Ch-2 Overview of Java

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Topics1.Brief History of Java

2.What is Java?

3.Differences to C++

4.Java Development Tools

5.The First Simple Program

6.Structure of a Class

7.Variables7.1.Declaration

7.2.Scope7.3Constants

8.Data Types8.1.Primitive Data Types

8.2.Reference Data Types

9.Operators

9.1.Arithmetic9.2.Comparison and Conditional

9.3.Assignment

9.4.Others

10.Type Conversions and Casting

11.Mathematical FunctionsChapter 2

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1.Brief History of Java0 Java was originally developed by Sun Microsystems, a US company,

in 1991. Its first name was Oakand it was designed for the

development of software for consumer electronic devices such astelevisions and video recorders. As such, one of the main goals forthe language was that it is simple, portable and highly reliable.

0 The team based the new language on the existing C and C++

languages, but they removed features of C and C++ that were seenas being the sources of problems.

0 In 1993, the WWW (World Wide Web) was starting to be used,transforming the previously text-based internet into a more

graphic-rich environment. The team developing the Oak languagecame up with the idea of creating small programs, called applets,that would be able to run on any computer that was connected tothe internet.

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cont0 In 1995, Oak was renamed Java due to some legal issues the

name does not have any particular meaning. Java was supported bymany IT companies, including the big internet browser developers,Netscape and Microsoft.

0 Since then, Java has become one of the main languages used forinternet programming and also as a general-purpose object-oriented language for stand-alone applications.


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2.What is Java?0 Java technology consists of a programming language and a

platform on which programmes can be run.

0 Java is different from other programming languages because a Javaprogram is both compiledand interpreted but in many otherlanguages, the program is either compiled or interpreted.

0 The Java language is object-orientedand programs written in Java

areportable that is, they can be run on many different platforms(e.g. Windows, Mac, Linux, Solaris).

0 Mac, short for Macintosh, is an operating system owned by theApple Corporation. The Mac OS has a GUI that looks like Windows

but has a different underlying architecture. It is a competitor toWindows. The Mac OS has to be run on Apple Mac computers,which, again, have a different underlying architecture to PCs likeIBM, Acer etc.


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...cont0 Solaris is a platform from Sun Microsystems. Like Windows, Solaris

has server and workstation operating systems.0 The Java compiler translates the program into an intermediate

language calledJava bytecodes.

0 Java bytecodes are platform-independent this means they can

be run on different operating systems (e.g. Windows and Mac).0 There are different Java interpreters for different platforms. The

interpreter parses and runs each of the Java bytecode instructions.


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Compiling and Interpreting of a Java program


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Compiling and Interpreting of a Java program0 Compilation only needs to happen one time, whereas

interpretation happens every time the program is executed.

0 The source code for a Java program is saved to one or more .javafiles.

0 The compiler generates a .class file for each .java file.

0 A .java file can be opened and viewed in any text editor, but a .class

file is bytecode so it cannot be viewed in the same way.0 Every Java interpreter is actually an implementation of somethingcalled theJava VM(Virtual Machine).

0 It is called a Virtual Machine because it is like a computer that doesnot physically exist

0 The VM exists only within the memory of the computer. The Javabytecodes are like a set of machine code instructions for the JavaVM. A web browser that is Java-enabled is an example of aninterpreter.


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cont0 The big benefit of the Java way of doing things is that the program

code can be written once, compiled and then run on many differentplatforms.

0 See the figure below for an illustration of this concept. There is oneJava VM, but many different implementations of it i.e. manydifferent interpreters for the Java bytecodes.


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cont0 Figure - a Java program can be compiled once and then run on

different platforms


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cont0 As mentioned above, Java includes a programming language and a

platform.

0 A platform refers to the hardware (PCs, servers, printers etc) andsoftware (operating systems, application software etc) in which aprogram is run. So, for example, the platform we use here in yourcomputer laboratory is Windows XP/7 running on IBM-compatible

PCs.0 The Java platform is a bit different to other platforms, in that it

consists of software only that means it is hardware-independent.This is what makes it possible to run compiled Java programs ondifferent platforms.

0 The Java platform consists of:0 The Java VM

0 The Java API (Application Programming Interface).


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cont0 The API provides libraries of pre-written, ready-to-use

components that provide standard functionality.

0 A library groups a number of related classes and interfacestogether into what is called a Javapackage. You can choose whatpackages you want to use in your program.

0 In order to write Java programs, you need to have the Java SDK

(Software Development Kit) installed on your PC. This includes theAPI and the VM, as well as compilers and debuggers for Java.

