XOM API for Processing XML with Java

[François Beausoleil <fbeausoleil AT ftml.net>]

Hi !

Elliotte Harold wrote:
> Wolfgang Hoschek wrote:
> > 12% speedup in ParentNode.java for changing the static declaration of 
> > childen from a List to an ArrayList (which it is anyway):
> > ParentNode.java:
> >
> >     ArrayList   children;
> >     //List   children;
>
> That's easy enough to do, and doesn't really harm anything. It's a minor 
> code smell at worst. However in my tests this had no noticeable effect. 
> Can anyone explain why this might have such a dramatic effect? i.e is 
> there any theoretical reason to believe this is a good idea? Is this 
> likely to be very different from one VM to another? Is this something 
> that might make a big difference in 1.5 and not 1.4? or the server VM 
> and not the client?

You all probably already know that, but List is an interface, whereas 
ArrayList is a concrete class.  The way the JVM maps functions to memory 
makes access to concrete methods an O(1) operation, whereas access to an 
interface methods is O(n/2) on average, where n is the number of methods 
defined in the concrete class and all parent classes.

Why the difference ?  All concrete classes have their methods at the 
same offset from the classes' base address.  This is an artefact of the 
compilation method, if I remember correctly.  On the other hand, when 
interface methods are thrown in, the compiler cannot map these to always 
the same offset, because the interface might be implemented by different 
concrete classes, which means that the methods will not always be at the 
same offset.

Anyway, I wasn't trying to offend anyone here.  Forgive me if you 
already knew all of that.

Have a nice day !
François