Why is Java prohibiting inheritance of inner interfaces?

Question

I.e. why is the following "cyclic dependency" not possible?

public class Something implements Behavior {
    public interface Behavior {
        // ...
    }
}

Since interfaces don't reference the outer class this should be allowed; however, the compiler is forcing me to define those interfaces outside the class. Is there any logical explanation for this behavior?

Answer 1

Relevant rules in spec:

http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.4

A class C directly depends on a type T if T is mentioned in the extends or implements clause of C either as a superclass or superinterface, or as a qualifier of a superclass or superinterface name.

http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html#9.1.3

An interface I directly depends on a type T if T is mentioned in the extends clause of I either as a superinterface or as a qualifier within a superinterface name.

Therefore if A extends|implements B.C, A depends on both C and B. Spec then forbids circular dependencies.

The motivation of including B in the dependency is unclear. As you mentioned, if B.C is promoted to top level C2, not much is different as far as the type system is concerned, so why A extends C2 is ok, but not A extends B.C? Granted a nested type B.C does have some prviledged access to B's content, but I can't find anything in spec that makes A extends B.C troublesome.

The only problem is when C is an inner class. Suppose B=A, A extends A.C should be forbidden, because there's a circular dependency of "enclosing instance". That is probably the real motivation - to forbid outer class from inheriting inner class. The actual rules are more generalized, because they are simpler, and make good sense anyway even for non-inner classes.

Answer 2

Imagine you are the compiler.

We are saying you to create a class Something. This class implements Behavior... But Behavior does not exist yet because Something is not already registered...

Do you understand the problem ?

See class as box which contains things. Behavior is contained in the box Something. But Something does not exist.

Answer 3

The simple fact that the language specs forbid it should be enough.

Some reasons I could think of:

• It wouldn't be useful.

• For whatever reasons you might want to use this, I'm sure there exist better options.

• Child classes should extend base classes, so why would you declare a base class inside its own child?

• It would be counter-intuitive having a separate class extend your inner-class.