case表达式必须是static final int的常量表达式？ [英] Case expressions must be constant expressions for static final int?

问题描述

我有一个最终类Ring 定义为：

final class Ring {
    public static final int OUT = 3;
    public static final int MID = 2;
    public static final int IN  = 1;
}

我还有公共类MorrisBoard 使用以下代码：

public class MorrisBoard {
    public static final Ring RING = new Ring();

    private boolean checkMillBy(int ring, int x, int y) {
    switch(ring) {
    case MorrisBoard.RING.OUT:
        //...
    case MorrisBoard.RING.MID: //etc.
        //...   
    }
    return false;
}

MorrisBoard.RING.OUT 引用一个在程序生命周期内不可变的变量。所有价值都是最终的。

MorrisBoard.RING.OUT references a variable which is immutable for the lifetime of the program. All values are final.

但是，我仍然会收到以下错误： case表达式必须是常量表达式。我对此感到困惑 - MorrisBoard.RING.OUT 是一个常量表达式。

However, I still get the following error: case expressions must be constant expressions. I'm confused by this - MorrisBoard.RING.OUT is a constant expression.

这里发生了什么？

推荐答案

替换

 case MorrisBoard.RING.OUT:

with

 case Ring.OUT:

所以这将是一个常数，如编译时确定。

So this will really be a constant as in "determined at compilation".

规范精确SwitchLabel必须是

The specification precises that a "SwitchLabel" must be

• 案例后跟一个常量表达式


• case 后跟枚举值的名称


• 或默认

• case followed by a constant expression
• case followed by the name of an enum value
• or default

什么是有效的常量表达式在规范中描述。它相当有限。

What is considered a valid constant expression is described here in the specification. It's fairly limited.

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