Depends on what you have defined already.
There is the XMLEncoder and XMLDecoder classes already in
Java 1.4, for simple cases. There is JAXB, which you can generate Java files or XML Schema when you have one or the other. In the case where both are already defined, you can use XPath, DOM, JDOM, Axis, to hard code it, or you can try Castor XML to map you object to the XML Schema.
Mark