Java泛型编译错误涉及Class <？延伸T>

Question

This program does not compile:

public class xx {
    static class Class1<C> {
        void method1(C p) {
        }
    }
    static class Class2<T> extends Class1<Class<? extends T>> {
        T object;
        void method2() {
            this.method1(this.object.getClass());
        }
    }
}

The error is:

xx.java:10: method1(java.lang.Class<? extends T>) in xx.Class1<java.lang.Class<? extends T>>
cannot be applied to (java.lang.Class<capture#215 of ? extends java.lang.Object>)
        this.method1(this.object.getClass());

Why does this happen? Why does the compiler seemingly believe that object.getClass() returns Class<? extends Object> Class<? extends Object> instead of Class<? extends T> Class<? extends T> ?

Answer 1

There is no upper bound set on T in your code, so ? extends T ? extends T is really tantamount to ? extends Object ? extends Object . Just yesterday I played with a similar example and hit this barrier. I had

static <T> T newInstance(T o) throws Exception {
  final Class<? extends T> c = o.getClass();
  return c.newInstance();
}

and it complained with the same error. Consider this: the return type of Object.getClass() is Class<?> and the compiler will want to capture the ? into a concrete type. But instead, we would like not to capture the ? , but to "capture the upper bound" T -- and there is no such thing in Java's generics.

Answer 2

Object.getClass() is defined to return a Class<? extends |T|> Class<? extends |T|> , where T is the statically known type of the receiver (the object getClass() is called on). Take special note of the vertical bars, the erasure operator . The erasure of a type variable is the erasure of its leftmost bound. In your case that's the implicit bound Object . So you get back a Class<? extends Object> Class<? extends Object> , not a Class<? extends T> Class<? extends T> .

Why is that?

Imagine T = List<Integer> , you could suddenly do the following without unchecked warning :

List<String> myStrings = new ArrayList<>();
List<Integer> myInts = new ArrayList<>();
List<Integer> myIntyStrings = myInts.getClass().cast(myStrings);
myIntyStrings.add(-1);
String myString = myStrings.get(0); // BANG!

But thankfully we do get a warning.. ;)

Answer 3

According to the documentation on getClass(), the returned object has type Class< ? extends |X| > Class< ? extends |X| > Class< ? extends |X| > , where |X| is the erasure of the type of the instance on which the method is called.

Therefore calling getClass() on an object of type T returns Class< ? extends Object > Class< ? extends Object > . We have no bound information about T in this API.

Usually APIs which use reflection on generic classes require that the client pass an additional argument of type Class< T > to the constructor or generic method in question.