Coupling computer science: Encyclopedia II - Coupling computer science - Low coupling

Coupling computer science - Low coupling

Coupling can be "low" (also "loose"), or "high" (also "tight"). Low coupling means that one module does not have to be concerned with the internal implementation of another module, and interacts with another module with a stable interface (see Information hiding). With low coupling, a change in one module will not require a change in the implementation of another module. Low coupling is a sign of a well structured computer system.

However, in order to achieve maximum efficiency, a highly coupled system is probably needed. In modern computing systems, performance can often be traded for lower coupling; the gains in the software development process are greater than the value of the running performance gain.

Low-coupling / high-cohesion is a general goal to achieve when structuring computer programs, so that they are easier to understand and maintain.

The concepts are usually related: low coupling implies high cohesion and vice versa. In the field of object-oriented programming, the connection between classes tends to get lower (low coupling), if we group related methods of a class together (high cohesion).

In object-oriented programming, coupling is a measure of how strongly one class is connected to another.

Coupling is increased between two classes A and B if:

• A has an attribute that refers to (is of type) B.
• A calls on services of a B object.
• A has a method which references B (via return type or parameter).
• A is a subclass of (or implements) B.

Disadvantages of high coupling include:

• A change in one class forces a ripple of changes in other classes.
• Difficult to understand a class in isolation.
• Difficult to reuse or test a class because dependent class must also be included.

One measure to achieve low coupling is functional design: it limits the responsibilities of modules. Modules with single responsibilities usually need to communicate less with other modules, and this has the virtuous side-effect of reducing coupling and increasing cohesion in many cases.

Adapted from the Wikipedia article "Low coupling", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki