Java泛型 - 不在范围内

Question

I have following sample that cannot compile and produces Error:() java: type argument GroupOfPartsDecorImpl<V> is not within bounds of type-variable GOP . Code is the following:

class MainContainerGroupPartDecorator<V, GOP extends GroupOfParts<V, PartDecorator<V, ? extends Part<V>>>> 
extends BaseContainerGroupPartDecorator<V, GOP> {
    public static <V> MainContainerGroupPartDecorator<V, GroupOfPartsDecorImpl<V>> getInstance() {
        return null;
    }
}

class BaseContainerGroupPartDecorator<V, GOP extends GroupOfParts<V, ?>> {
    void something() {}
}

class GroupOfPartsDecorImpl<V> implements GroupOfParts<V, PartDecorator<V, PartImpl1<V>>> {
    @Override
    public Collection<PartDecorator<V, PartImpl1<V>>> getParts() {
        return null;
    }
}

interface GroupOfParts<V, P extends Part<V>> {
    Collection<P> getParts();
}

class PartDecorator<V, P extends Part<V>> implements Part<V> {
    @Override
    public V getId() {
        return null;
    }
}

class PartImpl1<V> implements Part<V> {
    @Override
    public V getId() {
        return null;
    }
}

Since GOP is GOP extends GroupOfParts<V, PartDecorator<V, ? extends Part<V>>> GOP extends GroupOfParts<V, PartDecorator<V, ? extends Part<V>>> and GroupOfPartsDecorImpl should be in the end GroupOfParts<V, PartDecorator<V, Part<V>> why this error shows up?

Answer 1

The second generic parameter to GroupOfParts must be PartDecorator<V, ? extends Part<V>> PartDecorator<V, ? extends Part<V>> . And since generics are invariant by default there can be no deviation from this. But GroupOfPartsDecorImpl uses PartDecorator<V, PartImpl1<V>> , which is not the same, so it doesn't compile.

You can solve this by making the second parameter covariant in the declaration of MainContainerGroupPartDecorator :

class MainContainerGroupPartDecorator<V, GOP extends GroupOfParts<V,
    ? extends PartDecorator<V, ? extends Part<V>>>> 
    extends BaseContainerGroupPartDecorator<V, GOP> {

(Basically changing PartDecorator<V, ? extends Part<V>> to ? extends PartDecorator<V, ? extends Part<V>> )

Answer 2

The problem is your bounded wildcard in the 3rd level of your 2nd generic ? extends Part<V> ? extends Part<V> . You may specify it by an additional generic P

class MainContainerGroupPartDecorator<V, P extends Part<V>, GOP extends GroupOfParts<V, PartDecorator<V, P>>>
    extends BaseContainerGroupPartDecorator<V, GOP>
{
    public static <V> MainContainerGroupPartDecorator<V,PartImpl1<V>, GroupOfPartsDecorImpl<V>> getInstance() {
        return null;
    }
}