0 Java is a programming language which is based on the OOP (object-oriented paradigm).

0 'Paradigm' means a set of ideas, a concept or hypothesis.0 You should already be familiar with some OO concepts from your

study of C++. However, there are some differences between Javaand C++.


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3.Differences to C++0 Java is a true OO language, while C++ is C with an object-oriented

extension (C is not an OO language).

0 The following is a list of some major C++ features that wereintentionally omitted from Java or significantly modified. You maynot have covered all of these C++ features in your C++programming course, but you should be familiar with some of

them.

0 There are, of course, other differences between Java and C++,which you will discover as we go through the course.


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...cont0 The differences are:

0 Java does not support operator overloading .

0 Java does not have template classes.0 Java does not directly support multiple inheritance of classes, but it

does allow this to be achieved using a feature called an interface.

0 Java does not support global variables every variable and method

is declared within a call and forms part of that class (this relates tothe scope of variables.

0 Java does not use pointers (but similar functionality is achievedusing implicit references to objects).

0 Java has replaced the destructor function with a finalize() function.0 There are no header files in Java (but there is a way to import all

the classes from another package or library).


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4.Java Development Tools0 To get started with writing Java programs, you need to have the

Java SDK installed on your PC.

0 The current version of the Java itself is Java 2; the current versionof the SDK is 6.9. The latest version of the SDK can be downloadedfor free from the Sun web site (http://java.sun.com).

0 The SDK provides various tools for working with Java code, as wellas the Java VM and the API.

0 Some of the tools are:

0 Javac (Java compiler)

0 Java (Java interpreter i.e. to run a Java program)

0 Javadoc (for creating HTML-format documentation from Java sourcecode)

0 Jdb (Java Debugger)


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...cont0 The Java API consists of many built-in classes and methods that

you can use in your code. This reflects the fact that the Java

technology is built using Java itself.0 Some of the most commonly used packages are the following:

0 Language support package (java.lang) classes required forimplementing basic features of Java.

0 Utilities package (java.util) classes that provide various utilityfunctions such as date and time functions.

0 Input/Output package (java.io) classes required for manipulation ofinput/output to/from programs

0 Networking Package (java.net) classes for communicating with otherprograms/PCs over networks and internet

0 Applet Package (java.applet) classes that are used to create Javaapplets.


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5.The first simple Program1. /* This is a simple Java program. */

2. class Example {3. // Your program begins with a call to main().

4. publicstaticvoid main(String args[]) {

5. System.out.println(Hello comps!");

6. }

7. }

0 When the program is run, the following output is displayed:

Hello comps!


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cont0 Line 1: is multiple line comment which is like in C++.

/*multiple line comment*/

0 Line 2: class Example {0 This line uses the keyword class to declare that a new class is being

defined.

0 Example is an identifier that is the name of the class. The entire classdefinition, including all ofits members, will be between the openingcurly brace ({) and the closing curly brace (}).

0 The use of the curly braces in Java is identical to the way they are usedin C, C++, and C#.

0 For the moment, dont worry too much about the details of a classexcept to note that in Java, all program activity occurs within one. Thisis one reason why all Java programs are (at least a little bit) object-oriented.

0 Line 3: is the single-line comment which is like in C++.

// single line commentChapter 2


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cont0 Line 4:public static void main(String args[]) {

0 This line begins the main( ) method. As the comment preceding itsuggests, this is the line at which the program will begin executing.All Java applications begin execution by calling main( ). (This isjust like C/C++.)

0 The public keyword is an access specifier, which allows the

programmer to control the visibility of class members. When aclass member is preceded by public, then that member may beaccessed by code outside the class in which it is declared. (Theopposite ofpublic is private, which prevents a member from

being used by code defined outside of its class.)0 In this case, main( ) must be declared as public, since it must be

calledby code outside of its class when the program is started.


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cont0 The keyword static allows main( ) to be called without having to

instantiate a particular instance of the class. This is necessary since

main( ) is called by the Java interpreter before any objects are made.0 The keyword void simply tells the compiler thatmain() does not

return a value.

0 In main( ), there is only one parameter, albeit a complicated one.

String args[ ] declares a parameter named args, which is an array ofinstances of the class String. (Arrays are collections of similar objects.)Objects of type String store character strings.

0 In this case, args receives anycommand-line argumentspresent when

the program is executed. This program does not make use of thisinformation, but other programs which well see later will do.


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cont0 Line:5

System.out.println(Hello Comps!");

0 This line outputs the string Hello Comps! followed by a new lineon the screen.

0 Output is actually accomplished by the built-in println() method.In this case, println( ) displays the string which is passed to it.

