At this level, the design focuses on the packages and main components of a subsystem, and it should stay as a high-level design. You usually keep it in a design document or in the package—info.java class if it is relatively simple. The design of this level provides an abstract overview of a subsystem. You should only include only key classes and their relationship in the design. Any other details should be avoided as much as possible. When you look at the design of this level, you should be able to understand the structure and the main logic of that subsystem.

You do the abstraction level design when you start the implementation of a subsystem. Usually, the design at this level is, or is close to, stable. During the implementation, you might improve it as you gain more insight into the logic of the application. However, you should not need to change it often. Otherwise, this implies that there must be something wrong with the design itself.