There is a Java Overview pages that requires a general understanding of IT concepts, intended for programmers and non-programmers. It describes the different brands and buzzwords you might encounter when talking about Java.

The Design Patterns pages contain code examples and UML diagrams for developers.

The JUnit Overview is also intended for the developer, containing code samples etc.

Here you can learn more about JAXB , a simple to use Java API for binding object graphs to an XML-representation.