0 As you will see, println( ) canbe used to display other types ofinformation, too.

0 The line begins with System.out. - System is a predefined classthat provides access to the system, and outis the output stream

that is connected to the console


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6.Structure of a Class0 All Java code is written inside classes.

0 The class definition is the first part of the code that appears. Thisconsists of the access modifier for the class (public or private), theclass keyword and the name of the class.

0 By convention, class names begin with an upper case letter.Forexample, to define a class called Circle :

public class Circle

{

}


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cont0 The methods implement the behaviour of the objects belonging to

the class. Generally, methods return information about the state of

an object or they change the state of an object. These aresometimes called accessorand mutatormethods.

0 The order of these parts of a class is generally not important, butplacing the fields and then the constructors at the beginning does

make the class readable and easy to get around for programmers.Of course, comment blocks should also be used to document thecode.


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cont (Eg)public class Circle{

//data

private double radius;

//constructors

public void Circle ()

{ radius = 1.0;}

public void Circle (double r)

{ radius = r; }

//methods

public double calcArea()

{

return 3.14* radius * radius;

}

public void draw()

{

}

}Chapter 2


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7.1.Variables Declaration0 Remember that a variable is like a storage location to store a data value

and that a variable can take different values at different times during the

execution of a program.0 Some Java variable declarations include:

int x;

FirstJava myObject = new FirstJava ();

int sum = 0;0 A variable must be given an explicit name and data type.

0 The name must be a legal identifier(an identifier is simply a name givento an item used in a Java program). A legal identifier begins with a letterand can consist of alpha-numeric characters (letters and digits) and canalso include the underscore (_) and dollar ($) characters. Identifiers arecase-sensitive and cannot have spaces in them.

0 Some data types have default values that are used for initialisation if avariable is not explicitly initialised in code. For example, the integer data

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7.1.Variables Declaration(cont)0 A convention used in Java programming is that variable names begin

with a lowercase letter while class names begin with an uppercase letter.

If a variable or class name has more than one word in it, the words arejoined together and the first letter of each word after the first will beginwith a capital letter. This convention makes code easier to read andunderstand.

0 A variable declaration is used to give a variable a name and a data type

the format is to put the type followed by the name i.e. type name

0 A variable must be initialised before it is used (the compiler will notallow an uninitialized variable to be used). Initialisation simply meansassigning some initial value to a variable e.g.

0 int i ;0 i = 0; //to initialise an integer variable to the value 0.

0 However, declaration and initialisation are often combined into onestatement e.g.

0 int i = 0;


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7.2.Variables Scope0 A variable has scope.

0 Scope refers to the parts of the code in which the variable name

can be used without prefixing it with the class or object name. Thescope is determined by where in the class definition a variable isdeclared.

0 Generally, Java variables have either class scope or local scope.

0 Static variables or fields , which is another way of saying 'classvariable have class scope because they can be referenced withinthe class and its subclasses without prefixing with the class name.

0 Instance variables are variables that belong to instances (objects)

of a class. These also have class scope, because they must beprefixed with the instance name when used outside the class or itssubclasses.


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7.2.Variables Scope(cont)0 The difference between static, or class, variables and instance

variables is that an instance variable can take different values for

each objectwhile a class variable has the same value for all objectsof that class.

0 Class and instance variables are declared at the class level notwithin methods. But they can be used within methods in the class,

without prefixing with the class or instance name.0 Variables that are declared within a method or within a block of

code have localor lexical scope, that is, they can be used only withinthe enclosing block of statements or method. For example, in thefollowing block of code, the variable i is out of scope in the line thatbegins 'System.out.println' because it was declared within theblock of statements following the if statement, so its scope is thatblock. Thus the last line will not compile:


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7.2.Variables Scope(cont)0 if (...) { int i = 17; ...}

System.out.println("The value of i = " + i); // error

0 The scope of parameters to a method or cannot be used outside themethod or constructor.

0 Local variables do not have the public or private modifiers in frontof them. The lifetime of a local variable is only the time of themethod execution they are created when a method is called and

destroyed when the method finishes.To summarise:

0 Class variables are used to store data that is common to all objectsof the class.

0 Instance variables are used to store data that persists through thelifetime of an object.

0 Local variables are often used as temporary storage locationswhile a method or constructor completes its task.


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7.3.Variables Constants0 In Java, a variable declaration can begin with thefinalkeyword.

This means that once an initial value is specified for the variable,that value is never allowed to change.

