Why would we need to allow interfaces to extend only from java.lang.Object and not any other class? [duplicate]

Question

As per my understanding from some books on Java, interfaces cannot extend classes.

But all interfaces do inherit methods from Object class. Why is this so?

If Interface not extend from Object class. So how this code work it?

interface A
{    
 public boolean equals(Object o);

}

class InterfaceAndObjectClass
{

    public static void main(String[] args)
    {
        A a = null;

        a.equals(null);

        a.hashCode();

        a.toString();
    }
}

Please help me to explain how access the Object class method in Interface.

Answer 1

An interface implicitly contains all the methods of Object class. And since any class that implements that interface is a sub-class of Object, it contains the implementation of all those methods.

JLS 9.2

If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless an abstract method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface.

Answer 2

Eran is absolutely right. Actually Interface does not extend or implement Object class. It Implicitly contains all the methods of Object class. Note(further):

If you declare same (abstract) method(with same name-signature, return type, and throws type, compiler will consider it as overriding(its ok) but if you declared with different return type or different throws type compiler will will force you to keep same(compatible) return type or throws clause.

try to declare below equals method in your interface. public int equals(Object obj); //it will show compile time error return type is //not compatible. (it should be boolean not int)