All you need is a JDK (version 6 or higher) and one class file. (source)
You reference them by ID and they are downloaded automatically from the central repository.
Example: java x9 610 (will print 'hello')
You can also run any local Java code. Just say java x9 sourcefolder. You only have to make sure there is a "main" class in there.
Really, anything. All you need is a "translator" (and we love to build those!).
Just put the translator ID at the start of your program. Example here.
Examples:
Sometimes translators take little input and produce whole programs.
Example: #598 makes a whole webserver. Your program can even be empty apart from the reference to the translator. "!598" is a valid JavaX program that starts a webserver on port 8080.