1. Coupling and Cohesion 1

2. Desired Class/Object Interaction
Maximize internal interaction (Cohesion)
Easier to understand
Easier to test
Minimize external interaction (Coupling)
Can be used independently
Easier to replace

3. Characteristics of Good Design
Component independence
High Cohesion
Low Coupling

4. What is Coupling?
Coupling is a measure of the independence of components
Coupling is related to Cohesion
It is an indication of the strength of the inter-connections between the components in a design

5. Highly Coupled
These types of systems have multiple inter-connections with components dependent upon each other
Component A
Component B
Component C
Component D

6. Loosely Coupled
Loosely coupled systems are made up of components which are independent or almost independent
Component A
Component B
Component C
Component D

7. Uncoupled Uncoupled components have NO interconnections
No dependencies
Component A
Component B
Component C
Component D

8. The range of coupling measures
High Coupling
Content Coupling
Common Coupling
Control Coupling
Stamp Coupling
Loose Coupling
Data Coupling
Uncoupled
Low Coupling

9. Content Coupling
A component directly references (shares) the content of another module
Component p modifies a statement of component q
Component p refers to local data of component q
Content coupled components are inextricably interlinked
Changes to component p requires a change to component q (including recompilation)
Reusing component p requires using component q also

10. Common Coupling Using global variable (i.e. global coupling)
All components have read/write access to a global data block
Components exchange data using the global data block (instead of arguments)
A single component with write access where all other components have read access is not Common Coupling

11. Common Coupling (cont.)
Have to look at many components to determine the current state of a variable
Side-effects require looking at all of the code in a function to see if there are any global effects
Changes in one component to the declaration requires changes in all other components
Identical list of global variables must be declared for component to be reused
Component is exposed to more data than is needed

12. Control Coupling
A component passes control parameters to coupled components
May be good or bad depending upon the situation
Bad when the component must be aware of internal structure and logic of another component
Good if parameters allow factoring and reuse of functionality

13. Control Coupling Example
Acceptable
A sort function that accepts a comparison function as an argument
Component p calls component q and q returns a flag indicating an error (if any) occurred
Unacceptable
Component p calls component q and q returns a string indicating the error (if any) that occurred

14. Stamp Coupling
A component passes a data structure to another component that does not have access to the entire structure
Requires a second component to know how to manipulate the data structure (e.g., needs to know about the implementation)
May be necessary due to efficiency factors: this is a choice made by insightful designers, not lazy programmer

15. Stamp Coupling Example
Address verification
Address verification accepts a Customer data structure as an argument, “parses it” and verifies the address
bool VerifyAddress (Customer customer) {…}
Better
bool VerifyAddress (
String address1,
String address2,
String city,
String state,
String zipcode) {…}

16. Data Coupling
Two components are data coupled if there are homogeneous data items
Every argument is a simple argument or data structure in which all elements are used
Good, if it can be achieved
Easy to write contracts for this and modify component independently

17. Cohesion
The degree to which all elements of a component are directed towards a single task and all elements directed towards that task are contained in a single component
Internal glue with which component is constructed
All elements of a component are directed toward and essential for performing the same task
High is good

18. The range of Cohesion Functional High Cohesion Informational
Sequential
Communicational
Procedural
Temporal
Logical
Coincidental
Low Cohesion

19. Coincidental Cohesion
Parts of the component performs multiple, completely unrelated actions
May be based on factors outside of the design
Skillset or interest of developers
Avoidance of small components
No reusability
Difficult corrective maintenance or enhancement
Elements needed to achieve some functionality are scattered throughout the system
Example: a “Utilities” class

20. Logical Cohesion
Elements of components are related logically and not functionally
Several logically related elements are in the same component and one of the elements is selected by the caller
May include both high and low-level actions in the same class
May include unused parameters for certain uses
Interface is difficult to understand
In order to do something you have to wade through a lot of unrelated possible actions
Example: grouping all output (print, export) routines

21. Temporal Cohesion
Elements of a component are related by timing – grouped together and used during the same phase of execution
Difficult to change because you may have to look at numerous components when a change in a data structure is made
Increases chances of regression fault
Component unlikely to be reusable
Often happens in initialization or shutdown
Example: a function which is called after catching an exception which closes an open file, creates an error log and notifies the user

22. Procedural Cohesion
Elements of a component are related only to ensure a particular order of execution – procedures (methods) that are called one after another are kept together
Actions are still weakly connected and unlikely to be reusable
Changes to the ordering of steps or purpose of steps requires changing the component abstraction
Example: a function which checks file permissions and then opens the file

23. Communicational Cohesion
Component performs a series of actions related by a sequence of steps to be followed by the product and all actions are performed on the same data – procedures that access the same data are kept together
Action based on the ordering of steps on all the same data
Actions are related but still not completely separated
Module cannot be reused

24. Sequential Cohesion
Methods are together in a class because the output from one part is the input to another part
Occurs naturally in functional programming
Example: a function which reads data from a file and processed the data

25. Informational Cohesion
Component performs a number of actions, each with its own entry point, with independent code for each action, all performed on the same data
Different from logical cohesion
Each piece of code has a single entry and single exit
In logical cohesion, actions of module intertwined

26. Functional Cohesion
Every essential element to a single computation is contained in the component
Every element in the component is essential to the computation
Example: tokenizing a string of XML

27. ?

28. References Chawla, Jagnesh. “Cohesion & Coupling”
Pfleeger, S. “Software Engineering Theory and Practice” Prentice Hall 2001