如何在Java中将通用类类型转换为另一个通用类类型

Question

This is what I want:

interface A {}
abstract class B implements A {}
class C extends B { /*few fields*/ }
class D extends B { /*few different fields*/ }

public void doSomething(Class<A> clazz) {
    if(clazz.isAssignableFrom(B)) {
        doSomethingElse((Class<? extends B>)clazz); // casting is good, but cannot pass this way to doSomethingElse
    }
}

public <T extends B> void doSomethingElse(Class<T> clazz) {
    if(clazz.isAssignableFrom(C)) {
        // do something with C fields
    } else if(clazz.isAssignableFrom(D)) {
        // do something with D fields
    }
}

But obviously I cannot cast the class type this way.

Is there a way to do it? I am very curious...

Thanks!

Answer 1

First of all, a value of type Class<A> can only be the class object for the exact class A , it cannot be any other class. The class object for some other class B would be Class<B> , and Class<B> is never a subtype of Class<A> . You probably want a Class<? extends A> Class<? extends A> parameter instead.

Second, your use of .isAssignableFrom() seems backwards (besides the fact that it's syntactically invalid as B is not a valid expression). clazz.isAssignableFrom(B.class) means that the class represented by clazz is a superclass of B , in which case it makes no sense to cast clazz to Class<? extends B> Class<? extends B> . You probably want B.class.isAssignableFrom(clazz) instead.

Once you make the first change above (make clazz type Class<? extends A> ), the cast will compile, although it will be an unchecked cast. If you don't want an unchecked cast, instead of (Class<? extends B>)clazz , you can do clazz.asSubclass(B.class) .