Question

Let’s consider the following simple code in Java.

package temppkg;

interface Interface
{
    @Override
    public abstract boolean equals(java.lang.Object arg);

    @Override
    public abstract String toString();

    public void show();
}

final class Demo implements Interface
{
    public void show()
    {
        System.out.println("Method invoked.");
    }
}

final public class Main
{
    public static void main(String...args)
    {
        new Demo().show();
    }
}

In the above code snippet, the interface named Interface has some Object class methods from JDK and they are with the @Override annotation even though they are abstract. Now, the class Demo has implemented Interface and has not implemented the equals() and the toString(); methods. Still the compiler doesn’t complain and the program is running successfully. Why?
What is the relation between interfaces and the object class in Java?

Answer 1

The Java Language Specification clearly says that the members of an
interface are those which are declared in the interface and those
which are inherited from direct super interfaces. If an interface has
no direct superinterface then the interface implicitly declares a
public abstract member method corresponding to each public instance
method declared in the Object class, unless a method with the same
signature, same return type, and a compatible throws clause is
explicitly declared by that interface. This is what makes the
signatures of the Object methods available to the compiler and the
code compiles without any error. Remember if the interface tries to
declare a public instance method declared ‘final’ in the Object class
then it’ll result into a compile-time error. For example, ‘public
final Class getClass()’ is a public instance method declared ‘final’
in the Object class and therefore if an interface tries to declare a
method with this signature then the compilation will fail.

http://geekexplains.blogspot.com/2008/06/do-interfaces-really-inherit-from-class.html