couplingandcohesion-student (1).ppt

7/24/2019 CouplingandCohesion-student (1).ppt

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Coupling and Cohesion

Pfleeger, S., Software Engineering Theory

and Practice. Prentice Hall, 2001.


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Characteristics of Good Design

Component independence

High cohesion

Low coupling

Eception identification and handling

!ault pre"ention and fault tolerance


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Coupling# Degree of dependence

among components

$o dependencies Loosel% coupled&some dependencies

Highl% coupled&man% dependencies

High coupling ma'es modif%ingparts of the s%stem difficult, e.g.,

modif%ing a component affects all

the components to which the

component is connected.


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(ange of Coupling

High Coupling

Loose

Low

Content

Common

Control

Stamp

Data

Uncoupled


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Content coupling

Definition# )ne component references

contents of another

Eample# Component directl% modifies another*s data

Component refers to local data of another

component in terms of numerical

displacement

Component modifies another*s code, e.g.,

+umps into the middle of a routine


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Eample of Content Coupling&

Part of program handles loo'up for customer.

-hen customer not found, component adds

customer % directl% modif%ing the contents ofthe data structure containing customer data.


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Eample of Content Coupling&/

Part of program handles loo'up for customer.

-hen customer not found, component addscustomer % directl% modif%ing the contents of

the data structure containing customer data.

0mpro"ement#

-hen customer not found, component calls the1ddCustomer23 method that is responsile formaintaining customer data.


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Common Coupling

Definition# 4wo components share data

Gloal data structures

Common loc's

5suall% a poor design choice ecause

Lac' of clear responsiilit% for the data

(educes readailit%

Difficult to determine all the components that affecta data element 2reduces maintainailit%3

Difficult to reuse components

(educes ailit% to control data accesses


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Eample&

Each source process writes directly to global data

store. Each sink process reads directly from global

data store.

Process control component maintains current data

about state of operation. Gets data from multiple

sources. Supplies data to multiple sinks.


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Eample&/

Each source process writes directl% to gloal datastore. Each sin' process reads directl% fromgloal data store.

0mpro"ement Data manager component is responsile for datain data store. Processes send data to andre6uest data from data manager.

Process control component maintains current data

about state of operation. Gets data from multiple

sources. Supplies data to multiple sinks.


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Control Coupling

Definition# Component passes control

parameters to coupled components.

7a% e either good or ad, depending on

situation.

8ad when component must e aware of

internal structure and logic of another module

Good if parameters allow factoring and reuse

of functionalit%


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Eample

1cceptale# 7odule p calls module 6 and 6

passes ac' flag that sa%s it cannot complete

the tas', then 6 is passing data

$ot 1cceptale#7odule p calls module 6 and 6

passes ac' flag that sa%s it cannot complete

the tas' and, as a result, writes a specific

message.


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Stamp Coupling

Definition# Component passes a data structure

to another component that does not ha"e

access to the entire structure.

(e6uires second component to 'now how tomanipulate the data structure 2e.g., needs to

'now aout implementation3

7a% e necessar% due to efficienc% factors# this

is a choice made % insightful designer, not la9%

programmer.


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Eample&

4he print routine of the customer illing

accepts a customer data structure as anargument, parses it, and prints the name,

address, and illing information.

Customer billing system


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Eample&/

4he print routine of the customer illing accepts a

customer data structure as an argument, parses it,and prints the name, address, and illinginformation.

0mpro"ement4he print routine ta'es the customer name,address, and illing information as an argument.

Customer Billing System


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Data Coupling

Definition# 4wo components are data coupled

if there are homogeneous data items.

E"er% argument is simple argument or data

structure in which all elements are used

Good, if it can e achie"ed.

Eas% to write contracts for this and modif%

component independentl%.


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:e% 0dea in )+ect&)riented

Programming )+ect&oriented designs tend to ha"e low

coupling.


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Cohesion

Definition# 4he degree to which all elements of a

component are directed towards a single tas'

and all elements directed towards that tas' are

contained in a single component. 0nternal glue with which component is

constructed

1ll elements of component are directed toward

and essential for performing the same tas'

High is good


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(ange of Cohesion

High Cohesion

Low

Functional

Informational

Sequential

Communicational

Procedural

Temporal

Logical

Coincidental


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Coincidental Cohesion

Definition# Parts of the component are onl%

related % their location in source code

Elements needed to achie"e some

functionalit% are scattered throughout the

s%stem.

1ccidental

-orst form


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Eample

Print net line

(e"erse string of characters in second

argument

1dd ; to


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Logical Cohesion

Definition# Elements of component are

related logicall% and not functionall%.

