Classes and Methods

Classes

data field 1

method n

data field n

method 1

An object

...

...

State

Behavior

Data Fieldradius = 5

MethodfindArea

A Circle object

Class Definition

• Data Fields– Variables to store data items– Differentiate multiple objects of a class– They are called data members

• Methods– Operations of a class– They operate on variables of the class

Example

Circle

radius

findArea

circle1: Circle

radius = 2

new Circle()

circlen: Circle

radius = 5

new Circle()

...

Graphicalnotation forclasses

Graphicalnotation forobjects

Fields in Class Definition

• Class variables

• Instance variables

Fields in Class Definition

• Class Variable– Only one copy is shared with all the objects of

the class– Shared among all objects of a class– Exists even if no object has been created– Also called as static fields– Declared using the keyword static– If value is changed, it is reflected for all

objects.

Instance variables

• Variables associated with each object

• A separate value for each instance of a class

• For example, name , address etc.

Example

Class Sphere Definitionpublic class Sphere {// class variablestatic double PI=3.14;

// instance variablesdouble xCenter;double yCenter;double zCenter;double radius;}

Example

Methods in Class Definition

• Instance Methods

• Class Methods

Class Methods

• Execute class methods even when no objects of a class exist

• Declared using the keyword static.

• Static methods

• Can not refer to an instance variable

• Example: main method

• Buildin methods in standard class Math

Instance Methods

• Can execute when objects exist.

• Example: calculate area()

Accessing Variables and Methods

• Accessing Class Methods– double rootPi = Math.sqrt(Math.PI);– Objects can also access in the same way

• Instance variables and methods– double ballVolume = ball.volume();

Defining Classes

class Sphere {static final double PI = 3.14; //class variable with fixed valuestatic int count = 0; // Class variable to count objects

// Instance variablesdouble radius; // Radius of a spheredouble xCenter; // 3D coordinatesdouble yCenter; // of the centerdouble zCenter; // of a sphere// Plus the rest of the class definition...}

Defining Methods

• Self contained block of codes

• Reusable

• Can be executed from anywhere in the program

• Calling a method

Basic Structure of a method

return_type methodName( arg1, arg2, ..., argn )

{ code here

}

• Return Statement

return return_value; // To return a value from a method

Example

public static int max(int num1, int num2){

int result = 0;

if (num1 > num2) result = num1;else result = num2;

return result;}

modifier

returnValueType

methodName

parameters

return value

methodbody

methodheading

Parameters and Arguments

• Parameter– has a name and a type– Appear in definition of the method– Defines type of value to be passed to method when

called

• Arguments– Actual value that is passed to a method when

executed– Must be consistent with the type specified in

parameter definition

Example

public static void main(String[] args){

...

x = obj.mean( 3.0 , 5.0 );

}

double mean( double value1 , double value2 ){

double result = ( value1 + value2 )/ 2.0;

return result;

}

Example

Passed by Value

double d = 2.0;changeMe(d);System.out.println(d);

public void changeMe(double d){     //this has no effect on d outside of this method!     d = 345.0;}

Passed by Value

Class Method Definition

class Sphere {// Class definition as before...// Static method to report the number of

objects createdstatic int getCount() {return count; // Return current object count}}

Example Classclass Sphere {static final double PI = 3.14; // Class variable that has a fixed valuestatic int count = 0; // Class variable to count objects// Instance variablesdouble radius;double xCenter; // 3D coordinatesdouble yCenter; // of the centerdouble zCenter; // of a sphere// Static method to report the number of objects createdstatic int getCount(){return count; // Return current object count}// Instance method to calculate volumedouble volume() {return 4.0/3.0*PI*radius*radius*radius;}// Plus the rest of the class definition...}

class Sphere {static final double PI = 3.14; static int count = 0;

// Instance variablesdouble radius = 5.0; double xCenter = 10.0; double yCenter = 10.0; double zCenter = 10.0;

// Static method to report the number of objects createdstatic int getCount(){

return count; // Return current object count

}

// Instance method to calculate volume

double volume() {return 4.0/3.0*PI*radius*radius*radius;

}// Plus the rest of the class definition...

public static void main (String[] args){int count; double volume; count=Sphere.getCount();System.out.println(count);Sphere s1=new Sphere();volume=s1.volume();System.out.println(volume);

}

}

The Variable this

• Every instance method has a variable with the name this that refers to the current object for which the method is being called

void changeRadius(double radius) {// Change the instance variable to the argument valuethis.radius = radius;

}

Initializing data members: The ordinary way

class Sphere {static final double PI = 3.14; // Class variable that has a fixed valuestatic int count = 0; // Class variable to count objects// Instance variablesdouble radius = 5.0; // Radius of a spheredouble xCenter = 10.0; // 3D coordinatesdouble yCenter = 10.0; // of the centerdouble zCenter = 10.0; // of a sphere// Rest of the class...

}

Using Initialization Blocks

• There are two kinds of initialization blocks:– static initialization block – non-static initialization block

static initialization block

• A static initialization block is a block defined using the keyword static and is executed once when the class is loaded.

• A static initialization block can initialize only static data members of the class.

• A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword.

• Here is an example: static { // whatever code is needed for initialization goes here }

static initialization

block

class TryInitialization { static int[] values = new int[10]; static {System.out.println("Running initialization

block.");for(int i=0; i<values.length; i++) {values[i] = (int)(100.0*Math.random());}}// List values in the array for an objectvoid listValues() {System.out.println(); // Start a new linefor(int value : values) {System.out.print(" " + value); // Display values}System.out.println(); // Start a new line}public static void main(String[] args) {TryInitialization example = new

TryInitialization();System.out.println("\nFirst object:");example.listValues();example = new TryInitialization();System.out.println("\nSecond object:");example.listValues();}}

Running initialization block.First object:40 97 88 63 58 48 84 5 32 67Second object:40 97 88 63 58 48 84 5 32 67

Non-static initialization block

– non-static initialization block is executed for each object that is created and thus can initialize instance variables in a class.

– This block appears without the static keyword.

{ // whatever code is needed for initialization goes here } Running initialization block.

First object:66 17 98 59 99 18 40 96 40 21Running initialization block.Second object:57 86 79 31 75 99 51 5 31 44

LAB PRACTICE

• Compile & execute “TryInitialization” class with static initialization and non-static initialization block.

• Examine its output.

LAB PRACTICE

• Make a class Bicycle, identify its data members for example “speed=0”, “gear=1” etc.

• Add methods like changeGear(int), speedUp(int) , applyBrakes(),

printStates() etc. Make a main method to create 2 bicycle objects and call functions of the class in the following sequence.

1. Change speed2. Change gear3. Print states4. Apply brakes5. Print states