Question

Suppose I have the following interface:

public interface Interface<T extends Number>{
    public Vector<Interface<T>> getVector();
}

and the following class implementing that interface:

public abstract class C<T extends Number> implements Interface<T>{

    private Vector<C<T>> vector;
    public Vector<Interface<T>> getVector(){ //errror
         return this.vector;
    }
} 

Why is not legal returning a Vector<C<T>> meanwhile is legal ( obviously) returning a Vector<Interface<T>>. C is actually implementing Interface, so it should be possible, right? What am I missing?

EDIT:

why this work for non generics interface? Is this actually a generic related problem?

public interface Interface{
        public Interface getVector();
    }

public abstract class C implements Interface {

    private C;
    public Interface getVector(){ //errror
         return this.c;
    }
} 

Answer 1

Because the Vector is explicitly made up of Interface<T>, not things that extend Interface<T>, I believe this would work if you changed the definition to

 public Vector<? extends Interface<T>> getVector();

The problem is that for some V implements T or V extends T that Foo<V> is not a supertype of Foo<T>. The compiler does not test inheritance on the generic arguments unless you explicitly indicate that extension point.

Using Vector<? extends Interface<T>> means “allow any class that implements or extends Interface<T>, whereas Vector<Interface<T>> means a vector consisting only of Interface<T> items.

Perhaps it’s more concrete to consider that List<Integer> is not an acceptable replacement for List<Number> despite Integer extending Number for precisely the same reason.

update:

I tested this and the following compiles without any errors or warnings

interface Interface<T extends Number>{
    public Vector<? extends Interface<T>> getVector();
}

abstract class C<T extends Number> implements Interface<T>{

    private Vector<C<T>> vector;
    public Vector<? extends Interface<T>> getVector(){
         return this.vector;
    }
}