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Can explain coupling

Coupling is a measure of the degree of dependence between components, classes, methods, etc. Low coupling indicates that a component is less dependent on other components. High coupling (aka tight coupling or strong coupling) is discouraged due to the following disadvantages:

  • Maintenance is harder because a change in one module could cause changes in other modules coupled to it (i.e. a ripple effect).
  • Integration is harder because multiple components coupled with each other have to be integrated at the same time.
  • Testing and reuse of the module is harder due to its dependence on other modules.

In the example below, design A appears to have more coupling between the components than design B.



Can reduce coupling

X is coupled to Y if a change to Y can potentially require a change in X.

If the Foo class calls the method Bar#read(), Foo is coupled to Bar because a change to Bar can potentially (but not always) require a change in the Foo class e.g. if the signature of Bar#read() is changed, Foo needs to change as well, but a change to the Bar#write() method may not require a change in the Foo class because Foo does not call Bar#write().

code for the above example

Some examples of coupling: A is coupled to B if,

  • A has access to the internal structure of B (this results in a very high level of coupling)
  • A and B depend on the same global variable
  • A calls B
  • A receives an object of B as a parameter or a return value
  • A inherits from B
  • A and B are required to follow the same data format or communication protocol


Types of coupling

Can identify types of coupling

Some examples of different coupling types:

  • Content coupling: one module modifies or relies on the internal workings of another module e.g., accessing local data of another module
  • Common/Global coupling: two modules share the same global data
  • Control coupling: one module controlling the flow of another, by passing it information on what to do e.g., passing a flag
  • Data coupling: one module sharing data with another module e.g. via passing parameters
  • External coupling: two modules share an externally imposed convention e.g., data formats, communication protocols, device interfaces.
  • Subclass coupling: a class inherits from another class. Note that a child class is coupled to the parent class but not the other way around.
  • Temporal coupling: two actions are bundled together just because they happen to occur at the same time e.g. extracting a contiguous block of code as a method although the code block contains statements unrelated to each other