如何遍历通配符泛型? [英] How to iterate over a wildcard generic?

问题描述

如何遍历通配符泛型?基本上，我想内联以下方法:

How can I iterate over a wildcard generic? Basically I would like to inline the following method:

private <T extends Fact> void iterateFacts(FactManager<T> factManager) {
    for (T fact : factManager) {
        factManager.doSomething(fact);
    }
}

如果此代码位于如图所示的单独方法中，则它可以工作，因为通用方法上下文允许定义一个可以对其进行迭代的通配符类型(此处为 T ).如果尝试内联此方法，则该方法的上下文将消失，并且无法再遍历通配符类型.即使在Eclipse中自动执行此操作，也会失败，并显示以下(不可编译)代码:

If this code is in a separate method as shown, it works because the generic method context allows to define a wildcard type (here T) over which one can iterate. If one tries to inline this method, the method context is gone and one cannot iterate over a wildcard type anymore. Even doing this automatically in Eclipse fails with the following (uncompilable) code:

...
for (FactManager<?> factManager : factManagers) {
    ...
    for ( fact : factManager) {
        factManager.doSomething(fact);
    }
    ...
}
...

我的问题很简单:是否有一种方法可以迭代一些通配符类型，或者这是对泛型的限制(意味着不可能这样做)?

My question is simply: Is there a way to put some wildcard type one can iterate over, or is this a limitation of generics (meaning it is impossible to do so)?

推荐答案

类型参数只能在上定义

• 类型(即类/接口)
• 方法和
• 构造函数.

您将需要一个用于本地块的类型参数，这是不可能的.

You would need a type parameter for a local block, which is not possible.

是的，有时我也会错过类似的东西.

Yeah, I missed something like this sometimes, too.

但是，这里没有内联方法并没有真正的问题-如果它在内联有所帮助的地方出现了性能瓶颈，那么Hotspot会再次内联该方法(不在乎该类型).

But there is not really a problem with having the method non-inlined here - if it presents a performance bottleneck where inlining would help, Hotspot will inline it again (not caring about the type).

另外，使用单独的方法可以为其指定一个描述性名称.

Additionally, having a separate method allows giving it a descriptive name.

只是一个想法，如果您经常需要:

Just an idea, if you need this often:

interface DoWithFM {
   void <T> run(FactManager<T> t);
}

...
for (FactManager<?> factManager : factManagers) {
    ...
    new DoWithFM() { public <T> run(FactManager<T> factManager) {
        for (T fact : factManager) {
            factManager.doSomething(fact);
        }
    }.run(factManager);
    ...
}
...

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