1. Cohesion and Coupling
Chapter 5, Pfleeger
01/01/10

2. Coupling The amount of interaction among components in a system
What is it?

3. Indications of Coupling

4. Coupling Types Content coupling Common coupling External coupling
Control coupling
Stamp coupling
Data coupling
Uncoupled
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5. Content and Common Coupling
The components share data via direct access to data stores:
One component depends on the internal workings of another:
Usually a poor design choice because
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6. External Coupling Two components share something externally imposed
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7. Control Coupling
Component passes control parameters to coupled components.
May be either good or bad, depending on situation.
Bad if parameters indicate completely different behavior
Good if parameters allow factoring and reuse of functionality
Example of good: Sort that takes a comparison function as an argument.
The sort function is clearly defined: return a list in sorted order, where sorted is determined by a parameter.

8. Stamp Coupling Component A passes a data structure to component B
B only uses part of the data
Requires B to know how to manipulate the data structure (eg needs to know about implementation)
B has access to more information that it needs
Be sure you know the difference between the insightful designer and the lazy programmer.
Know what tradeoffs you are making in your design.

9. Data Coupling Component A passes data to component B
Not the same as Stamp Coupling:
Data (a representation), not a data structure (an implementation).

10. Example
Customer billing system
The print routine takes the customer name, address, and billing information as arguments.

11. Uncoupled Completely uncoupled components are not systems.
Systems are made of interacting components.

12. Key Idea in Object-Oriented Programming

13. Consequences of Coupling

14. Coupling between pairs of modulesQ R S T U p q r s t

15. Cohesion
The degree to which all elements of a component are directed towards a single task
The degree to which all elements directed towards a task are contained in a single component
The degree to which all responsibilities of a single class are related
Note: We are talking about the contents of a single component.
What is it

16. Types of Cohesion: low to high
Coincidental low
Logical
Temporal
Procedural
Communicational
Sequential
Functional high
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17. Coincidental Cohesion
Accidental
Worst form

18. Logical Cohesion
Several logically related elements are in the same component.
Not related functionally, but related logically.

19. Example A component reads inputs from tape, disk, and network.
All the code for these functions are in the same component.
Operations are related logically, but the functions are significantly different.

20. Temporal Cohesion Elements of a component are related by timing
Elements are grouped by when they are processed

21. Procedural Cohesion
Elements of a component are related only to ensure a particular order of execution
Common to pass partial results

22. Communicational Cohesion
Elements of a component are related by the data that they process
Component performs a series of actions related by a sequence of steps to be followed. All actions are performed on the same data.

23. Example Update record in data base and send it to the printer.
database.Update (record).
record.Print().
I don’t even want to think about it.

24. Sequential Cohesion
The output of one component is the input to another.
Good situation
Data flows between components
(different from procedural cohesion)
Occurs naturally in functional programming languages

25. Informational (Data) Cohesion
Module performs a number of actions, each with its own entry point, with independent code for each action, all performed on the same data.
Difference from logical cohesion

26. Functional Cohesion
Every essential element to a computation is contained in the component.
Every element in the component is essential to the computation.
Ideal situation.

27. Techniques for Improving Design
Increase cohesion
Design by contract
Prototype design

28. Problem: Classify cohesion for each module
Compute average daily temperatures at various sites
Initialize sums and open files
Create new temperature record
Store temperature record
Close files and print average temperatures
Read in site, time, and temperature
Store record for specific site
Edit site, time, or temperature field