List class wrapped by Map class do not support generic types in JAVA?

class definition

public interface ITest {
    Long foo();
}

public class Test implements ITest{

    @Override
    public Long foo() {
        return 0L;
    }
}

methods

    public Map<Long, List<? extends ITest>> moo() {
        List<Test> a = List.of(new Test());
        Map<Long, List<? extends ITest>> b = Map.of(1L, a);
        return b;
    }

    public Map<Long, List<? extends ITest>> koo() {
        List<Test> a = List.of(new Test());
        Map<Long, List<Test>> b = Map.of(1L, a);
        return b;
    }

    public List<? extends ITest> too() {
        List<Test> a = List.of(new Test());
        return a;
    }

I got an error only at koo() function. Why this could not work? List class that wrapped by Map class do not support generic?
The below is what I met.

Incompatible types. Found: 'java.util.Map<java.lang.Long,java.util.List<Test>>', required: 'java.util.Map<java.lang.Long,java.util.List<? extends ITest>>'

I think it works properly, but it didn’t.

>Solution :

Your too method compiles because a List<Test> is a subtype of the declared return type of the method, List<? extends ITest>. Your explicit ? extends in the return type makes List<Test> a subtype.

Your koo method does not compile because even though List<Test> is a subtype of List<? extends ITest>, a Map<Long, List<Test>> is not a subtype of Map<Long, List<? extends ITest>>. Nesting that generics relationship in another generic type is not allowed, because you’re using invariant generics.

If you removed ? extends from the return type of too, then that method wouldn’t compile because List<Test> is not a subtype of List<ITest>.

Like what is needed for too, add ? extends to the return type for koo to get this to compile. The type Map<Long, List<Test>> is a subtype of Map<Long, ? extends List<? extends ITest>> because of this added ? extends.

//               vvvvvvvvv
public Map<Long, ? extends List<? extends ITest>> koo() {