Java Classes methods and inheritance
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Transcript of 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
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
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super and this keywords
• super is used to access the super class members.
• this is used to access the currently executing objects members.
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super - example
class Car{ int speed;}class Ford extends Car{
int speed; void move(){
System.out.println(“Moving with car speed:”+super.speed)}
}
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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.
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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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