0 This is the equivalent of a constantin C++ or other languages.

0 For example, to declare a constant for the value of the maximumnumber of students allowed on a course:

finalintMAX_STUDENTS = 100;

0 The variable can be initialised after the declaration but if thedeclaration includes the final modifier, the initial value is the lastvalue that can be assigned.

finalintMAX_STUDENTS;

MAX_STUDENTS = 100;0 Note that the convention in Java programming is to use all

uppercase letters for constant names, so that they stand out in thecode. Underscores are used to separate the words if there is morethan one word in the constant's name.

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8.Data Types0 Every variable must have a data type the data type

determines the values that a variable can contain and also

what operations can be performed on it.

0 Java has two types of data type

0primitive and

0 reference.


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8.1.Primitive Data Type0 A variable ofprimitivetype contains a single value of the

appropriate size and format for its type: a number, a character, or

a boolean value. For example, an integer value is 32 bits of data ina format known as two's complement, the value of a char is 16 bitsof data formatted as a Unicode character, and so on.

0 A variable ofreferencetype has as its value an address, or a

pointer to the values or set of values that the variable represents.Classes, interfaces and arrays are reference types.

0 The primitive types can be divided into numeric and non-numeric types.

0 Numeric types can further be divided into integer and realnumber (or floating-point) types. Integer types hold wholenumbers, positive and negative. Real number types can holddecimal values e.g. 1.234, -6.754.


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Numeric data types in Java


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Numeric data types in JavaType Description Storage Size

Integer types byte Byte-length integer; -128 to 127 8 bits (1 byte)

short Short integer; -32768 to 32767 16 bits (2 bytes)

int Integer; -231 to (2311) 32 bits (4 bytes)

long Long integer; -263 to +(263-1) 64 bits (8 bytes)

Real number/floating-point types

float IEEE 754 floating point;+/- 1.4E-45 to +/- 3.4028235E+38

32 bits

double IEEE 754 floating point;

+/- 4.9E-324 to +/- 1.7976931348623157E+308

64 bits


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cont


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cont0 The bigger storage size types take up more memory (i.e. short takes more

memory than byte). So, you should select a data type appropriate to thevalues that will be held in a variable e.g. if the values will be in the range

30000 to +30000, use short rather than int. All Java number values aresigned which means they can be positive or negative.

0 Non-numeric data types are boolean and character.

0 The boolean type has only two possible values, representing the twoBoolean states true and false. Java reserves the words true andfalse forthese values.

0 Comparison operators (e.g. >,


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cont0 Literal primitive values can be used directly in code e.g.char myChar = 'A'; //to assign the character A to a char variable

int myInt = 5; // to assign the value 5 to an integer variable

0 The declarations above also show how a value can be assigned to avariable as part of the variable declaration e.g. the myChar variableis initially given the value of 'A'.

0 For the char type, there are escape sequences for special characterse.g. \b for a backspace, \n for a newline, \t for a tab, \\ for abackslash, \', \.

0 Generally speaking, a series of digits with no decimal point is typedas an integer. A long integer can be specified by putting an 'L' or 'l'after the number. 'L' is preferred as it cannot be confused with the

digit '1'.0 A series of digits with a decimal point is of type double (e.g. 34.543).

0 But you can specify that the number is a float by putting an 'f ' or 'F'after the number (e.g. 34.543f).


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cont


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cont0 A literal character value is any single Unicode character b/n single

quote marks. The two boolean literals are simply true and false.

0 Java has wrapper classes for each of the integer types these areByte, Short, Integer and Long. These classes define MIN_VALUE andMAX_VALUE constants that describe the range of each type. Theyalso have useful static methods that can be used to convert strings

to integer values e.g. Integer.parseInt() to convert a string to aninteger. The exercise below demos these wrapper classes.

0 The primitive types have default values so if a variable is notexplicitly initialised, it is automatically initialised to its

corresponding default value. These are 0 for int, 0.0 for floating-point values, and an empty string for char.


