An abstract class in Java need not implement any methods from its implementing interface. Why?

Question

Let\'s look at the following simple code snippet in Java.

interface Sum
{
    abstract public void showSum();
}

interface Mul
{
    abstract public void showMul         


        

Answer 1

Because it's abstract. An abstract class is a class which is allowed to declare some methods without providing any implementation of those methods, forcing concrete subclasses doing it. You could add the method

@Override
public abstract void showSum();

to the abstract class, but it would just be redundant with the method already declared in the interface.