Question

Java is not allowing me to add a subclass of the Type declaration in this class

public class Exam<T> {

    public  void set(Holder<? super T> hold){

    }
    public  T get(Holder<? extends T> holder){ return holder.get();}


    public static void main (String[] args){
        Exam<Question> eq = new Exam<Question>();
        eq.set(new Holder<Identification>());
    }
}

Where Identification is a subclass of Question.

and this how my holder class looks like

public class Holder<T> {
    T item;

    public void set(T item){ this.item = item; }
    public T get(){return item;}
}

ERROR

The method set(Holder<? super Question>) in the type Exam<Question> is not applicable for the arguments (Holder<Identification>)

Answer 1

The error looks pretty self-explanatory to me – the set method expects a Holder<? super Question> and you’re trying to give it a Holder of something that is a subclass of Question. As written, Exam.set could take a Holder<Object>, for example, but not a Holder<Identification>.

A good way to think about extends and super in generics is in terms of assignment: T extends Foo will accept any type T that you could use on the right hand side of an assignment to Foo without casting, i.e.

Foo something = new T();

(treat this as pseudocode – I know you’re not really allowed to new a type varaible). Conversely, T super Foo accepts any T you could use on the left hand side of an assignment without casting:

T myThing = new Foo();

In your specific example, Identification i = new Question() isn’t legal without a cast, so a Holder<? super Question> parameter can’t accept a Holder<Identification> value.