Question

This code won’t compile because there is an illegal reference to a static field.

public enum Foo {

  A,
  B;

  private Foo[] foos = new Foo[] { Foo.A };

}

Shouldn’t you be able to access static fields from a non-static field initializer? For example:

public class Foo {

  static int A;

  private int[] foos = new int[] { Foo.A };

}

This compiles fine.

Note, making foos static in the first example compiles.

Answer 1

Check out Java Language Specification, Third Edition, Section 8.9 at http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.9

It is a compile-time error to reference a static field of an enum type
that is not a compile-time constant (§15.28) from constructors,
instance initializer blocks, or instance variable initializer
expressions of that type. It is a compile-time error for the
constructors, instance initializer blocks, or instance variable
initializer expressions of an enum constant e to refer to itself or to
an enum constant of the same type that is declared to the right of e.

Discussion

Without this rule, apparently reasonable code would fail at run time
due to the initialization circularity inherent in enum types. (A
circularity exists in any class with a “self-typed” static field.)
Here is an example of the sort of code that would fail:

enum Color {
        RED, GREEN, BLUE;
        static final Map<String,Color> colorMap = 
        new HashMap<String,Color>();
        Color() {
            colorMap.put(toString(), this);
        }
    } 

Static initialization of this enum type would throw a
NullPointerException because the static variable colorMap is
uninitialized when the constructors for the enum constants run. The
restriction above ensures that such code won’t compile.