fastutil is a collection of type-specific Java classes that extend the Java Collections Framework by providing several containers, such as maps, sets, lists and prority queues, implementing the interfaces of the java.util package; it provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files.
fastutil provides a huge collection of specialized classes generated starting from a parameterized version; the classes are much more compact and much faster than the general ones. Please read the package documentation for more information.
Note that the jar file is huge, due to the large number of classes: if you plan to ship your own jar with some fastutil classes included, you should look at AutoJar or similar tools to extract automatically the necessary classes.
You have to "make sources" to get the actual Java sources; finally, "ant jar" and "ant javadoc" will generate the jar file and the API documentation.
The Java sources are generated using a C preprocessor. The gencsource.sh script reads in a driver file, that is, a Java source that uses some preprocessor-defined symbols and some conditional compilation, and produces a (fake) C source, which includes the driver code and some definitions that customize the environment.