Se"eral logicall% related elements are in

the same component and one of the

elements is selected % the client

component.


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Eample&

1 component reads inputs from tape, dis',

and networ'. 1ll the code for these

functions are in the same component.

)perations are related, ut the functions aresignificantl% different.


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Eample&/

1 component reads inputs from tape, dis', andnetwor'. 1ll the code for these functions are inthe same component. )perations are related,ut the functions are significantl% different.

0mpro"ement

1 de"ice component has a read operation thatis o"erridden % su&class components. 4hetape su&class reads from tape. 4he dis' su&

class reads from dis'. 4he networ' su&classreads from the networ'.


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4emporal Cohesion

Definition# Elements of a component are

related % timing.

Difficult to change ecause %ou ma% ha"e

to loo' at numerous components when a

change in a data structure is made.

0ncreases chances of regression fault

Component unli'el% to e reusale.


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Eample&

1 s%stem initiali9ation routine# this routine

contains all of the code for initiali9ing all of the

parts of the s%stem. Lots of different acti"ities

occur, all at init time.


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Eample&/

1 s%stem initiali9ation routine# this routinecontains all of the code for initiali9ing all ofthe parts of the s%stem. Lots of different

acti"ities occur, all at init time.

0mpro"ement

1 s%stem initiali9ation routine sends aninitiali9ation message to each component.

Each component initiali9es itself atcomponent instantiation time.


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Procedural Cohesion

Definition# Elements of a component are

related onl% to ensure a particular order of

eecution.

1ctions are still wea'l% connected and

unli'el% to e reusale


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Eample

...

(ead part numer from

data ase

update repair record onmaintenance file.

...

7a% e useful toastract the intent ofthis se6uence. 7a'ethe data ase andrepair recordcomponents handlereading and updating.7a'e component that

handles moreastract operation.


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Communicational Cohesion

Definition# 7odule performs a series of

actions related % a se6uence of steps to

e followed % the product and all actions

are performed on the same data


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Eample

5pdate record in data

ase and send it to

the printer.

dataase.5pdate

2record3.

record.Print23.


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Se6uential Cohesion

4he output of one component is the input

to another.

)ccurs naturall% in functional programming

languages

Good situation


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0nformational Cohesion

Definition# 7odule performs a numer of

actions, each with its own entr% point, with

independent code for each action, all

performed on the same data. Different from logical cohesion

Each piece of code has single entr% and single eit

0n logical cohesion, actions of module intertwined 1D4 and o+ect&oriented paradigm promote


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!unctional Cohesion

Definition# E"er% essential element to a

single computation is contained in the

component.

E"er% element in the component is

essential to the computation.

0deal situation.


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Eamples of Cohesion&

!unction 1

!unction

8!unction

D

!unction

C!unction

E

Coincidental

Parts unrelated

!unction 1

!unction 1*

!unction 1**

logic

Logical

Similar functions

4ime t>

4ime t> ? @

4ime t> ? /@

4emporal

(elated % time

!unction 1

!unction 8

!unction C

Procedural

(elated % order of functions


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Eamples of Cohesion&/

!unction 1 part

!unction 1 part /

!unction 1 part A

!unctional

Se6uential with complete, related functions

!unction 1

!unction 8

!unction C

Communicational

1ccess same data

!unction 1

!unction 8

!unction C

Se6uential

)utput of one is input to another


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p

q

t

r s

u

1

! "

# $

1ircraft t%pe Status flag

/ &&&&& List of aircraft

partsA !unction code &&&&&

= &&&&& List of aircraftparts

< Part numer Part manufacturer

B Part numer Part name

No. In Out

Interface Description

p, t, u access the same

database in update

mode

Problem: Define coupling between pairs of modules.


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6 r s t u

p

6 &&& &&&

r &&& &&&

s &&&

t

Coupling between pairs of modules


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Prolem# Classif% cohesion for

each module Compute a"erage dail% temperatures at "arious

sites

0nitiali9e sums and open files

Create new temperature record Store temperature record

Close files and print a"erage temperatures

(ead in site, time, and temperature

Store record for specific site

Edit site, time, or temperature field


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0n Class

P# -hat is the effect of cohesion on

maintenance

P/# -hat is the effect of coupling on

maintenance

PA# Produce an eample of each t%pe of

cohesion. ustif% %our answers.

P=# Produce an eample of each t%pe of

coupling. ustif% %our answers.

couplingandcohesion-student (1).ppt

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