Modules are used in Jia as a generic word for describing
some logical element of a system.
Modules always have an interface and an implementation.
Module is a hierarchical concept. Modules are composed of modules
1. A module realizes some logical function of a system.
2. The entities within a module usually have a high frequency
of interaction among themselves and relatively
low frequency of communication outside the
Modules are represented in Java as classes, packages and files.
The processes by which modules are defined are given in the next slides.