[]) {HashTableDemo hashTableDemo = new HashTableDemo();hashTableDemo.addWindowListener(new WindowAdapter() {

public void windowClosing(WindowEvent e) {System.exit(0);

}});

}}

class EmployeeDetails {

private String name;private int phoneNp;public EmployeeDetails(String fName, int phNo) {

name = fName;phoneNp = phNo;

}public String toString() {

return name + " " + phoneNp;}

}

Output

Download HashTableDemo.java

Java HashSet

• The HashSet class implements the Set interface.• It makes no guarantee that the order of elements will remain constant over time.• This class is not synchronized and permits a null element.• This class offers constant time performance for the basic operations (add, remove,

contains and size), assuming the hash function disperses the elements properly among the buckets.

• To prevent unsynchronized access to the Set: Set s = Collections.synchronizedSet(new HashSet(…));

Below is a HashSet Example showing how collections are manipulated using a HashSet

import java.util.*;

public class HashSetExample {

private static String names[] = { "bob", "hemanth", "hhh", "hero","shawn", "bob", "mike", "Rick", "rock", "hemanth", "mike","undertaker" };

public static void main(String args[]) {ArrayList aList;aList = new ArrayList(Arrays.asList(names));System.out.println("The names elements " + aList);HashSet ref = new HashSet(aList); // create a HashSetIterator i = ref.iterator();System.out.println();System.out.print("Unique names are: ");while (i.hasNext())

System.out.print(i.next() + " ");System.out.println();

}}

Output

The names elements [bob, hemanth, hhh, hero, shawn, bob, mike, Rick, rock, hemanth, mike, undertaker]

Unique names are: hhh hero bob Rick shawn hemanth rock mike undertaker