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In Java, you can extend from another interface using the extends keyword. But Java 8 has default methods. What happens if we extend another interface that has the same default methods as our interface?

It gets re-declared in the "child" interface.

Re-declare with different Implementation

Consider these codes. We have two (2) interfaces and one of them extends the other. The ParentInterface has a default method that returns the String "Parent interface". The other interface, ChildInterface, extends from the ParentInterface.

package com.turreta.interface1;

public class DefaultMethodsDemo1 {

    public static void main(String[] args) {
  
        // Using anonymous class
        ChildInterface ci = new ChildInterface() {
        };

        System.out.println(ci.getId());
    }
}

interface ParentInterface {

    default String getId() {
        return "Parent interface";
    }
}

interface ChildInterface extends ParentInterface {
    default String getId() {
        return "Child interface";
    }
}

package com.turreta.interface1;

public class DefaultMethodsDemo1 {

public static void main(String[] args) {

// Using anonymous class

ChildInterface ci = new ChildInterface() {

};

System.out.println(ci.getId());

}

interface ParentInterface {

default String getId() {

return "Parent interface";

}

interface ChildInterface extends ParentInterface {

default String getId() {

return "Child interface";

}

Output

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Child interface

1	Child interface

Re-declare as an abstract method

If we do not wish to use any default implementation, we can re-declare the default method as an abstract method. It is now up to the users of the interface to provide their own implementations of the abstract method.

public class DefaultMethodsDemo2 {

    public static void main(String[] args) {
        ChildInterface ci = new ChildInterface() {

            @Override
            public String getId() {
                return "Programmer's own implementation";
            }
        };

        System.out.println(ci.getId());
    }
}

interface ParentInterface {

    default String getId() {
        return "Parent interface";
    }
}

interface ChildInterface extends ParentInterface {
    String getId();
}