>> http://xml.apache.org/xalan-j/xpath_apis.html

Those are essentially the JAXP/TrAX APIs, which are a de facto 
semi-standard for invoking XPaths from Java... though other 
implementations may use other APIs.

If you just want to experiment with XPaths, most XPath implementations 
(including Xalan) come with sample programs which will let you execute 
an XPath from the command line (or which can easily be tweaked into that 
form.)

As Martin said, you could also consider finding a tool which supports 
XPath and using it as your "experimental" environment. I originally 
learned XPath mostly in the context of using it from XSLT.

Whether you want to jump straight to the 2.0 versions of these depends 
in part on where you're going to be using them. 2.0 does add a lot of 
nice features, but it isn't supported everywhere yet (as the Apache code 
illustrates), so it may be worth considering learning 1.0 first and then 
adding on the 2.0 changes rather than learning 2.0 and then discovering 
what has to be trimmed out in a 1.0 environment. And in fact, for the 
most common uses of XPath, the difference between 1.0 and 2.0 isn't very 
significant. (That may change as people start using Schema-sensitive 
paths more often, admittedly.)