Loose coupling between modules
High cohesion within modules

Coupling

The degree of interdependence between software modules.

Advantages

1. A change in one module does not have a ripple effects requiring a change in another.
2. Promotes code re-use and ease of testing by reducing dependencies.

Cohesion

The degree to which elements inside a module belong together.

• If methods of a class are similar in many aspects, the class has high cohesion.

Advantages

1. Improved code readability
2. Improved code reusability
3. Complexity is manageable

Examples

Bad: The only relationship between the parts is that they have been grouped together.

• Ex: A “utilities” class

Good: Parts are grouped because they operate on the same data.

• Ex: A class that manipulates XML.

Robustness—the ability of a system to cope with bad input and/or errors during execution. Redundancy—one way to support a system’s robustness.