为什么我得到一个“变量可能没有被初始化"?我的开关块中的编译器错误? [英] Why do I get a "variable might not have been initialized" compiler error in my switch block?

问题描述

我在使用 switch 块时遇到变量可能尚未初始化"错误.

I'm encountering "a variable might not have been initialized" error when using switch block.

这是我的代码:

public static void foo(int month)
{
    String monthString;
    switch (month)
    {
        case 1: monthString = "January";
                break;
        case 2: monthString = "February";
                break;
    }
    System.out.println(monthString);
}

错误:

Switch.java:17: error: variable monthString might not have been initialized
        System.out.println (monthString);

据我所知，当您尝试访问尚未初始化的变量时会发生此错误，但是当我在 switch 块中为其分配值时我没有初始化它吗?

To my knowledge this error occurs when you try access a variable you have not initialized, but am I not initializing it when I assign it the value in the switch block?

同样，即使月份是编译时常量，我仍然会收到相同的错误:

Similarly, even if the month is a compile-time constant, I still receive the same error:

public static void foo()
{
    int month = 2;
    String monthString;
    switch (month)
    {
        case 1: monthString = "January";
                break;
        case 2: monthString = "February";
                break;
    }
    System.out.println(monthString);
}

推荐答案

如果 month 不是 1 或 2 则没有在被引用之前初始化 monthString 的执行路径中的语句.编译器不会假定 month 变量保留其 2 值，即使 month 是 final.

If month isn't 1 or 2 then there is no statement in the execution path that initializes monthString before it's referenced. The compiler won't assume that the month variable retains its 2 value, even if month is final.

JLS，第 16 章, 讨论明确赋值"以及变量在被引用之前可能被明确赋值"的条件.

The JLS, Chapter 16, talks about "definite assignment" and the conditions under which a variable may be "definitely assigned" before it's referenced.

条件布尔运算符 &&、|| 和 ? 的特殊处理除外: 对于布尔值常量表达式，流分析中不考虑表达式的值.

Except for the special treatment of the conditional boolean operators &&, ||, and ? : and of boolean-valued constant expressions, the values of expressions are not taken into account in the flow analysis.

变量monthString在被引用之前没有明确赋值.

The variable monthString is not definitely assigned prior to being referenced.

在 switch 块之前初始化它.

Initialize it before the switch block.

String monthString = "unrecognized month";

或者在 switch 语句中以 default 的情况对其进行初始化.

Or initialize it in a default case in the switch statement.

default:
    monthString = "unrecognized month";

或者抛出异常

default:
    throw new RuntimeExpception("unrecognized month " + month);

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