Question

The code below makes complete sense to me – its about adding an element of some type which is supertype of type T and type S is definitely such a super type , so why the compiler refuses to add ‘element’ into the collection ?

class GenericType<S,T extends S>{
   void add1(Collection<? super T> col ,S element ){
        col.add(element);  // error
       // The method add(capture#9-of ? super T) in the type 
       // Collection<capture#9-of ? super T> is not applicable for the arguments (S)
    }
}

Answer 1

Collection<? super T> does not mean “a collection that can contain T and any superclass of it” – it’s actually not possible to formulate that restriction. What it means is “a collection that can only contain instances of some specific class which is a superclass of T” – basically it ensures that you can add a T to the collection.

The method can be called with a Collection<T>, yet you want to add an S to it.