XOM API for Processing XML with Java

[Remko Popma <remko.popma AT azzurri.jp>]

--- Elliotte Rusty Harold <elharo AT metalab.unc.edu> wrote:
> 
> At 10:33 AM +0200 9/19/02, Laurent Bihanic wrote:
> 
> 
> >  - Code compactness: As I already said on the jdom-interest mailing 
> >list, there are cases (e.g. TreeNode.getChild()) where instanceof 
> >tests make the code very unclear and thus harder to maintain.
> >I know Elliotte does not like the idea of a int getNodeType() method 
> >but we need to find a way to remove these ugly tests.
> >
> 
> 
> I agree the situation is imperfect. I think there's a bit of a 
> mismatch between the XML model and the Java model, but I don't see 
> how we can do any better than I've done.
> 
> Why would
> 
> if (node.getNodeType() == Node.ELEMEBT_TYPE)
> 
> be preferable to
> 
> if (node instanceof Element)
> 
> ?
> 
> They're both ugly. I think the second one is less ugly. 

I agree.

> Do you see something different? What do you think the code should 
> look like? (Actually what I think the code should look like is 
> polymorphic method dispatching, where I can pass a Node to a 
> process() method and have the runtime choose the overloaded method to 
> dispatch based on the node's subtype, but that's not a feature Java 
> gives us so we have to deal. :-( )

If this is really an issue, you could use the "double dispatch" pattern, eg:

public interface ICallback {
    public handleElement(Element element);
    public handleDocument(Document document);
    public handleComment(Comment comment);
    public handleAttribute(Attribute attribute);
    // etc for all Node subclasses
}

then, in the Node class you can add this method:
public abstract void process(ICallback callback);

and every Node subclass would call the appropriate method 
in the ICallback interface:

class Element {
    public void process(ICallback callback) {
        callback.handleElement(this);
    }
}

class Document {
    public void process(ICallback callback) {
        callback.handleDocument(this);
    }
}

class Comment {
    public void process(ICallback callback) {
        callback.handleComment(this);
    }
}

You could improve usability with a CallbackAdapter class that
provides an empty implementation for every method (like 
java.awt.MouseAdapter).

This would make for very clean client code, at the expense of
growing the API with a method that may not be obvious at first glance.
Good class and method names (suggestions for better names than
"ICallback" and "process"?), and some good examples may help out a little 
here.

Remko