[jdom-interest] suggested JDOM2 improvements

Rolf Lear jdom at tuis.net
Fri Jan 20 06:50:53 PST 2012

No, no static type analysis.

JDOM has 'always' had the 'Filter' concept. You could, for example, do:

List comments = element.getContent(new

In order to make the above 'generic' in JDOM2, the getContent() has to
return an appropriate type for whatever the Filter returns. I 'extended'
the Filter class to have a generic return type. Thus, it is now possible

List<Comment> comments = element.getContent(Filters.comment());

The Filter implementations all follow the rules:
1. if the content to be filtered does not match the filter, then the
content is discareded.
2. if the content matches the filter, then it is explicitly cast to the
generic type of the filter.

What this means is that you are guaranteed that the generic type of the
Filter results is accurate, and it is impossible to 'force' Filter results
to have badly-loaded result lists.

Filter instances can do more than just type-checking on the input data,
but can also do anything else to filter the content, like checking for
particular names, etc.

With the XPath library, I intend to apply the same Filter concept to the
XPath results.

Since the user knows the XPath expression, they will also know the
anticipated return type. If they want to select Elements then they can
apply an Element filter. If they want to select 'everything' then
they can use a 'passthough' filter which 'does no filtering' (but as a
result can only 'cast' to Object).

Essentially the Filter concept is a way to coerce unknown data in to a
user defined type while ensuring the results will never generate
class-cast, and providing an opportunity to discard what you do not want.
It is ideal for XPath results.

The 'user' creates their own filter
, or reuses one of the 'common' filters accessible in the 'Filters' class

Most Filter implementations take a Class instance (matching the generic
type of the Filter) as a constructor argument, and any values that match
the filter are cast using the Class.cast() method.


On Fri, 20 Jan 2012 14:31:07 +0000, Michael Kay <mike at saxonica.com> wrote:
>>public XPathCompiled<Object> compile(String xpath);
> I started introducing generics for this in Saxon 9.4 and the experience 
> wasn't wholly positive; it left a lot of cases where there were warnings

> that needed to be ignored. That may be because I found generics to be 
> deeper and more bewildering than I expected.
> It's not at all clear to me how your types such as 
> XPathCompiled<Element> are supposed to work. Do they rely excessively on

> the ability of the XPath engine to do static type analysis of the 
> supplied expression?
> Michael Kay
> Saxonica

More information about the jdom-interest mailing list