Java Classes methods and inheritance

Classes, Methods and Inheritance

SCJP / OCJP objectives : 1.1, 1.2, 1.3, 1.4, 5.5

By,Srinivas Reddy.Swww.JAVA9S.comwww.JAVA9S.com

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Declaring ClassesSyntax:package com.java9s.ocjp;import com.sun.sample;class Car{ int speed; State void move(){ behaviour //code related to move }}Save the file with Car.java.If there are multiple classes in a file, the file name should

be the name of the class with public access modifier.

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Declaring classes - Rules• The package statement should be the first

statement in a file if the class belongs to a package.

• Import statements comes next to package statement.

• The order packageimportclass should be maintained.

• Comments can come anywhere in the java file.• A java file can have any number of non public

class files


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Declaring classes - Rules

• package and import statements declared in a file apply to all the classes defined in the file.

• A separate .class file will be generated for each class defined in the java file.

• Any name can be given to a file when there is no public class declared in it.


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Creating Objects

Car c = new Car();

DeclarationInstantiation

C is the reference which holds the memory address of the Car object

C

CarSpeed=50


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Creating Objects

Car a = new Car(); Car b = new Car();Car c = new Car();Car d = new Car();

Car e = d;

abcd

e

d.speed =60;System.out.println(e.speed) ;-> 60


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Methods

• Methods are members of a class.• Methods have the behavior of an object.• Methods can be declared with or without

arguments.• Two variants for a method:– Methods that return something– Methods that don’t return anything - void


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Methods – Return typeSyntax:type methodName(arguments){//code that decides the methodsreturn x;}E.g., int addition(int a, int b){

int c = a+b;return c;}


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Methods – void type

Syntax:void methodName(arguments){//Method code.. No need to return anything.}E.g., void saveToFile(String message){//Code related to saving message to file..}


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Method – without argument

Method with No arguments and with a return type:Date getCurrentDate(){return Calender.get(Calender.DAY_OF_MONTH);}

Method with no argument and no return typevoid printCurrentDate(){System.out.println(Calendar.get(Calender.DAY_OF_MONTH));}


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Inheritance

• Inheritance is a way to reuse code from already existing types or objects.

• Inheritance is implemented between two classes using extends keyword.

• When a class extends another class, extending class is called subclass and extended class is super class.


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Inheritance

class Car{ void move(){

System.out.println(“Moves”);}

}class Ford extends Car{ void moveFast(){ System.out.println(“Moves Fast”);

}}

Ford f = new Ford();f.moveFast();f.move();

•Car is super class•Ford is subclass.


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Inheritance

• With inheritance, all the members of super class are available to the subclass objects.

• When a class extends another class, it has an IS-A relation with its super class.

• instanceof keyword can be used to confirm IS-A relationship.


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Right or Wrong??

class Car{ }class Ford extends Car{ }class BMW extends Car{ }

Ford f = new Ford();BMW b = new BMW();Car c = new Car();

f instanceOf Carf instanceOf BMW b instanceOf Fordb instanceOf Carc instanceOf Ford


super and this keywords

• super is used to access the super class members.

• this is used to access the currently executing objects members.


super - example

class Car{ int speed;}class Ford extends Car{

int speed; void move(){

System.out.println(“Moving with car speed:”+super.speed)}

}


this - exampleclass Ford{

int price; setFordPrice(int price){

this.price = price;}

}Ford a = new Ford();a.setFordPrice(3000);‘this’ is not mandatory. But if you have local variables

declared inside a method, to avoid confusion, this can be used to refer to members of the object.


int price;setFordPrice(int price){this.price = price;}

a

HAS-A relationship

class Student{Pen p = new Pen();

}class Pen{}Student HAS-A pen.

HAS – A relationship helps to reducethe complexity of the classes by the composition of the other classes.


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Java Classes methods and inheritance

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