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++.

score 12 · Accepted Answer · edited May 23 '17 at 10:27

12

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.

edited May 23 '17 at 10:27

Community

1
1

answered Nov 04 '09 at 06:37

Greg Hewgill

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8

An enum is an intrinsically static beast. In the example above, you only ever want there to be one Enum.AD object. But if you declared an enum inside a non-static class, you'd (in theory) need a different instance of Enum.AD for each synamic scope. That doesn't make sense ... therefore it is forbidden. – Stephen C Nov 04 '09 at 06:59

Java: how come this does not compile?

1 Answers1