Generic Composite Interface

[Rick Beaver]

Hello All

Quick question on my generic composite interface.

I have the following interface

public interface CompositeNode<E extends CompositeNode> {
    public void addChild(E);
    
    public E getChild(int index);
}

And a concrete class

public class Org implements CompositeNode {
    private List<Org> children = new ArrayList<Org>():
 
    public Org getChild(int index) {
        return children.get(index);
    }
 
    public void addChild(Org child) {
        if (!children.contains(child)) {
            children.add(child);
        }
    }
}

The compiler complains that I have not implemented the method addChild from the CompositeNode interface - if I change the argument type of the addChild method in the implementation to CompositeNode it compiles OK (after changing the list).

The getChild method works OK so I am not understanding why the addChild is no good.

Can anyone help?

Thanks in advance

[Jeff Albertson]

public class Org implements CompositeNode<Org> {
...
}

[Rick Beaver]

Works great, thanks Jeff

[Jim Yingst]

Also:

[Rick]: The getChild method works OK so I am not understanding why the addChild is no good.

When overriding or implementing a method, you can always (as of JDK 5) provide a more specific return type than the original method declaration did. That is, if the original return type was a reference type, the overriding return type may be a subclass or subinterface of the original. This is true regardless of whether the class is generic or not.

[Rick Beaver]

Thanks Jim, that I didn't know. That's two things I have learned today... I can take tomorrow off

I am trying to think of a situation where returning a subtype in an overridden method would be useful - mostly overridden classes will be called polymorphically so the client would normally not be aware of the more specific subtype.

Can you think of an example of where this may be useful to aid my understanding of it's use?

Thanks

Originally posted by Jim Yingst:
Also:

[Rick]: The getChild method works OK so I am not understanding why the addChild is no good.

When overriding or implementing a method, you can always (as of JDK 5) provide a more specific return type than the original method declaration did. That is, if the original return type was a reference type, the overriding return type may be a subclass or subinterface of the original. This is true regardless of whether the class is generic or not.