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* Uses the wrapper classes to get the min// display them all


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Uses the wrapper classes to get the minand max values for each number type.*/public class MaxVariablesDemo {

public static void main(String args[]) {

// integersbyte largestByte = Byte.MAX_VALUE;short largestShort = Short.MAX_VALUE;int largestInteger = integer.MAX_VALUE;

long largestLong = Long.MAX_VALUE;

// real numbersfloat largestFloat = Float.MAX_VALUE;double

largestDouble=Double.MAX_VALUE;

// other primitive typeschar aChar = 'S';boolean aBoolean = true;

System.out.println("The largest bytevalue is" + largestByte);

System.out.println("The largest shortvalue is " + largestShort);

System.out.println("The largest integer

value is " + largestInteger);System.out.println("The largest long

value is " + largestLong);System.out.println("The largest float

value is " + largestFloat);System.out.println("The largest double

value is " + largestDouble);

if (Character.isUpperCase(aChar)) {System.out.println("The character "

+ aChar + " is upper case.");} else {

System.out.println("The character "+ aChar + " is lower case.");

}System.out.println("The value of

aBoolean is " + aBoolean);}

}


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0 The output will be

The largest byte value is127

The largest short value is 32767

The largest short value is 2147483647

The largest float value is 9223372036854775807

The largest float value is 3.4028235E38

The largest float value is 1.7976931348623157E308

The character S is upper case.

The value of aBoolean is true


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8.2.Reference Data Types

0 Arrays and classes are reference data types in Java.

0 While a variable that has a primitive data type holds exactly onevalue, a variable that has a reference data type is a reference to thevalue or set of values represented by the variable.

Classes

0 When a class is defined, it can then be used as a data type. Thevariable declaration for a reference data type is the same as for aprimitive data type. For example, if a program has a class namedCustomer, a new variable, myCustomer1, to hold an object of type

Customer can be declared like this:Customer myCustomer1;


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Cont0 However, to assign a reference to the variable, the newkeyword

must be used new creates a new object from the specified class.

The class name is followed by parentheses in which anyarguments required by the class constructor are passed e.g.

Customer myCustomer1 = new Customer();

0 The above line creates a new object from the Customer class; the

constructor for the Customer class in this case does not take anyarguments.

0 This is the same as the following lines where the object iscreated by a separate assignment:

Customer myCustomer1;

MyCustomer1 = new Customer();


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Arrays


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Arrays0 An array is also a reference type, a structure that can hold multiple values

but all the values must be of the same type. That type can be a primitivedata type or an object type, or it can be other arrays.

0 In Java, an array is actually an object but one that has specialised syntaxand behaviour.

0 An array is declared by specifying the data type of the values it will hold,followed by [] to indicate that it is an array. For example:

byte[] arrayOfBytes; //array of values of type byte

byte[][] arrayOfArrayOfBytes ; //an array of arrays of byte[] type

Customer[] arrayOfCustomers; //array of objects of the class//Customer

0 The C++ syntax for declaring a variable of array type is also supported byJava i.e.

Byte arrayOfBytes[]; //array of values of type byte

0 However, this syntax can be confusing, so it should be avoided in Java.Chapter 2


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Arrays(cont)


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Arrays(cont)0 In the above examples, we can say that byte[] and Customer[] are types.

0 After an array has been declared, it must be created. Because an array isactually an object in Java, the newkeyword must be used to create thearray (new is used to create any object). The size of the array i.e. howmany elements it can hold must be specified. For example, to declare anarray called 'arrayOfBytes' that will hold 1024 bytes and to create it:

byte[] arrayOfBytes = newbyte[1024];

0 To declare an array of 50 strings:String[] lines = newString[50];

0 When the array is created in this way, each of the values in the array isinitialised to the corresponding default value for the data type of the

value (the default value for an object type is null, meaning the object hasan empty reference).

0 In Java, arrays have a 0-based index, that is, the first element is at index 0.The elements are accessed by the array name followed by the index insquare brackets.


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Arrays(cont)


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Arrays(cont)0 For example, to declare an array named 'responses', with two

elements whose values are 'Yes' and 'No':

String[] responses = new String[2];responses[0] = "Yes";

responses[1] = "No";

//to read the elements

System.out.println ("The answer should be " + responses[0] + " or "+ responses[1] + "!");

0 If the size of an array is n, then the highest index for it is (n-1) e.g.an array of size 5 holds 5 elements, with the last one being

accessed at index [4].0 If the code tries to read an element past the end of the array e.g. to

read index [5] in an array of size 5, the interpreter throws anArrayIndexOutOfBoundsException at runtime.


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Arrays cont0 The size, or length, of an array can be accessed in code by reading the

value of the length property of the array object e.g.

int sizeOfResponsesArray = responses.length;

0 This is commonly used to loop through all the values in an array e.g.int[] valuesArray; //assume this array is created and initialised

//somewhere else

int totalValue = 0; //store the sum of the array elements

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

totalValue += valuesArray [i];

0 The above example creates an array of values and then uses a for loop toiterate through the values and add them up. As the index for the array is0-based, the last element is at the index [array.length-1].

