XOM API for Processing XML with Java

[Elliotte Rusty Harold <elharo AT metalab.unc.edu>]

> Visitor is straightforward to implement (see below)
> Then the first NodeVisitor example could be the Serializer (with the 
> pleasant side-effect that the ugly method "public void 
> write(TreeNode node) throws IOException"
> would vanish).

I'm somewhat familiar with the visitor pattern. I did explore it when 
I was first designing XOM.  I'm still not convinced it really fits 
the XML problem space well. I don't like adding the extra method to 
Node just to support this. I may be wrong here. So far nobody's shown 
me convincingly how Visitor would make their life easier than using 
the more traditional navigation techniques. I'm inclined to agree 
with the DOM FAQ here:

Visitor was considered for inclusion in the Traversal module of the 
Level 2 DOM. There are negative as well as positive consequences to 
implementing the Visitor pattern. One of Visitor's advantages over 
Iterator is that Visitor can handle structures where the objects 
don't share a common ancestor class, which is not an issue when 
everything you're looking at is derived from Node. Since most of the 
things a Visitor could do can be emulated with a switch statement 
driven by an iterator, we decided to defer this issue.

Or at least I agree until someone shows me how much easier visitor 
would make important operations.

It's also a question of programmer familiarity. I think Visitor is 
one of those issues like interfaces vs. classes, push vs. pull, and 
pointers vs. stack variables, where the more advanced solution may be 
marginally better and more extensible for some uses, but really 
exceeds a lot of working programmers' comfort level. The level of 
abstraction and indirection can just get too high. Putting the client 
more in control is a lot more comfortable for most programmers since 
they can more easily see and visualize how the code flows. I am 
willing to trade some level of extensibility and generality in 
exchange for simplicity.
--

+-----------------------+------------------------+-------------------+
| Elliotte Rusty Harold | elharo AT metalab.unc.edu | Writer/Programmer |
+-----------------------+------------------------+-------------------+
|          XML in a  Nutshell, 2nd Edition (O'Reilly, 2002)          |
|              http://www.cafeconleche.org/books/xian2/              |
|  http://www.amazon.com/exec/obidos/ISBN%3D0596002920/cafeaulaitA/  |
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