Java : Abstract Class VS Interfaces

Abstract Class VS Interfaces

 

 

Abstract Class VS Interfaces

Inheritance :

class animal{

void eat(){System.out.print("animal eat");}

}

class dog extends animal {

}

class Test {

public static void main(String[] args){

dog d=new dog();

d.eat(); // instance dog can inherit animal property

}

}

Abstract Class :

abstract class animal{

void eat(){System.out.print("animal eat");}

abstract void noise(); // No definition

}

//dog class can inherit all properties except method declared as abstract

//Abstract method needs to be defined explicitly in the subclass ie., dog

class dog extends animal {

void noise(){System.out.print("bark");}

}

class Test {

public static void main(String[] args){

dog d=new dog();

d.eat();

d.noise();

}

}

Ref : http://www.javacoffeebreak.com/faq/faq0084.html

To summarise:

1.Use abstract classes- can inherit all methods except ones declared as abstract.Abstract method should be declared explicitly in the calling method Ex :all animals eat,sleep but make different sound(sound can be made abstract which is explicitly defined in subclass)

2.Use interfaces - here all methods are abstract by default and everything has to be implemented (once implemented gives you a skeleton on things needs to be defined )

Interface:

An interface can contain any number of methods.

An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

All of the methods in an interface are abstract.

An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

An interface is not extended by a class; it is implemented by a class.

/* File name : Animal.java */

interface Animal {

public void eat();

public void travel();

}

/* File name : MammalInt.java */

public class MammalInt implements Animal{

public void eat(){System.out.println("Mammal eats"); }

public void travel(){ System.out.println("Mammal travels");}

public int noOfLegs(){return 0; }

public static void main(String args[]){

MammalInt m = new MammalInt();

m.eat();

m.travel();

}

}

4) NOTE:

Using an abstract class, you can have functions, that dont need to get implemented by the classes inheriting the abstract class. (http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html)

Using an interface, every subclass has to define every method, provided by this interface. (http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html)

Use an abstract class when you want to make one or more methods not abstract.

If you want to keep all abstract, use an interface.

5) Links

http://www.javatpoint.com/abstract-class-in-java

http://www.programmerinterview.com/index.php/java-questions/interface-vs-abstract-class/

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Uses of Interface

One reason I use interfaces is because it increases the flexibility of the code. Let's say we got a method that takes an object of class type Account as parameter, such as:

public void DoSomething(Account account) {

// Do awesome stuff here.

}

The problem with this, is that I have to send only Account object as a parameter.

Take this below example, which instead uses an account of type= Interface as parameter.

public void DoSomething(IAccount account) {

// Do awesome stuff here.

}

This solution is not fixed towards an implementation, which means that I can pass it a SavingsAccount or a JointAccount or CurrentAccount (both who has implemented the IAccount interface ie., ) and get different behavior for each implemented account.

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Uses of Interface

One of the many uses I have read is where its difficult without multiple-inheritance-using-interfaces in Java :

class Animal

{

void walk() { }

....

.... //other methods and finally

void chew() { } //concentrate on this

}

Now, Imagine a case where:

class Reptile extends Animal

{

//reptile specific code here

} //not a problem here

but,

class Bird extends Animal

{

...... //other Bird specific code

} //now Birds cannot chew but Bird classes can also call chew() method which is unwanted

Better design would be:

class Animal

{

void walk() { }

....

.... //other methods

}

Animal does not have the chew() method and instead is put in an interface as :

interface Chewable {

void chew();

}

and have Reptile class implement this and not Birds (since Birds cannot chew) :

class Reptile extends Animal implements Chewable { }

and in case of Birds simply:

class Bird extends Animal { }

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Rules for using Abstract ,Interface ,extending a class

Java interface is like a 100% pure abstract class.You MUST implement all abstract methods

Concrete class

-normal class - when your new class doesn’t pass the IS-A test for any other type.

When To extend a class ?

-when u need to add new behaviors and Passes IS-A.

How to implement Has -A functionality in an object ?

-Instance Variable or class variable

When To Use an abstract class -

A template for a group of subclasses and you have at least some implementation code that all subclasses could use.

when you want to guarantee that nobody can make objects of that type.

Abstract class uses methods as well as abstract methods

Use an interface -

when 100% abstract and static methods is allowed

when you want to define a role that other classes can play

--------------------------------------------------------

interface tree{

production();

}

class mango implements tree{

production(){}

}

class grape implements tree{

production(){}

}

psvm[]{

tree t=new mango();

tree t=new grape();

}

now user can create a new tree type like strawberry , orange etc., which has implement tree and use --

tree t=new orange();

ref:

http://catchbug.blogspot.in/2015/06/jave-excellent-interface-example.html