interface in Java

What is an interface in Java?

Interface looks like a class but it is not a class. An interface can have methods and variables just like the class but the methods declared in interface are by default abstract. Also, the variables declared in an interface are public, static & final by default.

What is the use of interface in Java?

As mentioned above they are used for full abstraction. Since methods in interfaces do not have body, they have to be implemented by the class before you can access them. The class that implements interface must implement all the methods of that interface. Also, java programming language does not allow you to extend more than one class, However you can implement more than one interfaces in your class.

Syntax:
Interfaces are declared by specifying a keyword “interface”. E.g.:

interface MyInterface
{
   /* All the methods are public abstract by default
    * As you see they have no body
    */
   public void method1();
   public void method2();
}

class implements interface but an interface extends another interface.

interface A
{
   public void aaa();
}
interface B
{
   public void aaa();
}
class Central implements A,B
{
   public void aaa()
   {
        //Any Code here
   }
   public static void main(String args[])
   {
        //Statements
    }
}

 

Interface and Inheritance

interface Inf1{
   public void method1();
}
interface Inf2 extends Inf1 {
   public void method2();
}
public class Demo implements Inf2{
   /* Even though this class is only implementing the
    * interface Inf2, it has to implement all the methods 
    * of Inf1 as well because the interface Inf2 extends Inf1
    */
    public void method1(){
 System.out.println("method1");
    }
    public void method2(){
 System.out.println("method2");
    }
    public static void main(String args[]){
 Inf2 obj = new Demo();
 obj.method2();
    }
}