java 如何进行检查演员表？
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How to perform a checked cast?
提问by AllenKll
I am new to Generics and am having an issue.
我是泛型的新手，遇到了一个问题。
Consider the following code:
考虑以下代码：
public class A {}
public class B extends A {}
public <T extends A> T getB()
{
A test = new B();
Class<B> clazz = B.class;
if (clazz.isInstance(test))
{
return (T)test;
}
return null;
}
This generates an Unchecked cast warning. on the return (T)test;line.
but clearly I am checking the type with the if (clazz.isInstance(test))line.
这会生成 Unchecked cast 警告。就return (T)test;行了。但显然我正在检查if (clazz.isInstance(test))线路的类型。
Is there a way to do a "checked cast"?
有没有办法做一个“检查演员”？
I'm not looking to just suppress the warning but actually implement a checked cast. Unfortunately I can't find information on how to perform a checked cast.
我不希望只是抑制警告，而是实际实施已检查的强制转换。不幸的是，我找不到有关如何执行检查转换的信息。
回答by Jon Skeet
Is there a way to do a "checked cast"?
有没有办法做一个“检查演员”？
Sure, although it's important to note that it doesn't reallyhelp you here, because your method is hard-coded to use Bin a few places. You can perform the cast with:
当然，尽管重要的是要注意它在这里并没有真正帮助您，因为您的方法是硬编码的，可以B在一些地方使用。您可以通过以下方式执行演员表：
clazz.cast(test)
... but that will cast to B, not T. In particular, suppose I ran:
...但这将转换为B，而不是T。特别是，假设我跑了：
public class C extends A {}
...
C c = foo.<C>getB();
How would you expect that to work?
你希望它如何工作？
You might want to change your code to something like:
您可能希望将代码更改为以下内容：
public <T extends A> T getB(Class<T> clazz)
{
A test = // get A from somewhere
return clazz.isInstance(test) ? clazz.cast(test) : null;
}
Then that's fine, because clazz.castwill return a value of type T, which you're fine to return.
那很好，因为clazz.cast将返回一个 type 值T，您可以返回它。
