为什么在switch语句中有每个枚举常量的用例时，为什么仍必须提供默认值？ [英] Why, when I have cases for every enum constant in a switch statement, must I still provide a default?

问题描述

我试图在Java中声明一个枚举，并在switch语句中使用该类型的变量，其中涵盖了该类型的枚举常量的所有可能情况。

I'm trying to declare an enum in Java and use a variable of that type in a switch statement, where all possible cases for enum constants of that type are covered.

enum MyEnum {
    FOO, 
    BAR
}

private static void test(MyEnum e) {
    String msg;
    switch (e) {
        case FOO:
            msg = "foo";
            break;
        case BAR:
            msg = "bar";
            break;
    }
    System.out.println("Enum is: " + e + " msg is: " + msg); //compiler error
}

为什么编译器无法检测到此开关将总是初始化 msg （或抛出NullPointerException，因为 e 为 null ）？

Why is the compiler not capable of detecting that this switch will always initialize msg (or throw a NullPointerException because e is null)?

推荐答案

想象一下 MyEnum 是一个单独的类。这样就可以重新编译 MyEnum 类，并添加新值，而无需重新编译 EnumSwitchTest （这样就不会得到任何错误）。

Imagine if MyEnum was a separate class. Then it would be possible to recompile the MyEnum class, and add new values, without recompiling EnumSwitchTest (so not getting any errors).

然后，另一个类就有可能使用新值调用 test 。

Then it would be possible for another class to call test with the new value.

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