Ppt la10 java programming_01

Java Programming(J2EE LC)

Day 1

ER/CORP/CRS/LA10/003

Version 1.00Copyright © 2005, Infosys

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Course Objective

To introduce Java Architecture & appreciate basic syntax in Java Language

To apply Object Oriented Concepts using Java

To illustrate how to make use of standard Java Class Library and create

reusable classes.

To introduce Exception Handling in Java

To learn what is required to package and deploy a Java application

To introduce User Interface Concepts & Event Handling

Java Applets



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Session Plan

Day 1

– Review of Object Oriented Concepts

– Java architecture

– The basic constructs in Java



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Class and Object

What is a Class?

– A class is a blueprint or prototype that defines the variables and the

methods (functions) common to all objects of a certain kind.

What is an Object?

– An object is a representative or specimen of a class. Software objects are

often used to model real-world objects you find in everyday life.



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Example : Objects and Classes

Daria

R002

Jane

R003

Brittany

R004

Jodie

R001

classobject

class Student

char name

int rollNo

setName()

setRollNo()

calcMarks()



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Features of OOP

Abstraction:

– The process of extracting the essential information and hiding the irrelevant details

Encapsulation:

– The process of binding code and data together in the form of a capsule

Inheritance:

– The feature by which one class acquires the properties and functionalities of another

class

Polymorphism:

– The feature that allows the same interface to be used for a general set of actions



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Object Oriented Program

Data

Function

Data

Function

Data

Function

Data

Function

State (Data) is kept accessible only to a

set of functions. Behavior of the

object is exposed using methods.

An object communicates with another object by

passing messages (invoking methods)



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Introduction to Java

A language developed by Sun Microsystems

A general-purpose language

High-level language

Developed initially for consumer devices

Popular platform to develop enterprise applications

– Finds use in internet based applications



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Features of Java

Object-oriented

Simpler language

– Compared to earlier OO languages like C++, it is simple

– Designed considering the pitfalls of earlier languages

Robust

Architecture Neutral / Portable

– Example: Java code compiled on Windows can be run on Unix without recompilation

Secure

– Built -in security features like absence of pointers and confinement of the java program within its

runtime environment

Support for Multithreading at language level

Designed to handle Distributed applications



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Platform independence

Java is a language that is platform independent.

A platform is the hardware and software environment in which a program

runs

Once compiled, code will run on any platform without recompiling or any

kind of modification.

– “Write Once Run Anywhere”

This is made possible by making use of a Java Virtual Machine commonly

known as JVM



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Java Virtual Machine (JVM) (1 of 2)

The source code of Java will be stored in a text file with extension .java

The Java compiler compiles a .java file into byte code

The byte code will be in a file with extension .class

The .class file that is generated is the machine code of this processor.

– Byte code is a binary language

The byte code is interpreted by the JVM

JVM can be considered as a processor purely implemented with software.

The interface that the JVM has to the .class file remains the same irrespective of the

underlying platform.

– This makes platform independence possible



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Java Virtual Machine (JVM) (2 of 2)

The JVM interprets the .class file to the machine language of the underlying

platform.

The underlying platform processes the commands given by the JVM



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Source File (HelloWorld.java)

Java Architecture:

Compiler (javac)

Machine Code or Byte code (HelloWorld.class)

JVM

Class Loader

Byte Code Verifier

InterpreterJIT Code

Generator

Runtime

Operating System

Hardware



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Installing and using Java

Before we begin, something on installation

– Java 2 SDK (v1.4 or higher)

– Can be downloaded freely from http://java.sun.com

– Also available in the intranet

Setting Environment variables for Java

Environment Variable:

– A variable that describes the operating environment of the process

– Common environment variables describe the home directory, command search path

etc.



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Environment variables used by JVM

JAVA_HOME: Java Installation directory

– This environment variable is used to derive all other env. variables used by JVM

– In Windows: set JAVA_HOME=C:\jdk1.4.3

– In UNIX: export JAVA_HOME=/var/usr/java

CLASSPATH

– In Windows: set PATH=%PATH%;%JAVA_HOME%\lib\tools.jar;.

– In UNIX: set PATH=$PATH:$JAVA_HOME/lib/tools.jar:.

PATH

– Path variable is used by OS to locate executable files

– In Windows: set PATH=%PATH%;%JAVA_HOME%\bin

– In UNIX: set PATH=$PATH:$JAVA_HOME/bin

This approach helps in managing multiple versions of Java – Changing JAVA_HOME will

reflect on CLASSPATH and PATH as well

Set these environment variables on the command prompt and type ‘javac’

– Displays all the options of using ‘javac’



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Source File Layout - Hello World

We will have the source code first

Type this into any text editorpublic class HelloWorldApp {

public static void main(String[]args){

System.out.println(“Hello World!”);

}

}

Save this as HelloWorldApp.java

Important :

– Take care!! cAsE of file name matters



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To Compile

Open a command prompt

Set Environment variables (explained earlier)

Go to the directory in which you have saved your program.

