Same method in Interface and Abstract class

Question

I came to situation :

public interface Intr {
    public void m1();
}

public abstract class Abs {
    public void m1() {
        System.out.println("Abs.m1()");
    }
    // public abstract void m1();
}

public class A extends Abs implements Intr {

    @Override
    public void m1() {
        // which method am I overriding, well it is Abs.m1() but why?
        // if method implemented is Abs.m1(), then why I am not getting error for Intr.m1() not implemented.
    }

}

Answer 1

You are satisfying both conditions at once; ie. the one implementation is at the same time fulfilling the abstract class requirements and the interface requirements.

As a note, unless you are using Intr in another inheritance chain, you don't need it. Also, it might make sense to move the implements Intr up to the abstract class definition.

Answer 2

You can only override methods defined in another class.

Methods declared in an interface are merely implemented. This distinction exists in Java to tackle the problem of multiple inheritance. A class can only extend one parent class, therefore any calls to super will be resolved without ambiguity. Classes however can implement several interfaces, which can all declare the same method. It's best to think of interfaces as a list of "must have"s: to qualify as a Comparable your cluss must have a compareTo() method but it doesn't matter where it came from or what other interfaces require that same method.

So technically you override Abs.m1() and implement Intr.m1() in one fell swoop.

Note that this would be fine too:

public class B extends Abs implements Intr {

    //m1() is inherited from Abs, so there's no need to override it to satisfy the interface
}