调用“ Class.forName”时未选中的投射警告 [英] Unchecked Cast Warning when calling 'Class.forName'

问题描述

我的代码如下

package com.foo;

public class TestComposition {
    public static void main(String[] args) {
        try {
            Class<Foo> fooClass = 
                    (Class<Foo>) Class.forName("Foo");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

'块会产生警告，指出

The assignment within the 'try' block results in a warning stating

Type safety: Unchecked cast from Class<capture#1-of ?> to 
     Class<Foo>

这是为什么？

推荐答案

首先，让我们弄清楚问题出在哪里-它在演员表中。这是一个简短的示例：

Well, to start with let's be clear where the problem is - it's in the cast itself. Here's a shorter example:

public class Test {    
    public static void main(String[] args) throws Exception {
        Object x = (Class<Test>) Class.forName("Test");
    }
}

这仍然有相同的问题。问题在于强制类型转换实际上不会进行任何测试-因为强制类型转换将有效地转换为原始的 Class 类型。对于 Class< T> 来说，这有点令人惊讶，因为实际上对象确实知道所涉及的类，但是要考虑类似的情况：

This still has the same problem. The issue is that the cast isn't actually going to test anything - because the cast will be effectively converted to the raw Class type. For Class<T> it's slightly more surprising because in reality the object does know the class involved, but consider a similar situation:

List<?> list = ... get list from somewhere;
List<String> stringList = (List<String>) list;

该演员表不会检查它是否真的是 c $ c> List< String> ，因为该信息由于类型擦除而丢失。

That cast isn't going to check that it's really a List<String>, because that information is lost due to type erasure.

现在您的情况，实际上有一个相当简单的解决方案-如果您无论如何在编译时都知道类名，只需使用：

Now in your case, there's actually a rather simpler solution - if you know the class name at compile-time anyway, just use:

Class<Foo> fooClass = Foo.class;

如果您可以提供一个更现实的示例，而不是 我们可以帮助您确定最合适的选择。

If you can provide a more realistic example where that's not the case, we can help you determine the most appropriate alternative.

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