Interface in Java

An interface is a blueprint of a class. It has static constants and abstract methods.

The interface is a mechanism to achieve fully abstraction in java. There can be only abstract methods in the interface. It is used to achieve fully abstraction and multiple inheritance in Java.

Interface also represents IS-A relationship.

It cannot be instantiated just like abstract class.

Why use Interface?

There are mainly three reasons to use interface. They are given below.

• It is used to achieve fully abstraction.
• By interface, we can support the functionality of multiple inheritance.
• It can be used to achieve loose coupling.

The java compiler adds public and abstract keywords before the interface method and public, static and final keywords before data members.

In other words, Interface fields are public, static and final bydefault, and methods are public and abstract.

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Understanding relationship between classes and interfaces

As shown in the figure given below, a class extends another class, an interface extends another interface but a class implements an interface.

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Simple example of Interface

In this example, Printable interface have only one method, its implementation is provided in the A class.

1. interface printable{  
2. void print();  
3. }  
4.   
5. class A implements printable{  
6. public void print(){System.out.println("Hello");}  
7.   
8. public static void main(String args[]){  
9. A obj = new A();  
10. obj.print();  
11.  }  
12. }  

Output:Hello

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Multiple inheritance in Java by interface

If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple inheritance.

1. interface Printable{  
2. void print();  
3. }  
4.   
5. interface Showable{  
6. void show();  
7. }  
8.   
9. class A implements Printable,Showable{  
10.   
11. public void print(){System.out.println("Hello");}  
12. public void show(){System.out.println("Welcome");}  
13.   
14. public static void main(String args[]){  
15. A obj = new A();  
16. obj.print();  
17. obj.show();  
18.  }  
19. }  

Output:Hello
       Welcome

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Q) Multiple inheritance is not supported in case of class but it is supported in case of interface, why?

As we have explained in the inheritance chapter, multiple inheritance is not supported in case of class. But it is supported in case of interface because there is no ambiguity as implementation is provided by the implementation class. For example:

1. interface Printable{  
2. void print();  
3. }  
4.   
5. interface Showable{  
6. void print();  
7. }  
8.   
9. class A implements Printable,Showable{  
10.   
11. public void print(){System.out.println("Hello");}  
12.   
13. public static void main(String args[]){  
14. A obj = new A();  
15. obj.print();  
16.  }  
17. }  

Output:Hello

As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class A, so there is no ambiguity.

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Note: A class implements interface but One interface extends another interface .

1. interface Printable{  
2. void print();  
3. }  
4.   
5. interface Showable extends Printable{  
6. void show();  
7. }  
8.   
9. class A implements Showable{  
10.   
11. public void print(){System.out.println("Hello");}  
12. public void show(){System.out.println("Welcome");}  
13.   
14. public static void main(String args[]){  
15. A obj = new A();  
16. obj.print();  
17. obj.show();  
18.  }  
19. }  

Output:Hello
       Welcome

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Que) What is marker or tagged interface ?

An interface that have no member is known as marker or tagged interface. For example: Serializable, Cloneable, Remote etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.

1. //How Serializable interface is written?  
2.   
3. public interface Serializable{  
4. }  

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Nested Interface

Note: An interface can have another interface i.e. known as nested interface. We will learn it in detail in the nested classes chapter. For example:

1. interface printable{  
2.  void print();  
3.  interface MessagePrintable{  
4.    void msg();  
5.  }  
6. }  

More about Nested Interface