implicit super-interface in Java?

Question

So we all know that all classes implicitly extend Object. How about interfaces? Is there an implicit super-interface? I say there is. The following code compiles:

java.io.Serializable s1 = null;
java.io.Serializable s2 = null;

s1.equals(s2);

The equals method is not declared in Serializable, but in Object. Since interfaces can only extend other interfaces, and Object is a class, not an interface, there must be some implicit interface that is being extended. And the Object class must then implicitly implement this implicit interface (wow, that was weird to write).

So, the question is, how correct is this?

Answer 1

Since interfaces can only extend other interfaces, and Object is a class, not an interface, there must be some implicit interface that is being extended.

No. Citing the Java Language Specification:

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 a method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface. It is a compile-time error if the interface explicitly declares such a method m in the case where m is declared to be final in Object.

The difference between this and your "implicit super interface" is that Object has a number of final and protected methods, and you couldn't have those modifiers in an interface.