为什么我会收到“变量可能尚未初始化"的信息?我的开关块中出现编译器错误? [英] Why do I get a "variable might not have been initialized" compiler error in my switch block?

问题描述

使用开关块时遇到可能未初始化变量"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块中为其赋值时，我是否没有对其进行初始化?

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?

类似地，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，则在引用之前，执行路径中没有用于初始化monthString的语句.即使month是final，编译器也不会假定month变量保留其2值.

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章讨论了确定赋值"以及在引用变量之前可以确定赋值"变量的条件.

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

除了条件布尔运算符&&，||和?的特殊处理外:和布尔值常量表达式，在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.

在引用之前并没有明确分配变量monthString.

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

在switch块之前将其初始化.

String monthString = "unrecognized month";

或在switch语句的default情况下将其初始化.

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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