LOW COUPLING – HIGH COHESION
LOW COUPLING
Low Coupling is an evaluative pattern, which dictates how to assign responsibilities to
support:
 Lower dependency between the classes,
 Change in one class having lower impact on other classes,
 Higher reuse potential.
 Coupling is a measure of how strongly one element is connected to, has knowledge of, or
relies upon other elements. An element with Low Coupling is not dependent upon too
many other elements. But how many is too many?
 High coupling is not really the problem; it is high coupling to potentially unstable (likely
to be changed) objects.
 Problems with high coupling
Forced local changes because of changes in related classes
Harder to understand in isolation
Harder to reuse because it requires other classes
 Assign responsibility so coupling remains low
 This reduces the impact of change
NextGen example
 If we need to create a Payment instance and associate it with Sale, who should do it?
Since Register records a payment, the Creator pattern suggests it
 On the other hand, Sale could create Payment and that would not increase the coupling
 In practice, level of coupling alone is not sufficient
Forms of Coupling
 X has an attribute of type Y
 A type X object calls on services in Y
 X has a method that references an instance of Y
 X is a direct or indirect subclass of Y
 Y is an interface that X implements
HIGH COHESION
High Cohesion is an evaluative pattern that attempts to keep objects appropriately
focused, manageable and understandable. High cohesion is generally used in support of Low
Coupling. High cohesion means that the responsibilities of a given element are strongly related
and highly focused. Breaking programs into classes and subsystems is an example of activities
that increase the cohesive properties of a system. Alternatively, low cohesion is a situation in
which a given element has too many unrelated responsibilities. Elements with low cohesion often
suffer from being hard to comprehend, hard to reuse, hard to maintain and averse to change.