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!

I downvoted the question. It is easily answerable if you do a tiny bit of work yourself. You don't even need google, just use ctrl-space in any Java IDE and see for yourself. — Dariusz
– Dariusz, Commented Jul 3, 2013 at 8:01
@Dariusz You're free to do whatever you wish. The thing is, sometimes code will "work" even if you don't really know why or how well it works. That's why I asked here. — Paz
– Paz, Commented Jul 3, 2013 at 12:11

sanbhat · Accepted Answer · 2013-07-03 07:39:13Z

3

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));

answered Jul 3, 2013 at 7:39

sanbhat

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Comments

Suresh Atta · Accepted Answer · 2013-07-03 07:32:53Z

3

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

answered Jul 3, 2013 at 7:32

Suresh Atta

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2 Comments

Prasad Kharkar Over a year ago

is this different with passing collections as generics? Because when we write someMethod(ArrayList<E>) then only arrayLists with type E can be passed.

johnchen902 Over a year ago

@PrasadKharkar Good spotted! Yes they are different.

Prasad Kharkar · Accepted Answer · 2013-07-03 07:35:09Z

0

If you use

public void set(E extends SomeType){
 this.data=data;
 }

then you can pass any object that implements or extends SomeType Remeber that SomeType can also be an interface here, even if it is strange, we need to write E extends SomeType.

answered Jul 3, 2013 at 7:35

Prasad Kharkar

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Comments

Matyas · Accepted Answer · 2013-07-03 07:44:03Z

0

Or will it work...?

Or have you tried it out if it works?

        final Node<Number> n =  new Node<Number>(new Integer(666));
        System.out.println(n.get());
        n.set(new Integer(777));
        System.out.println(n.get());

answered Jul 3, 2013 at 7:44

Matyas

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