How I can make a efficient data structure consisting of XML contents?

I have posted a question here: How can i parse the following XML using JDOM

I am trying to read the XML posted in the above question so that I can retieve any question or answers just by calling some combination of IDs.

How can I build some of data structures which contains the whole XML, and then开发者_如何转开发 how do I make the search for questions or answers more efficient when done on the data structure?