Java编译器错误解惑：“内部类不能有静态声明” - 除了简单类型

Question

While coding, I have encountered a strange Java Compiler behaviour.

When compiling the class (source below), the compiler emits an error (" inner classes cannot have static declarations ") on the NULL class variable. This is as expected!

However, no error is generated on the ZERO class variable. This I don't understand!

Why this difference, which seems to allow static declarations of simple types, but not Objects, in inner classes.

(javac -version: 1.6.0_24)

public class Outer {
    public static final Runnable HELLO = new Runnable() {
        // No compiler error
        public static final int ZERO = 0;

        // Causes compiler error: "inner classes cannot have static declarations"
        public static final Object NULL = null;

        @Override
        public void run() {
            System.out.println("Hello " + ZERO + NULL);
        }
    };
}

Answer 1

问题是内部类不能有一个静态初始化块，这是初始化非平凡常量和非常量所必需的。