ASTGen (http://sourceforge.net/projects/astgen) is a SourceForge project developed by the JavaPLT team at Rice to simplify the definition of large composite type hierarchies and accompanying visitors. A standalone preprocessor generates Java sources from the contents of a .ast file. The documentation on SourceForge provides a good overview.

While there are no official releases or version numbers, the tool had a major upgrade with many new features in 2007. Its use in DrJava (by DynamicJava and Language Levels) is still based on the earlier instance, and it will take some effort (but probably not much) to update to the latest version. In the meantime, some redundant casts generated by the earlier version produce warnings under javac 6.