Type javac HelloWorldApp.java

– If it says bad command or file name then check the path setting

– If it does not say anything, and you get the prompt, then the compilation was

successful.



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To execute

Type in the command prompt

java HelloWorldApp

The result



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Compilation & Execution

Java Program (.java)

Java Complier (javac)

Byte Code (.class file)

Interpreter (java) Interpreter (java) Interpreter (java)

Win 32 Linux Mac



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Best Practices

One .java file must contain only one class declaration

The name of the file must always be same as the name of the class

Stand alone Java program must have a public static void main defined

– it is the starting point of the program.

– Not all classes require public static void main

Code should be adequately commented

Must follow indentation and coding standards



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Java Keywords – (For Reference Only)

The reserved keywords defined in the Java language

abstract const finally implements public this

boolean continue for instanceof throw transient

break float if null short void

byte default import int super volatile

case do false return switch while

catch double interface package synchronized

char else long private static

class extends goto protected try

true final new native throws



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Data Types in Java

Java is a Strongly typed language

– What is typecasting?

– Unlike C, at runtime type checking is strictly enforced

– Impossible to typecast incompatible types

Two types of variables

– Primitive type

– Reference type



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Primitive Data Types in Java

Integer data types

byte (1 byte)

short (2 bytes)

int (4 bytes)

long (8 bytes)

Floating Type

float (4 bytes)

double (8 bytes)

Textual

char (2 bytes)

Logical

boolean (1 byte) (true/false)

Notes:

All numeric data types are signed

The size of data types remain the

same on all platforms (standardized)

char data type in Java is 2 bytes

because it uses UNICODE character

set. By virtue of it, Java supports

internationalization

UNICODE is a character set which

covers all known scripts and

language in the world



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Reference Types in Java (1 of 3)

Objects, Arrays are accessed using reference variables in Java

A reference variable is similar to a pointer (stores memory address of an

object)

Java does not support the explicit use of addresses like other languages

Java does not allow pointer manipulation or pointer arithmetic



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Reference Types in Java (2 of 3)

A reference type cannot be cast to primitive type

A reference type can be assigned ‘null’ to show that it is not referring to any

object

– ‘null’ is a keyword in Java



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Reference Types (3 of 3)

Use of a reference variable is similar to using the remote to operate the

television

TV (object)

ChangeChannel(6)

Remote Control

(reference variable)

ChangeVolume(INCREASE)



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Comment entry in Java

A single line comment in Java will start with //

// This is a single line comment in Java

A multi line comment starts with a /* and ends with a */

/* This is a multi line

comment

in Java */



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Variables in Java

Using primitive data types is similar to other languages

int count;

int max=100;

In Java variables can be declared anywhere in the program

for (int count=0; count < max; count++) {

int z = count * 10;

}

In Java, if a local variable is used without initializing it, the compiler will show an error

BEST PRACTICE: Declare a variable in program only when required. Do not declare variables upfront like in C.



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Variables -- declaration and initialization (1 of 2)

Variables must have a type

Variables must have a name

int count

type

name



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Variables -- declaration and assignment (2 of 2)

Assigning a value to the variable

count =10;

Declaration and initialization may be combined in a single step

int count =10;



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Give it a try….

What will be the output of the following code snippet when you try to compile

and run it?

class Sample{

public static void main (String args[]){

int count;

System.out.println(count);

}

}



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Typecasting of primitive data types

Automatic, non-explicit type changing is known as Conversion

– Variable of smaller capacity can be assigned to another variable of bigger capacity

int i = 10;

double d;

d = i;

Whenever a larger type is converted to a smaller type, we have to explicitly

specify the type cast operator

double d = 10

int i;

i = (int) d;

– This prevents accidental loss of data

Type cast operator



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Operators and Assignments

Arithmetic Operators:

The Bitwise operators:

Relational Operators:

Boolean Logical operators:

The ternary operator:



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Constituents of a Class

The main method may or may not be present depending on whether the class is a starter class

public class Student {

private int rollNo;

private String name;

Student(){

//initialize data members

}

Student(String nameParam){

name = nameParam;

}

public int getrollNo (){

return rollNo;

}

}

Data Members (State)

Constructor

Method (Behavior)



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Access Modifiers – private and public

Data members are always kept private

– It is accessible only within the class

The methods which expose the behavior of the object are kept public

– However, we can have helper methods which are private

Key features of object oriented programs

– encapsulation (binding of code and data together)

