java - 为什么这个枚举代码是对静态字段的非法引用？

Question

此代码无法编译，因为存在对静态字段的非法引用。

public enum Foo {

  A,
  B;

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

}

你不应该能够从非静态字段初始化器访问静态字段吗？例如:

public class Foo {

  static int A;

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

}

这编译得很好。

请注意，在第一个示例中将 foos 设置为静态编译。

Answer 1

查看 Java 语言规范，第三版，第 8.9 节 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.

讨论

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.