如何在 Java 中将泛型列表作为参数传递，并为该列表定义子类型作为值？

Question

I am trying to use generics to create some flexible data handling, but I am stumbling across a compiler error which I don't know how to get around. Example code:

import java.util.ArrayList;
import java.util.List;

public class Spielwiese {
    
    class A {
        public A() { }
    }
    
    class B extends A {
        public B() { }
    }
    
    class C<T extends A> {
        T value = null;
        public C() {}
        public void setValue(T v) {
            value = v;
        }
    }

    public static void doSomething(List<C<? extends A>> stuff) { }
    
    public static void main(String[] argv) {
        List<C<B>> stuff = new ArrayList<>(); 
        doSomething(stuff); // compiler error: "The method doSomething(List<C<? extends A>> stuff) is not applicable for the arguments (List<C<B>>)"
    }
}

Why doesn't this work, even though B extends A as required?

Answer 1

@Sweeper posted the solution:

public static void doSomething(List<? extends C<? extends A>> stuff) { }

This allows to pass a List<C<B>> as a parameter just nicely.