Java: how come this does not compile?

Question

How come this code doesnt compile?

class A
{
  class B
  {
    public enum Enum   <-- this line
    {
      AD,
      BC
    }
  }
}

Compiler reports:

enum declarations allowed only in static contexts.

But then when I put the Enum inside class A, everything is okay.

This is quite surprising. I dont think I have this problem in C++.

Answer 1

You can fix this by making B static:

static class B { ...

This mirrors more closely what C++ does with nested classes. By default (without static), instances of B contain a hidden reference to an instance of A.

A good explanation of the differences can be found at Java inner class and static nested class.