Why can't outer classes extend inner classes?

Question

Why can\'t I do this/is there a workaround to accomplish this:

package myPackage;

public class A {
    public class B {

    }
}

---

Answer 1

You can easily extend nested static classes

Update: You've mentioned that you don't want this first solution, but the phrasing of the question may lead people to it who are willing to have the inner class be static, so I'll leave this in the hopes that it's useful to them. A more proper answer to your exact question is in the second section of this answer.

You can, but the inner class has to be static, because if it's not, then every instance of the inner class has a reference to the enclosing instance of the outer class. A static nested class doesn't have that reference, and you can extend it freely.

public class Outer {
    public static class Inner {

    }
}

public class InnerExtension extends Outer.Inner {

}

---

But you can also extend nested non-static classes

package test;

public class Outer {
    public class Inner {
        public String getFoo() {
            return "original foo";
        }
    }
}

package test;

public class Extender {
    public static void main(String[] args) {
        // An instance of outer to work with
        Outer outer = new Outer();

        // An instance of Outer.Inner 
        Outer.Inner inner = outer.new Inner();

        // An instance of an anonymous *subclass* of Outer.Inner
        Outer.Inner innerExt = outer.new Inner() {
            @Override
            public String getFoo() {
                return "subclass foo";
            }
        };

        System.out.println("inner's class: "+inner.getClass());
        System.out.println("inner's foo: "+inner.getFoo());
        System.out.println();
        System.out.println("innerExt's class: "+innerExt.getClass());
        System.out.println("innerExt's foo: "+innerExt.getFoo());
    }
}

inner's class: class test.Outer$Inner
inner's foo: original foo

innerExt's class: class test.Extender$1
innerExt's foo: subclass foo