Desirable characteristics of a design (minimal complexity, ease of maintenance, minimal connectednes

Minimal complexity

The primary goal of design should be to minimize complexity. Avoid making “clever” designs. Clever designs are usually hard to understand. Instead make “simple” and “easy-to-understand” designs.

Ease of maintenance

Ease of maintenance means designing for the maintenance programmer. Continually imagine the questions a maintenance programmer would ask about the code you’re writing.

Loose coupling

Loose coupling means designing so that you hold connections among different parts of a program to a minimum. Use the principles of good abstractions in class interfaces, encapsulation, and information hiding to design classes with as few interconnections as possible.

Extensibility

Extensibility means that you can enhance a system without causing violence to the underlying structure. You can change a piece of a system without affecting other pieces.

Reusability

Reusability means designing the system so that you can reuse pieces of it in other systems.

High fan-in

High fan-in refers to having a high number of classes that use a given class. High fan-in implies that a system has been designed to make good use of utility classes at the lower levels in the system.

Low-to-medium fan-out

Low-to-medium fan-out means having a given class use a low-to-medium number of other classes. High fan-out (more than about seven) indicates that a class uses a large number of other classes and may therefore be overly complex. Researchers have found that the principle of low fan-out is beneficial whether you’re considering the number of routines called from within a routine or the number of classes used within a class.

Portability

Portability means designing the system so that you can easily move it to another environment.

Leanness

Leanness means designing the system so that it has no extra parts. Extra code has to be developed, reviewed, tested, and considered when the other code is modified.

Stratification

Stratification means trying to keep the levels of decomposition stratified so that you can view the system at any single level and get a consistent view. Design the system so that you can view it at one level without dipping into other levels.

For example, if you’re writing a modern system that has to use a lot of older, poorly designed code, write a layer of the new system that’s responsible for interfacing with the old code. Design the layer so that it hides the poor quality of the old code, presenting a consistent set of services to the newer layers. Then have the rest of the system use those classes rather than the old code.

Standard techniques

The more a system relies on exotic pieces, the more intimidating it will be for someone trying to understand it the first time. Try to give the whole system a familiar feeling by using standardized, common approaches.