HSQLDB指南中记录了用户定义的功能。请参阅本部分以Java编写的功能。

http://hsqldb.org/doc/2.0/guide/sqlroutines-chapt.html#src_jrt_routines

你也可以在SQL中编写您的函数，这在指南的同一章中记录。

I want to write a simple function in HSQL so that it is backwards compatible with DB2 functions. In theory I should be able to write a custom function in java and hook it into HSQL. Is there any direction/documentation on just that?

User defined functions are documented in the HSQLDB Guide. See this section for functions written in Java.

http://hsqldb.org/doc/2.0/guide/sqlroutines-chapt.html#src_jrt_routines

You can also write your functions in SQL, which is documented in the same chapter of the Guide.