The jininut.zip file contains the complete source code for each example
referenced in the book. When code for an example is unchanged from a
previous example, the previous code is repeated; as a result, each directory
contains a complete, self-contained program (either client or server). The
directories are arranged by example name (e.g. Example8-3).

jininut.zip is a JAR file: you may unpack it with the jar command like this:

jar xf jininut.zip

It is also a zip file, of course, and you may use a standard zip utility to
extract it.