How to do Collections.EMPTY_SET in Generics?

843793 · Oct 24, 2002 4:45AM

In non-Generic code, I sometimes use Collections.EMPTY_SET:

public class Test
{
    private Set setWhichCanBeNull;

    public Test( Set setWhichCanBeNull )
    {
        this.setWhichCanBeNull = setWhichCanBeNull;
    }

    public Set getSet()
    {
        return ( setWhichCanBeNull == null )
            ? Collections.EMPTY_SET
            : setWhichCanBeNull;
    }
}

If I write this using Generics, Collections.EMPTY_SET does not work, since it does not know what type I am interested in:

public class Test<E>
{
    private Set<E> setWhichCanBeNull;

    public Test( Set<E> setWhichCanBeNull )
    {
        this.setWhichCanBeNull = setWhichCanBeNull;
    }

    public Set<E> getSet()
    {
        return ( setWhichCanBeNull == null )
            ? Collections.EMPTY_SET
            : setWhichCanBeNull;
    }
}

Is there any way to specify something like this:

Collections<E>.EMPTY_SET

or am I stuck with having to write:

    public Set<E> getSet()
    {
        return ( setWhichCanBeNull == null )
            ? new HashSet<E>()
            : setWhichCanBeNull;
    }

Thanks.

Geoff

843793 · Oct 24, 2002 5:16PM

I wish I knew. I sent this same question to the JSR 14 expert group during the public review process. Apparently the question was either too trivial or too much of a stumper to warrant a response.

Another question I asked, which seems even more problematic, was the question of how addAll() will work. In the public review collections API, it was declared as:

interface Collection<E> {
  void addAll(Collection<E> stuff);
}

That prevents the following from working:

   List<String> x;
   List<Object> y;
   ...
   y.addAll(x);

Yikes!! That alone would be reason to vote down the JSR in my mind. Really, there should be a way to declare addAll() something like this:

interface Collection<E> {
  void addAll(Collection<T extends E> stuff);
}

Does anyone know if this problem was/will be fixed in the final draft for JSR 14?

843793 · Oct 24, 2002 10:43PM

This won't work?

interface Collection<E> {
  <T extends E> void addAll(Collection<T> stuff);
}

843793 · Oct 25, 2002 9:07AM

It works with the actual prototype compiler (v 1.2).

843793 · Oct 26, 2002 6:29PM

The short answer is: you can't do that. However, you
can make a method that returns the empty set.

843793 · Nov 1, 2002 1:02PM

schapel wrote:

This won't work?

interface Collection<E> {
<T extends E> void addAll(Collection<T> stuff);
}

joerg.wassmer wrote:

It works with the actual prototype compiler (v 1.2).

Great! I just downloaded the new prototype, and it does indeed work. This was broken in the version I downloaded about 8 months ago, and I'm very glad to see that it was fixed.

So here's a stumper. Suppose I have the following:

public interface Event { }

public class EventSuper implements Event { }

public class EventSub extends EventSuper { }

public class EventListener<E extends Event>
    {
    public void handleEvent(E event)
        { System.out.println("got event: " + event); }
    }

public class EventBroadcaster<E extends Event>
    {
    public void addListener(EventListener<E> listener)
        { listeners.add(listener); }
        
    public void removeListener(EventListener<E> listener)
        { listeners.remove(listener); }
    
    public void broadcast(E event)
        {
        for(Iterator<EventListener<E>> i = listeners.iterator(); i.hasNext(); )
            i.next().handleEvent(event);
        }
    
    private List<EventListener<E>> listeners = new LinkedList<EventListener<E>>();
    }

Really, I ought to be able to do this:

    EventBroadcaster<EventSub> b = new EventBroadcaster<EventSub>();
    EventListener<EventSuper> l = new EventListener<EventSuper>();
    b.addListener(l);
    b.broadcast(new EventSub());

How do I finesse the EventBroadcaster and EventListener to allow that?

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