0 Throwing an exception is part of the runtime error handling of Java. Wewill look at how to properly handle exceptions later in the course.

0 The for syntax includes the initialisation, test and update steps for theloop initialise the counter to 0, loop until i reaches the size of the array,increment i by 1 for each iteration.


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Assigning Values to Arrays


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Assigning Values to Arrays0 The nullliteral can be used to indicate that an object that is of a

reference data type does not yet exist i.e. to initialise an object to

be empty or null. For example:char[] password = null; //sets the password array of

//characters to be null

0 Literal values can also be used to specify the values of an array, asin the String array example given above:

String[] responses = new String[2];

responses[0] = "Yes";

responses[1] = "No";

0 An array can also be initialised by the following syntax, where thearray object is created and the elements initialised in onestatement. The element values are separated by commas.

int[] someNumbers = {1, 2, 3, 4}


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Assigning Values to Arrays(cont)


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Assigning Values to Arrays(cont)0 The newkeyword is not used but the object is implicitly created.

0 The length of this array is now 4 again, this is implicit from thespecified elements.

0 The statement above could also be written as:int[] someNumbers = new int[4];

someNumbers[0] = 1;

someNumbers[1] = 2;

someNumbers[2] = 3;someNumbers[3] = 4;

0 In fact, the Java compiler compiles the single statement into Java bytecodes that are equivalent to the above. The implication of this is that ifyour program needs to include a large amount of data into an array, it

may not be most efficient to include the data literally in the array, as theexamples above do. This is because the compiler has to create a lot ofJava byte codes to initialise the array, and then the interpreter has toexecute all that code.

0 It may be more efficient to store the data in an external file and read it

into the program at runtime.


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9.Operators

0 This section covers arithmetic, comparison, conditional andassignment operators.

0 Java also has a group of operators called shift or bitwiseoperators these operate on the individual bits (1s and 0s) of

integer values. We will not cover these at this point.


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9.1. Arithmetic0 Java supports the standard arithmetic operators for all integer

and floating-point numbers. These are:

0 + (addition), - (subtraction), * (multiplication), / (division), %(modulo).

0 All of these are binary operators i.e. they take two operands andthe operator appears between the two operands (this is calledinfix notation), as follows:

Operand1 operator operand2

0 The subtraction operator can also be used as a unary operator when it is placed in front of a number e.g. 5. When used inthis way, it effectively multiplies the single operand by 1.


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9 1 Arithmetic(cont)


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9.1. Arithmetic(cont)

0 The addition operator can also be used to concatenate twostrings. If either one of the two operands is a string, then theother one is converted to a string and the strings areconcatenated. Try the following code in a main method to seethe addition operator operating on numbers and on strings.

System.out.println (3+4); //prints out 7

System.out.println ("The value is: " + 4); //prints out 'Thevalue is: 4'

System.out.println ("The value is: " + 3 + 4); //prints out 'Thevalue is: 34', because there is at least one string operand

System.out.println ("The value is: " + (3+4)); //prints out 'Thevalue is 7' because the parentheses indicate that the sum of3+4 is evaluated first, resulting in the value 7


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9.1. Arithmetic(cont)0 The division operator divides the first operand by the second e.g.

12/4 evaluates to 3.

0 If both operands are integers, the result is an integer and anyremainder is lost. If you want to do division and get the remainder,at least one of the operands should be typed as a floating-pointnumber the result will then be a floating-point.

0 When an integer and a floating-point number are used as operandsto any arithmetic operator, the result is a floating-point number.This is because the integer is implicitly converted to a floating-point value before the operation takes place.

0 The table below summarises the data types for the results ofarithmetic operations, based on what the data types of theoperands are.


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9.1. Arithmetic(cont)

Data Type of Result Data Type of Operands

longNeither operand is a floator a double (integer

arithmetic); at least one operand is a long.

int Neither operand is a floator a double (integerarithmetic); neither operand is a long.

double At least one operand is a double.

Float At least one operand is a float; neither operand isa double.


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9.1. Arithmetic(cont)0 There are also shortcut arithmetic operators, to easily increment

and decrement a value by 1. These are + + and - -. Where the

operator is placed relative to the operand i.e. before or after itaffects the behaviour of the operator.

0 When the operator is placed before the operand, it increments ordecrements the operand and the operation evaluates to the

incremented value of the operand. This is called apre-incrementoperator.

0 When the operator is placed after the operand, it increments ordecrements the operand but the expression evaluates to the value

of the operand before it was incremented/decremented. This iscalled apost-incrementoperator.