– State (data) is hidden and Behavior (methods) is exposed to external world

Access modifiers (public, private etc) will be covered in more details in the later

slides



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Creating Objects in Java

The new operator creates the object and returns a reference to it

Memory allocation of objects happens in the heap area

Reference returned can be stored in reference variables

E.g. :

Student obj1;

obj1 = new Student();

Or

Student obj2 = new Student(“Jack”);

obj1 is reference variable

new keyword creates an object and returns

a reference to it



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Constructors (1 of 2)

A constructor is a special method that is used to initialize a newly created

object

Called just after the memory is allocated for the object

Can be used to initialize the objects to required or default values at the time of

object creation

It is not mandatory for the coder to write a constructor for the class

When writing a constructor, remember that:

– it has the same name as the class

– it does not return a value not even void

– It may or may not have parameters (arguments)



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Constructors (2 of 2)

If no user defined constructor is provided for a class, compiler initializes

member variables to its default values. Examples:

– numeric data types are set to 0

– char data types are set to null character(‘\0’)

– reference variables are set to null



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Lifetime of objects (1 of 2)

Student obj1 = new student();

Student obj2 = new student();

The two Student objects are now living on the heap

-- References: 2

-- Objects: 2

Student obj3 = obj2;

-- References: 3

-- Objects: 2

2

1

heap

obj1

obj2

2

1heap

obj1

obj2

obj3



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Lifetime of objects (2 of 2)

obj3 = obj1;

-- References: 3

-- Objects: 2

obj2 = null;

-- Active References: 2

-- null references: 1

-- Reachable Objects: 1

-- Abandoned objects: 1 2

1heap

obj1

obj2

ob3

2

1

obj1

obj2

ob3 heap

Null reference

This object can be garbage collected (Can be Removed

from memory)



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Garbage Collection

In C language, it is the programmer’s responsibility to de-allocate memory

allocated dynamically

– free() function is used to de-allocate

In Java, JVM will automatically do the memory de-allocation

– Called “Garbage collection”

– However programmer has to ensure that reference to the object is released

• If a reference variable is declared within a function, the reference is invalidated soon as the

function call ends

• Other way of explicitly releasing the reference is to set the reference variable to null

• Setting a reference variable to null means that it is not referring to any object

An object which is not referred by any reference variable is removed from

memory by the garbage collector

Primitive types are not objects. They cannot be assigned null



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Variables and their scope (1 of 3)

Instance variables (Also known as member variables)

– Declared inside a class

– Outside any method or constructor

– Belong to the object

– Stored in the heap area along with the object to which they belong

– Lifetime depends on the lifetime of the object

Local variables (Also known as stack variables)

– Declared inside a method

– Method parameters are also local variables

– Stored in the program stack along with method calls and live until the call ends



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class Student{

int rollNo;

String name;

public void display (int z){

int x=z+10;

}

}

rollNo and name are instance variables, to be

stored in the heap

z and x are local variables to be stored in

the stack



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Strings in Java

Unlike C, String is a system defined class in Java

Declaring “Hello World” in code will create and object of type string with data “Hello

World” and returns a reference to it.

String is defined in the Java API under the package ‘java.lang’

– Packages are covered in detail later.

– The fully qualified name of String class in Java is ‘java.lang.String’

Unlike C, the string is of fixed length and memory for the string is managed totally by the

String class

– Prevents buffer overruns

– NULL terminator not used in strings

Unlike C, string is not a simple array of characters

– internally it is stored as an array, but not exposed to programmer



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Control Structures in Java

Similar to the “C” Programming Language

Conditionals

– if-else, else if statements

– switch case statements

Loops

– for

– while

– do-while



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Coding standards and Best practices for naming classes and variables

Class name should begin with uppercase and camel casing

– Eg. Student, ArrayList

Name should tell what the variable, method or class does

No short form of words

Variable name should start with lower case and to follow camel casing

– Eg. int numberOfStudents;

Method names should begin with lowercase and follow camel casing

– Eg. void displayUserChoice()



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Commenting code in Java (1 of 3)

File header

– Description of the file

/* This java file contains a class with a method to compute the

* Sum of two numbers. This method is invoked from another class

* by passing the necessary values as parameters

*/



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Class header

– description of the class

– Date

– @ author

– @ version

– Note: Do not type class name in header

/**

* This class contains a method to print sum of two numbers.

