CPSC 3720 - DAY 12
FEBRUARY 13, 2017

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COUPLING
--------

	.One component modifies another.
	.Example:
		.Component directly modifies another's data
		.Component modifies another's code, e.g., jumps (goto) into
		the middle of a routine
	.Example:
		.Program looks up customer info
		.Can't find it
		.Adds customer directly to the data structure.

	
Common Coupling
	.More than one component share data such as global data structures.
	.Usually a poor design choice because:
		.Lack of clear responsibility for the data
		.Reduces readability
		.Difficult to determine all the components that affect a data
		element (reduces maintainability)
		.Difficult to reuse components
		.Reduces ability to control data access.

External Coupling
	Two components share something externally imposed
		External file
		Device interface
		Protocol
		Data format

Control Coupling
	Component passes control parameters to coupled components.
		ex. flags in UNIX
	May be either good or bad, depending on situation
		Bad if the parameter indicates completely different behavior
		Good if parameters allow factoring and reuse of functionality
	Good example: 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.

Stamp Coupling

	Component passes a data structure to another component that does not
	have access to the entire structure.  
	Requires second component to know how to manipulate the data structure
	(e.g. needs to know about implementation)
	The second has access to more information than it needs.
	May be necessary due to efficiency factors.

	Example: Customer Billing System

	The print routine of the customer billing system accepts customer data
	structure as an argument, parses it, and prints the name, address, and
	billing information.

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

Cohesion

	A number of dependencies within the system.
	A subsystem contains many related objects and task
		Cohesion is high
	A subsystem contains many unrelated objects
		Cohesion is low
	We want HIGH cohesion
		Often contradicts with low coupling.