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9.1. Arithmetic(cont)0 Consider the code sample below:

int i,j;

i = 1;j = ++i; //pre-increment - sets j to 2 and i to 2

i = 1;

j = i++; //post-increment sets j to 1 and i to 2

0 These operators are useful as shortcuts for incrementing ordecrementing number values e.g. x++, x-- and are commonly usedto increment/decrement the counter that controls a loop e.g. a forloop that loops 10 times:

for (int i=0; i


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9.2.Comparision and Conditional

Operator Usage Returns true if

> op1 > op2 op1 is greater than op2

>= op1 >= op2 op1 is greater than or equal to op2

< op1 < op2 op1 is less than op2


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9.2.Comparision and Conditional(cont)

Operator Name Usage

&& Conditional AND op1 & op2

Returns true if op1 and op2 are both trueonly evaluates op2 if

op1 is true

| | Conditional OR op1 | | op2

returns true if op1 or op2 is trueonly evaluates op2 if op1 is

false

! Boolean NOT !op

Returns true if op is false i.e. changes the boolean value of the

operand

& Boolean AND op1 & op2Like && but always evaluates both operands

| Boolean OR op1 | op2

Like || but always evaluates both operands, even if the first one is

true

^ Boolean XOR op1 ^op2

Exclusive ORreturns true only if exactly one of the operands is

true. Returns false if both are true or both are false

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Java also has conditional (logical) operators. These can be used tocombine multiple comparison expressions into a single, morecomplex expression. The operands for a conditional operator mustbe boolean and the result is also a boolean.



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9.2.Comparision and Conditional(cont)0 Note the difference between && and &, and between | | and |.

0 If the right-hand operand in a && or | | operation carries out some

action, such as reading in some input or updating a value,remember that if the operand is not evaluated, the action will notbe carried out.

0 For example, suppose there is an object named customer1 which

has a method increaseBalance() which returns a Boolean true ifthe increase is successful and false if it is not. Take a statement thatchecks for the value of a variable (beingInt) being greater than agiven value AND the increaseBalance() method being successfullyinvoked. The statement can be written as follows, using aconditional AND:


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9.2.Comparision and Conditional(cont)someInt int = 8

depositAmt int;

depositAmt = 100;.

if (someInt >= 10 && customer1.increaseBalance(depositAmt) )

{

}

0 In this case, the value of someInt is 8, which is less than 10, so theexpression 'someInt>=10' is false. This means that the second

expression, customer1.increaseBalance() will not be evaluated. So,the balance will not be increased. This could be a problem, if it isnecessary to ensure that the balance is increased. In that case, aboolean AND operator (&) should be used as it will evaluate bothexpressions, even if the first one is false.


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9 3 Assignment


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9.3. Assignment0 The basic assignment operator is = e.g.

int x = 5; //assigns the value of 5 to the integer variable x

0 There are also several short-hand assignment operators. Theseare used to perform some operation on an operand and to assignthe result to a variable, all in one go.

0 For example:

int i;i = 2;

i = i + 3;

0 //the following statement is the same as the above twostatements

i += 2;

0 Any of the arithmetic operators can be used in this way i.e.

+=, -=, *=, /=, %=.Chapter 2


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9 4 Others(cont)


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9.4. Others(cont)Object Member Access (.)

0 The dot (.) operator is used to access the data and methods of an object.The data fields and methods of an object are also known as members of

the object.0 For example:

Person aPerson = new Person();

String theName = new String();

theName = aPerson.name //evaluates to the value of the name data fieldof the Person object

Method Invocation (( ))

0 A method can be accessed using the dot operator, and it is invoked by

using the ( ) operator after the method name. Any arguments orparameters to the method are placed inside the brackets. For example:

Person aPerson = new Person();

aPerson.increaseSalary(100); //invokes the method increaseSalary,

passing 100 as the paramter)

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9.4. Others(cont)


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9.4. Others(cont)Object Creation (new)

0 As already seen, newis used to create a new object or array. The ne

keyword is followed by the class name and a list of arguments to bepassed to the object constructor. The arguments are placed insidebrackets. For example, if the constructor for the Person class takesthe name and father's name as arguments:

Person aPerson = new Person("Firstname", "Fathersname");


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10. Type Conversions and Casting


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10. Type Conversions and Casting0 Java carries out implicit conversions between number types. For

example, if a short literal value is assigned to an int data type, Java

automatically converts the value to int:int x = 32767; //32767 is a literal value of type short, but isconverted to int

0 This is called a widening conversionbecause the value is beingconverted to a type that has a wider range of legal values.

