You could certainly use CSV or XLS (some helpful libraries are linked
here), but since the data isn't really tabular that would be a bit of a hack.
I might use XML, either creating it directly through one of the DOM libraries (JAXP, XOM, JDOM, dom4j, ...) or -if your data structures are amenable to that- by way of the java.beans.XMLEncoder/XMLDecoder class. Examples of that can be found
here under "Serialization". Don't let the term "serialization" confuse you - they write and read XML.
[ March 19, 2007: Message edited by: Ulf Dittmer ]