java Lecture3-Classes and Methods

8/2/2019 java Lecture3-Classes and Methods

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Classes and Methods

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Classes

data field 1

method n

data field n

method 1

An object

...

...

State

Behavior

Data Field

radius = 5

Method

findArea

A Circle object



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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

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ExampleCircle

radius

findArea

circle1: Circle

radius = 2

new Circle()

circlen: Circle

radius = 5

new Circle()

...

Graphical

notation for

classes

Graphical

notation for

ob ects

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Fields in Class Definition Class variables

Instance variables



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Fields in Class Definition Class Variable

Only one copy is shared with all the objects ofthe 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.

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Instance variables Variables associated with each object

A separate value for each instance of aclass

For example, name , address etc.

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ExampleClass Sphere Definition

public class Sphere {// class variable

static double PI=3.14;

// instance variables

double xCenter;

double yCenter;

double zCenter;double radius;

}

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Methods in Class Definition Instance Methods

Class Methods


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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

Builtin methods in standard class Math

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Instance Methods Can execute when objects exist.

Example: calculate area()


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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();

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Defining Classesclass Sphere {

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

// Instance variables

double radius; // Radius of a spheredouble xCenter; // 3D coordinates

double yCenter; // of the center

double zCenter; // of a sphere// Plus the rest of the class definition...

}

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Defining Methods Self contained block of codes

Reusable

Can be executed from anywhere in the

program Calling a method

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Basic Structure of a method

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

{ code here}

Return Statement

return return_value; // To return a valuefrom a method

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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

method

heading





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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 whenexecuted

Must be consistent with the type specified inparameter definition





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Examplepublic 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;}

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Passed by Valuedouble 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;}

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Class Method Definitionclass Sphere {

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

objects created

static int getCount() {

return count; // Return current object count

}}

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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...}

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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);

}

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The Variable this Every instance method has a variable with

the name this that refers to the currentobject for which the method is being called

void changeRadius(double radius) {

// Change the instance variable to the

argument value

this.radius = radius;}

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Initializing data members: The

ordinary way

class Sphere {static final double PI = 3.14; // Class variable that hasa fixed value

static int count = 0; // Class variable to count objects// Instance variablesdouble radius = 5.0; // Radius of a spheredouble xCenter = 10.0; // 3D coordinates

double yCenter = 10.0; // of the centerdouble zCenter = 10.0; // of a sphere// Rest of the class...

}





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Using Initialization Blocks There are two kinds of initialization blocks:

static initialization blocknon-static initialization block

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static initialization block A static initialization block is a block defined

using the keyword static and is executed oncewhen the class is loaded.

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

A static initialization blockis a normal block ofcode enclosed in braces, { }, and preceded bythe static keyword.

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


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static

initializationblock

class TryInitialization {

static int[] values = new int[10];

static {

System.out.println("Running initializationblock.");

for(int i=0; i


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Non-static initialization blocknon-static initialization block is executed for

each object that is created and thus caninitialize 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

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LAB PRACTICE Compile & execute

TryInitialization class with staticinitialization and non-static

initialization block. Examine its output.

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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 andcall functions of the class in the following sequence.1. Change speed

2. Change gear

3. Print states

4. Apply brakes

5. Print states

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Solution

class Bicycle{

int speed = 0;int gear = 1;void changeGear(int newValue){ gear = newValue;

}void speedUp(int increment){ speed = speed + increment;}void applyBrakes(){ speed = 0;}

void printStates(){ System.out.println(" speed:"+speed+" gear:"+gear);}}

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Solution

class BicycleDemo{

public static void main(String[] args){ // Create two different Bicycle objectsBicycle bike1 = new Bicycle();Bicycle bike2 = new Bicycle(); // Invoke methods on those objects

bike1.speedUp(10);bike1.changeGear(2);bike1.printStates();bike1.applyBrakes();bike1.printStates();

bike2.speedUp(10);bike2.changeGear(2);bike2.speedUp(10);bike2.changeGear(3);

bike2.printStates();}}

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Solution

The output of this test prints speed,and gear for the two bicycles:

speed:10 gear:2

speed:20 gear:3

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LAB PRACTICE1. Create a class Box that uses a parameterized

method to initialize the dimensions of abox.(dimensions are width, height, depth ofdouble type).

2. The class should have a method that canreturn volume.

3. Obtain an object and print the corresponding

volume in main() function.

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Solution

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Solution

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Output

Volume is 3000.0Volume is 162.0

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java Lecture3-Classes and Methods