JDK17+：为什么不可能在多个类型模式上进行单个 switch 分支匹配？

发布于 2025-01-19 20:28:21 字数 640 浏览 0 评论 0原文

只是想知道这是/是什么设计的理由。

我可以做，

int x = ...
switch( x ) {
    case 1,2 -> { ... stuff ...}
    default -> { ... something else ...}
}

但是尝试做类似

 Object x = ...
 switch( x ) {
   case String, Integer x -> { ... stuff ...}
   default -> { ... something else ...}
 }

不编译的事情（我还尝试了诸如“字符串x，integer x”和“ string | integer x”之类的东西，但它们都没有编译）。

如果我理解 jls 正确地，它根本不支持 - 有人知道为什么是这种情况吗？

原文

Just wondering what the design rationale for this is/was.

I can do

int x = ...
switch( x ) {
    case 1,2 -> { ... stuff ...}
    default -> { ... something else ...}
}

but trying to do something like

 Object x = ...
 switch( x ) {
   case String, Integer x -> { ... stuff ...}
   default -> { ... something else ...}
 }

does not compile (I also tried things like "String x, Integer x" and "String|Integer x" but neither of them compile).

If I understood the JLS correctly, it's simply not supported - does anybody know why this is the case and if this will ever be fixed ?

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影子是时光的心 2025-01-26 20:28:22

再次阅读JEP-420后，我发现了一个解释：

我仍然认为，如果分支机构实际上没有指案例标签，例如在这样的代码中，这是可能的（ApplicationState是一个密封类带有几个子类，请注意，我在这里无法使用枚举，因为一些州需要持有可变数据）：

private static ApplicationState checkValidTransition(ApplicationState from, ApplicationState to )
{
    final boolean isValidTransition = switch(from) {
        case Uninitialized ignored -> to instanceof ServletContextInitializing;
        case Destroyed ignored -> to instanceof ServletContextInitializing;
        case ServletContextInitializing ignored -> to instanceof SpringContextInitializing || to instanceof Destroyed;
        case SpringContextInitializing ignored -> to instanceof SpringContextInitializing || to instanceof Destroyed || to instanceof WebApplicationInitFinished;
        case WebApplicationInitFinished ignored -> to instanceof Destroyed;
    };
    if ( ! isValidTransition ) {
        throw new IllegalStateException( "Internal error, illegal state transition " + from + " -> " + to );
    }
    return to;
}

After reading JEP-420 again I found an explanation:

I still think this should be possible if the body of the branch does not actually refer to the case labels, for example in code like this (ApplicationState is a sealed class with several subclasses, note that I cannot use an enum here as some of the states need to hold mutable data):

private static ApplicationState checkValidTransition(ApplicationState from, ApplicationState to )
{
    final boolean isValidTransition = switch(from) {
        case Uninitialized ignored -> to instanceof ServletContextInitializing;
        case Destroyed ignored -> to instanceof ServletContextInitializing;
        case ServletContextInitializing ignored -> to instanceof SpringContextInitializing || to instanceof Destroyed;
        case SpringContextInitializing ignored -> to instanceof SpringContextInitializing || to instanceof Destroyed || to instanceof WebApplicationInitFinished;
        case WebApplicationInitFinished ignored -> to instanceof Destroyed;
    };
    if ( ! isValidTransition ) {
        throw new IllegalStateException( "Internal error, illegal state transition " + from + " -> " + to );
    }
    return to;
}

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