Java Classes methods and inheritance

Classes, Methods and
Inheritance
SCJP / OCJP objectives : 1.1, 1.2, 1.3,
1.4, 5.5

By,
www.JAVA9S.com Srinivas Reddy.S

Declaring Classes
Syntax:
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 packageimportclass 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();

Instantiation
Declaration

C is the reference which holds the Car
Speed=50
memory address of the Car object
C
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Creating Objects
Car a = new Car(); a
Car b = new Car(); b
Car c = new Car(); c
Car d = new Car(); d

Car e = d; 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 type
Syntax:
type methodName(arguments){
//code that decides the methods
return 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 type
void 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(){ •Car is super class
System.out.println(“Moves”); •Ford is subclass.
}
}
class Ford extends Car{
Ford f = new Ford();
f.moveFast();
void moveFast(){
f.move();
System.out.println(“Moves Fast”);
}
}

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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{ }

f instanceOf Car
Ford f = new Ford(); f instanceOf BMW
BMW b = new BMW();
Car c = new Car();
b instanceOf Ford
b instanceOf Car
c 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 - example
class Ford{
int price; int price;
setFordPrice(int price){
setFordPrice(int price){ this.price = price;
this.price = price; }

}
} a
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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HAS-A relationship
class Student{
Pen p = new Pen();
}
class Pen{
}
Student HAS-A pen.
HAS – A relationship helps to reduce
the complexity of the classes by the
composition of the other classes.
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Java Classes methods and inheritance

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