Question

How to initialize a class inheriting from an inner class? Why foo1.this() won’t compile?

class Foo1 {
    class Inner {
    }
}

class Foo2 extends Foo1.Inner {
    //Foo2(Foo1 foo1) {foo1.this();} //won't compile
    Foo2(Foo1 foo1) {foo1.super();}
}

Answer 1

I’m not sure I understand what foo1.this(); is supposed to do. Perhaps you are trying to call the default constructor of Inner. Since Inner is a not a static class, it does not have a no argument constructor. It only looks that way. In reality, it has a constructor which takes a Foo1 argument containing the reference to its parent object.

The call to foo1.super(), however, will work just fine. It will call the constructor of the new Inner instance with foo1 as implicit parent reference. That is, the new Inner instance will be tied to the given Foo1 instance.

As others have pointed out, you can of cause make your Inner class static, in which case it does not contain an implicit reference to a Foo1 instance. Then you can simply call super(); in your Foo2 constructor.