Parameters of a Java generic method

Question

I'm trying to figure out the whole Java generics topic.

More specifically this issue:

public class Node<E>{
    private E data;
    public Node(E data){
        this.data=data;
    }
    public E get(){
        return this.data;
    }
    public void set(E data){
        this.data=data;
    }
}

How can I add an "extends" wildcard specifying that the set method can receive E or any inheriting class of E (in which case the Node will hold a upcasted version of the parameter).

Or will it work even if I leave it the way it is?

(I might be a bit confused with the invariant aspect of generic types.)

Thanks!

Answer 1

Your class is already doing what you require. Lets demonstrate by example. Lets say you have created Node (Number is super class of Integer, Long etc);

Node<Number> numberNode = new Node<Number>(1);

You can call set method by passing its subclasses also

numberNode.set(new Integer(1));
numberNode.set(new Long(1));

Answer 2

You declared your class Node<E> where it already accepts any inheriting class of E.