Java switch语句：需要常量表达式，但它是常量 [英] Java switch statement: Constant expression required, but it IS constant

问题描述

所以，我正在研究这个有一些静态常量的类：

So, I am working on this class that has a few static constants:

public abstract class Foo {
    ...
    public static final int BAR;
    public static final int BAZ;
    public static final int BAM;
    ...
}

然后，我想要一种方法来获取基于常量的相关字符串：

Then, I would like a way to get a relevant string based on the constant:

public static String lookup(int constant) {
    switch (constant) {
        case Foo.BAR: return "bar";
        case Foo.BAZ: return "baz";
        case Foo.BAM: return "bam";
        default: return "unknown";
    }
}

然而，当我编译时，我得到需要常量表达式每个3个案例标签上的错误。

However, when I compile, I get a constant expression required error on each of the 3 case labels.

我知道编译器需要在编译开关的编译时间，但为什么不是 Foo.BA _ 常量？

I understand that the compiler needs the expression to be known at compile time to compile a switch, but why isn't Foo.BA_ constant?

推荐答案

我知道编译器需要在编译时知道表达式来编译一个开关，但为什么不是Foo.BA_常量？

I understand that the compiler needs the expression to be known at compile time to compile a switch, but why isn't Foo.BA_ constant?

虽然从字段初始化后执行的任何代码的角度看它们是常量，但它们不是编译时常量 JLS要求;请参阅§15.28常量表达式用于定义常量表达式所需的内容。这是指§4.12.4最终变量它定义了一个常量变量，如下所示：

While they are constant from the perspective of any code that executes after the fields have been initialized, they are not a compile time constant in the sense required by the JLS; see §15.28 Constant Expressions for a definition of what is required of a constant expression. This refers to §4.12.4 Final Variables which defines a "constant variable" as follows:

我们调用一个原始类型或类型String的变量，它是最终的并且用编译时常量表达式（第15.28节）一个常量变量。变量是否是常量变量可能对类初始化（第12.4.1节），二进制兼容性（第13.1节，第13.4.9节）和明确赋值（第16节）有影响。

We call a variable, of primitive type or type String, that is final and initialized with a compile-time constant expression (§15.28) a constant variable. Whether a variable is a constant variable or not may have implications with respect to class initialization (§12.4.1), binary compatibility (§13.1, §13.4.9) and definite assignment (§16).

在您的示例中，Foo.BA *变量没有初始值设定项，因此不符合常量变量的条件。修复很简单;将Foo.BA *变量声明更改为具有编译时常量表达式的初始值设定项。

In your example, the Foo.BA* variables do not have initializers, and hence do not qualify as "constant variables". The fix is simple; change the Foo.BA* variable declarations to have initializers that are compile-time constant expressions.

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