Java：从X到Y的未选中强制转换/如何实现castOrNull [英] Java: Unchecked cast from X to Y / how to implement castOrNull

问题描述

我已实现此功能：

 static <X,Y> Y castOrNull(X obj) {
  try {
   return (Y)obj;
  }
  catch(ClassCastException e) {
   return null;
  }
 }

这给我编译器警告：

Type safety: Unchecked cast from X to Y

这我不太明白。不是 try / catch 我在这里做一个检查吗？我可以忽略这个警告吗？

Which I don't exactly understand. Isn't the try/catch which I am doing here a check for it? Can I ignore the warning?

我的函数能否正常工作？我如何正确实现它？

Will my function work as expected or not? How would I implement it correctly?

我也尝试过一个 obj instanceof Y 检查，但是不工作因为Java处理泛型的方式。

I also tried with a obj instanceof Y check but that doesn't work because of the way Java handle generics.

Btw。，这个函数对我来说很有用（让一些其他代码更干净）。我不知道这样的函数在Java中是否已经存在？

Btw., this function seems quite useful to me (to make some other code more clean). I wonder if such a function may already exist in Java?

我想使用它的一个例子：

One example where I want to use it:

    void removeEmptyRawStrings() {
        for(Iterator<Entity> e = entities.iterator(); e.hasNext();) {
            RawString s = castOrNull(e.next());
            if(s != null && s.content.isEmpty()) e.remove();
        }
    }

我在我的代码中经常遇到这样的情况。我认为这是比其他任何东西更可读和更简单。但是如果你有任何关于如何让代码更简单的话，请给我一个更好的建议。

I have cases like these quite often in my code. And I think this is more readable and simpler than anything else. But please give me a better suggestion if you have any about how to make that code even more simple.

推荐答案

是用于动态铸造的通用参数 Y 被视为 Object 。它永远不会抛出CCE。

So the problem here is that the generic parameter Y when used for dynamic casting is treated as Object. It will never throw a CCE. You get a CCE thrown in the calling method, as you have broken static type safety.

还有 X code>在这里是完全没有意义的：

Also X is entirely pointless here:

几乎肯定正确的解决方案不是尝试这样的。 null 是错误的。

Almost certainly the correct solution is not to attempt anything like this. null is bad. Casting is bad.

但是，如果你决定写废话，可以传递 Class 对象：

However, if you are determined to write nonsense, you can pass the Class object:

public static <T> T evilMethod(Class<T> clazz, Object obj) {
    try {
        return clazz.cast(obj);
    } catch (ClassCastException exc) {
        return null;
    }
}

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