Method Design Issues: Implementation Hiding, Cohesion, and Coupling

To design effective methods, you should consider several program qualities:

• You should employ implementation hiding; that is, a method’s client should not need to
  understand a method’s internal mechanisms.
  

• You should strive to increase cohesion.
  

• You should strive to reduce coupling.
  

Understanding Implementation Hiding
An important principle of modularization is the notion of implementation hiding , the
encapsulation of method details. That is, when a program makes a request to a method, it
doesn’t know the details of how the method is executed.

Increasing Cohesion
To help determine the appropriate division of tasks among methods, you want to analyze
each method’s cohesion , which refers to how the internal statements of a method serve to
accomplish the method’s purpose. In highly cohesive methods, all the operations are related,
or “go together.” Such methods are functionally cohesive —all their operations contribute to
the performance of a single task. Functionally cohesive methods usually are more reliable
than those that have low cohesion; they are considered stronger, and they make programs
easier to write, read, and maintain.

Reducing Coupling
Coupling is a measure of the strength of the connection between two program methods; it
expresses the extent to which information is exchanged by methods. Coupling is either tight
or loose, depending on how much one method relies on information from another. Tight
coupling , which occurs when methods depend on each other excessively, makes programs
more prone to errors. With tight coupling, you have many data paths to keep track of, many
chances for bad data to pass from one method to another, and many chances for one method
to alter information needed by another method. Loose coupling occurs when methods do
not depend on others. In general, you want to reduce coupling as much as possible
because connections between methods make them more difficult to write, maintain,
and reuse.