* Date: 12-Jan-2005

* @author E&R Dept, Infosys Technologies Limited

* @version 1.0

*/



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Method header

– description of the method

– @param

– @return

– Note: Do not type method name in header

/**

* Computes the sum of the two integer variables passed

* as parameters

* @param number1 The First number

* @param number2 The Second number

* @return the sum of the two numbers passed as arguments

*/



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Arrays in Java (1 of 7)

An array is a data structure which defines an ordered collection of a fixed

number of homogeneous data elements

The size of an array is fixed and cannot increase to accommodate more

elements

Arrays in Java are objects and can be of primitive data types or reference

variable type

All elements in the array must be of the same data type

20 10045 7050

An array holding 5 int elements

An array holding 4 rabbit objects



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Declaring Array Variables

<elementType>[] <arrayName>;or<elementType> <arrayName>[];where <elementType> can be any primitive data type or reference type

Example:int intArray[];Pizza[] mediumPizza, largePizza;



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Constructing an Array<arrayName> = new <elementType>[<noOfElements>];

Example:int intArray[];Pizza mediumPizza[], largePizza[];

intArray = new int[10]; mediumPizza = new Pizza[5]; largePizza = new Pizza[2];

Declaration and Construction combined int intArray[] = new int[10]; Pizza mediumPizza[] = new Pizza[5];



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Declaring and Initializing an Array

<elementType>[] <arayName> = {<arrayInitializerCode>};

Example:

int intArray[] = {1, 2, 3, 4};

char charArray[] = {‘a’, ‘b’, ‘c’};

Pizza pizzaArray[] = {new Pizza(), new Pizza()};



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Unlike C, Java checks the boundary of an array while accessing an element in

it

Java will not allow the programmer to exceed its boundary

If x is a reference to an array, x.length will give you the length of the array

So setting up a for loop as follows is very common in Java

for(int i = 0; i < x.length; ++i){

x[i] = 5;

}



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Multidimensional arrays are arrays of arrays.

To declare a multidimensional array variable, specify each additional index

using another set of square brackets.

-- Ex: int twoD[ ][ ] = new int[4][5] ;



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When you allocate memory for a multidimensional array, you need only specify

the memory for the first (leftmost) dimension

You can allocate the remaining dimensions separately.

In Java the length of each array in a multidimensional array is under your

control.

Ex:

– int twoD[][] = new int[4][];

– twoD[0] = new int[5];






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this keyword(1 of 2)

class Counter{

private int data;

Counter(int var1){

data=var1;

}

public void increment(){

++data;

}

public int getData(){

return data;

}

}

class CounterTest{

public static void main(String args[]){

Counter a = new Counter(10);

a.increment();

System.out.println(a.getData());

System.out.println(a.getData());

Counter b = new Counter(25);

b.increment();

System.out.println(b.getData());

}

}



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this reference (2 of 2)

In the Counter example, when we call a.increment(), the increment() method

will say ++data

Whose data is it?

The methods of a class will automatically have a reference called this

The ‘this’ reference will always refer to the object that called the method



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static (1 of 4)

static keyword can be used in 3 scenarios:

– For class variables

– For methods

– For a block of code



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static (2 of 4)

static variable

– It is a variable which belongs to the class

– A single copy to be shared by all instances of the class

– For using static variables, creation of instance is not necessary

– Accessed using <class-name>.<variable-name> unlike instance variables which are accessed

as <object-name>.<variable-name>

static method

– It is a class method

– Accessed using class name.method name

– For using static methods, creation of instance is not necessary

– A static method can only access other static data and methods. It cannot access non-static

members



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static (3 of 4)

Class Duck {

private int size;

private static int duckCount;

public Duck(){

duckCount++;

}

public void setSize (int s){

size = s;

}

public int getSize (int s){

return size;

}

public static void main(String args[]){

System.out..println(“Size of the duck is;” + size);

}

}

The static duckCount variable is initialised to 0, ONLY when the

class is first loaded, NOT each time a new instance is made

Each time the constructor is invoked ie an object gets created, the static

variable duckCount will be incremented thus keeping a count of the total no of

Duck objects creataed

Which duck? Whose size? A static method cannot access anything

non-static

Compilation error



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static (4 of 4)

static block

– The static block is a block of statement inside a Java class that will be executed

when a class is first loaded and initialized

• A class is loaded typically after the JVM starts

• In some cases a class is loaded when the program requires it

– A static block helps to initialize the static data members just like constructors help to

initialize instance members

class Test{

static {

//Code goes here

}

}



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Java Native API (JNI)

Even though Java is platform independent, it can call a platform specific

function that is written in another language like C or C++

Such methods are qualified with the keyword native, to show that it is native

code and not implemented using Java

Java provides an API specially to handle native (platform-specific) code

The library has to be loaded in the static block using the method call

System.loadLibrary



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Referring to Java documentation

Java provides a rich set of library classes

Demo on how to refer to library classes



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Summary:

– Review of Object Oriented Concepts

– Java architecture

– The basic constructs in Java

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