Failing to compile correlated Java Generics parameters with wildcards

Question

The following little Java example won't compile for unclear reasoning:

package genericsissue;

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

interface Attribute<V> {}

interface ListAttribute extends Attribute<List<?>> {}

public class Context {
    public <T, A extends Attribute<T>> void put(Class<A> attribute, T value) {
        // implementation does not matter for the issue
    }

    public static void main(String[] args) {
        Context ctx = new Context();
        List<?> list = new ArrayList<String>();
        ctx.put(ListAttribute.class, list);
    }
}

The line with ctx.put produces following error:

Context.java:18: <T,A>put(java.lang.Class<A>,T) in genericsissue.Context cannot be applied to (java.lang.Class<genericsissue.ListAttribute>,java.util.List<capture#35 of ?>)

If working without wildcards the attribute pattern works fine.

Is there any explanation why the compiler does not accept the value with wildcard typing?

Answer 1

The problem is, the argument type of list is not really List<?>. Compiler does a "wildcard capture" first to convert its type to List<x> for some x. Usually this is more informative and helpful. But not in your case. It drives type inference to think that T=List<x>, but ListAttribute does not extend Attribute<List<x>>

You can provide explicit type arguments to work around it

ctx.<List<?>, ListAttribute>put(ListAttribute.class, list);
      (T)      (A)

Answer 2

Replace

public <T, A extends Attribute<T>>

With

public <T, A extends Attribute<? super T>>