0 A narrowing conversionoccurs when a value is converted to atype that is not wider than it. Sometimes this type of conversion isnot safe e.g. it is safe to convert the integer value 13 to a byte, butnot to convert 13000 to a byte, because the byte type can only hold

numbers between 128 and 127.0 Because of this, the Java compiler produces a compile error when

the code attempts a narrowing conversion. This happens even ifthe actual value being converted would fit into the narrower range.


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10. Type Conversions and Casting(cont


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10. Type Conversions and Casting(cont0 For example:

int i = 13;

byte b = i; //not allowed by the compiler because byte is a narrower type

(even though 13 is an allowed value for byte)0 There is, however, one exception to this rule: an integer literal (an int

value) can be assigned to a byte or a short variable, but only if the literalfalls inside the range of the byte/short variable. So this line would beallowed:

byte b = 13; //allowed because 13 is an integer literal0 The error produced by the compiler for a narrowing conversion includes

the message 'possible loss of precision.

0 The above are examples of implicit conversion Java carries out theconversion automatically, if it is ok to do so. However, a conversion from

one type to another can be forced using a cast. This can be used when anarrowing conversion would occur, and if the programmer knows thatdata will not be lost.

0 A cast is performed by putting the type to convert to in parenthesesbefore the value to be converted.Chapter 2 Comp321: Object Oriented

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10. Type Conversions and Casting(cont0 Taking the example above, there the narrowing conversion of the

int value to a byte is not allowed by the compiler, this can be forced

using a cast as follows:int i = 13;

byte b = (byte) i; //force the int value 13 to be converted to a byte

0 A castis often used to convert a floating-point value to an integer.When this occurs, the fractional part of the floating-point value istruncated to leave an integer.

0 For example:

int i;

i = (int) 13.456; //forces the double literal to the int value 13

0 Casting can also be used with reference data types, but there aresome restrictions.

0 As with primitive types, the Java interpreter automatically carriesout widening conversions. Narrowing conversions must be made

explicit using a cast.


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0 ype Co ve s o s a d Cast g(co t0 When converting reference data types, the following rules apply:

An object can be converted to the type of its superclass, or any ancestor

class in its class hierarchy. This is a widening conversion.An object can be converted to the type of its own subclass. This is a

narrowing conversion, so it requires a cast.

An object cannot be converted to an unrelated type i.e. to a class that the

object is not a subclass or superclass of.All Java classes automatically inherit from the Object superclass (this

gives them special methods such as toString(), clone() and equals()).

0 For example, the String class is a subclass of Object. So a string value canbe assigned to a variable of type Object. If assigning the value to a String

variable, a cast is required.

Object o = " a string"; //a widening conversion

//later in the code, the value of o can be cast back to a String

String s = (String) o; //a narrowing conversion, so requires a castChapter 2 Comp321: Object OrientedProgramming

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11. Mathematical Functions


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11. Mathematical Functions0 In Java, mathematical functions such as square root and trigonometric

functions are implemented as methods on the Math class (equivalent of

the C functions). This class is part of the built-in java.langpackage because it is a built-in class, it does not have to be imported touse it. The Math class also provides constants for the mathematicalvalues PI and E.

0 For example:

double x,y;

x = 9;

y= Math.sqrt(x); //computes the square root of x

//use PI to compute the area of a circle

double radius;

radius = 6;

double circleArea = Math.PI * radius * radius;

0 Some useful Math methods and constants are listed in the table below.Chapter 2 Comp321: Object OrientedProgramming

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Note: x and y are parameters of type double; a and b can be of type int, long, float or double


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Method Description Method Description

sin (x) Trigonometry Sin Log (x) Returns the natural logarithm of x

cos (x) Trigonometry Cos sqrt (x) Returns the square root of x

tan (x) Trigonometry Tan ceil (x) Returns the smallest whole numbergreater than or equal to x (rounding

up)

asin (x) Trigonometry aSin floor (x) Returns the greatest whole number

less than or equal to x (rounding

down)

acos (x) Trigonometry aCos rint (x) Truncated value of x

atan (x) Trigonometry aTan abs (a) Returns the absolute value of a

pow (x, y) Returns x to the power of y (xy) max (a,b) Returns the maximum value of a

and b

exp (x) Returns e to the power of x (ex) min (a,b) Returns the minimum value of a

and b

Constant Value & Description

PI Double, 3.141592653589793 (access

using Math.PI)

E Double, 2.718281828459045 (access

using Math.E)Chapter 2Comp321: Object Oriented

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Questions?

End of chapter 2


Ch-2 Overview of Java

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