• An interface in java is a blueprint of a class. It has static constants and abstract methods only.

• The interface in java is a mechanism to achieve fully abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve fully abstraction and multiple inheritance in Java.

• Java Interface also represents IS-A relationship.

• It cannot be instantiated just like abstract class.

Learn about Java in detail

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

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

            interface printable{
    void print();
}

class A6 implements printable{
    public void print() {
        System.out.println("Hello JBK");
    }

    public static void main(String args[]){
        A6 obj = new A6();
        obj.print();
    }
}

    Output:
     Hello JBK
    

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

            interface Printable{
    void print();
}

interface Showable{
    void show();
}

class A7 implements Printable, Showable{
    public void print()
    {
      System.out.println("Hello");
    }
    public void show()
    {
      System.out.println("Welcome JBK");
    }
    public static void main(String args[])
    {
      A7 obj = new A7();
      obj.print();
      obj.show();
    }
}

    Output:
     Hello
     Welcome JBK
    

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.

Example :

            interface Printable {
    void print();
}

interface Showable{
    void print();
}

class TestTnterface1 implements Printable, Showable {
    public void print()
    {
        System.out.println("Hello JBK");
    }
    public static void main(String args[])
    {
      TestTnterface1 obj = new TestTnterface1 ();
      obj.print();
    }
}

    Output:
     Hello JBK
    

As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class TestTnterface1, so there is no ambiguity.

A class implements interface but one interface extends another interface.

            interface Printable {
    void print();
}

interface Showable extends Printable {
    void show();
}

class Testinterface2 implements Showable {
    public void print()
    {
        System.out.println("Hello");
    }
    public void show()
    {
        System.out.println("Welcome JBK");
    }
    public static void main(String args[])
    {
        Testinterface2 obj = new Testinterface2 ();
        obj.print();
        obj.show();
    }
}

    Output:
     Hello
     Welcome JBK
    

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.

            // How Serializable interface is written?

public interface Serializable {

} 

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

Example :

            interface printable {
    void print();
    interface MessagePrintable {
        void msg();
    }
}

Learn more about Interface