What are final inner classes?

Question

What does it means to declare a non-static inner class as final ?

I have tried many links on google and stackoverflow.com as well but all of them seem to be dealing about inner classes accessing final members not final inner classes itself. I found this link on google but even it doesn't explains it.

Thanx in advance!

Answer 1

There is no semantic difference between making a top-level class final and making an inner class final : it tells the compiler that you cannot inherit from the class. Marking classes final is sometimes done to let the compiler skip a virtual table lookup, but this is often regarded as premature micro-optimization.

Answer 2

It has the same semantics as an outer class being declared final: the class cannot be extended.

Consider this example:

public class MyClass {
  public class A {
  }
  public class B extends A {
  }
}

If you add the final modifier to A it will generate a compilation error.

Answer 3

Well, inner classes are not any way different from outer classes in that context. So the following code is perfectly valid.

class Outer {
 int some_member;

 class Inner {
 void method();
 }
}

class OuterExtendsInner extends Outer.Inner{

}

As we all know, the purpose of declaring a class final is that we prevent any outside intruder from subclassing the class and exploit its facilities, the same way we can do in case of inner classes.