为什么Class.getAnnotation（）要求我进行演员表？ [英] Why is Class.getAnnotation() requiring me to do a cast?

问题描述

我正在使用Java 1.6.0_25。

I'm using Java 1.6.0_25.

我有一个定义的注释：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Resource {
    String value();
}

以后当我使用getAnnotation时：

And later when I use getAnnotation:

Resource resource = (Resource)cls.getAnnotation(Resource.class);

编译器和IDE同意我必须转换结果，但getAnnotation在Java 1.5文档中声明as：

the compiler and IDE agree that I must cast the result, but getAnnotation is declared in the Java 1.5 documentation as:

public <A extends Annotation> A getAnnotation(Class<A> annotationClass);

由于Resource.class是Class类型，在我看来这意味着cls.getAnnotation（ Resource.class）应该返回类型Resource，我应该需要强制转换。

Since Resource.class is of type Class, it seems to me that this means that cls.getAnnotation(Resource.class) should return type Resource, and I should need to cast.

我使用getAnnotation找到的所有示例都没有强制转换，所以我必须是做错了。

All examples I've found using getAnnotation don't have a cast, so I must be doing something wrong.

推荐答案

cls 的类型是什么？它是原始 Class 还是 Class< Something> ？

What is the type of cls? Is it raw Class or is it Class<Something>?

如果是原始类型 Class ，则必须进行强制转换。如果您至少 Class<？> 而不是 Class ，则不再需要演员。

If it's the raw type Class, then the cast is necessary. If you make it at least Class<?> instead of Class, you won't need the cast anymore.

Class cls = Example.class;

// Error: Type mismatch, cannot convert from Annotation to Resource
Resource anno = cls.getAnnotation(Resource.class);

Class<?> cls2 = Example.class;
Resource anno = cls2.getAnnotation(Resource.class); // OK

Class<Example> cls3 = Example.class;
Resource anno = cls3.getAnnotation(Resource.class); // OK

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