Java泛型：为什么someObject.getClass（）不返回Class&lt ;?延伸T> [英] Java generics: why someObject.getClass() doesn't return Class<? extends T>?

问题描述

我期望从编译时间方面以及从运行时方面来看， .getClass（）不会提供正确的问题

但是我一定错了。

  public class _GetClassGenerics2 {
 
 static class MyClass {
} 
 
 public static void main（String [] args）{
 MyClass myInstance = new MyClass ）; 
 //这里工作
 class< ;? extends MyClass> type = myInstance.getClass（）; 
 
 myMethod（myInstance）; 
} 
 
 public static< T extends MyClass> void myMethod（T instance）{
 Class< ;?延伸T> type = instance.getClass（）; 
 // java.lang.RuntimeException：不可编译的源代码 - 不兼容的类型
 //必需：java.lang.Class< ;?延伸T> 
 // found：java.lang.Class< capture＃1 of？ extends _GetClassGenerics2.MyClass> 
} 
 
} 
  

strong>它不能使用类< T> 和类< ;? oracle.com/javase/7/docs/api/java/lang/Object.html#getClass（）rel =nofollow> getClass 方法的Javadoc ：

实际的结果类型是 Class< ;?
extends | X |> 其中| X |是调用 getClass 的表达式
的静态类型的擦除
。对于
示例，在此
代码片段中不需要转换

您的代码片段中的$ c> | X | 是 MyClass ，因此 instance.getClass（）只能分配给 Class< ;?扩展MyClass> 或类<？> 。

措辞是因为当你说对于具有< T extends MyClass> 的类型T的变量，可以有多个扩展 MyClass ，因此能够满足 T extends MyClass 条件。没有运行时信息，没有办法知道在方法中是否传递了 MyClass 的具体实现子类。因此，为了提供通用解决方案，它返回< ;?扩展MyClass> ，因为对于 MyClass 的任何子类都是如此。

I would expect that from the aspect of compile time as well as from the aspect of runtime it wouldn't be a problem for .getClass() to provide a correctly-typed return value.

But I must be wrong.

public class _GetClassGenerics2 {

  static class MyClass {
  }

  public static void main(String[] args) {
    MyClass myInstance = new MyClass();
    // here it works
    Class<? extends MyClass> type = myInstance.getClass();

    myMethod(myInstance);
  }

  public static <T extends MyClass> void myMethod(T instance) {
    Class<? extends T> type = instance.getClass();
// java.lang.RuntimeException: Uncompilable source code - incompatible types
//  required: java.lang.Class<? extends T>
//  found:    java.lang.Class<capture#1 of ? extends _GetClassGenerics2.MyClass>
  }

}

EDIT: It doesn't work with Class<T> and Class<? super T> either.

解决方案

As per the Javadoc of the getClass method:

The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment

Here, the value for |X| in your code snippet is MyClass, hence instance.getClass() is assignable to only Class<? extends MyClass> or Class<?>.

The reason for this specific wording is because when you say that for this variable having type T where <T extends MyClass>, there can be multiple classes which extend MyClass and hence capable of satisfying the T extends MyClass criteria. Without runtime information there is no way of knowing which concrete implementation subclass of MyClass was passed in the method. Hence to provide a generic solution, it returns <? extends MyClass> since that would hold true for any subclass of MyClass irrespective of what class instance is passed in.

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