Coupling CohesionChandan R. RupakhetiSteve Chenoweth(Chapter 18)

Outline• Coupling and Cohesion• Command and Query Separation Principle

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Types of Coupling• Tight to loose coupling• Content: Module A has access to local data of Module B• Common: Global variable shared between two modules• External: Share an externally imposed data format or

communication protocol• Control: Module A controls the control flow of module B through

flags• Stamp: Modules share a composite data structure but uses only a

part of it• Data: Modules share data through parameters• Message: Modules communicate via message passing (e.g.

Observer)• No Coupling: Modules do not communicate at all 3

Types of Cohesion• Worst to best cohesion• Coincidental: When parts of a module is grouped arbitrarily, e.g.

Utilities class• Logical: Grouped because they are logically categorized to do the

same thing, e.g. MouseListener• Temporal: Grouped at runtime, e.g. exception processing function

that does multiple task, e.g. close file, log error, and notify users• Procedural: Grouped because they follow a certain sequence of

execution, e.g. function which checks file permission and opens it• Communicational: Grouped because they operate on the same

data• Sequential: Grouped because the output from one part is the

input to another part• Functional: Grouped because they all contribute to a single well-

defined task of the module

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Command-Query Separation Principle• Each method should be either a command or a query• Command Method• Performs an action, typically with side effects, but has no return

value• Query Method• Returns data but has no side effects

• Why is this principle important?• Principle of least surprise, side effects only happen in “void”

methods• Provides for most flexible interface, e.g. a value can be queried

multiple times without changing it

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Things to Do Next• Look at the quiz for today, and the zipped program.

• Answer the related questions about coupling!

• Answer the last question about cohesion!

• Turn in after class, as usual, but please also be ready to discuss your answers in class!

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