CouplingandCohesion CSE VI SEM

7/31/2019 CouplingandCohesion CSE VI SEM

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Coupling and Cohesion

Prof. ASHISH MISHRA


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Characteristics of Good Design

Component independence

High cohesion

Low coupling

Exception identification and handling

Fault prevention and fault tolerance


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Coupling: Degree of dependence

among components

No dependencies Loosely coupled-some dependencies

Highly coupled-many dependencies

High coupling makes modifying

parts of the system difficult, e.g.,

modifying a component affects all

the components to which the

component is connected.


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Range of Coupling

High Coupling

Loose

Low

Content

Common

Control

Stamp

Data

Uncoupled


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Content coupling

Definition: One component references

contents of another

Example:

Component directly modifies anothers data

Component refers to local data of another

component in terms of numerical

displacement

Component modifies anothers code, e.g.,

jumps into the middle of a routine


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Example of Content Coupling-1

Part of program handles lookup for customer.

When customer not found, component adds

customer by directly modifying the contents ofthe data structure containing customer data.


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Example of Content Coupling-2

Part of program handles lookup for customer.

When customer not found, component addscustomer by directly modifying the contents of

the data structure containing customer data.

Improvement:

When customer not found, component calls the

AddCustomer() method that is responsible formaintaining customer data.


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Common Coupling

Definition: Two components share data

Global data structures

Common blocks

Usually a poor design choice because

Lack of clear responsibility for the data

Reduces readability

Difficult to determine all the components that affecta data element (reduces maintainability)

Difficult to reuse components

Reduces ability to control data accesses


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Example-1

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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Example-2

Each source process writes directly to globaldata store. Each sink process reads directlyfrom global data store.

ImprovementData manager component is responsible fordata in data store. Processes send data to andrequest 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.

May be either good or bad, depending on

situation.

Bad when component must be aware of

internal structure and logic of another module

Good if parameters allow factoring and reuseof functionality


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Example

Acceptable: Module p calls module q and q

passes back flag that says it cannot complete

the task, then q is passing data

Not Acceptable: Module p calls module q and q

passes back flag that says it cannot complete

the task 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 have access

to the entire structure.

Requires second component to know how tomanipulate the data structure (e.g., needs to

know about implementation)

May be necessary due to efficiency factors: this

is a choice made by insightful designer, not lazy

programmer.


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Example-1

The print routine of the customer billing

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

address, and billing information.

Customer billing system


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Example-2

The print routine of the customer billing

accepts a customer data structure as anargument, parses it, and prints the name,address, and billing information.

ImprovementThe print routine takes the customer name,address, and billing information as anargument.

Customer Billing System


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Data Coupling

Definition: Two components are data

coupled if there are homogeneous data

items.

Every argument is 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.


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Key Idea in Object-Oriented

Programming Object-oriented designs tend to have low

coupling.


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Cohesion

Definition: 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 component are directed toward

and essential for performing the same task

High is good


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Range of Cohesion

High Cohesion

Low

FunctionalInformational

Sequential

Communicational

Procedural

Temporal

Logical

Coincidental


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Coincidental Cohesion

Definition: Parts of the component are only

related by their location in source code

Elements needed to achieve some

functionality are scattered throughout the

system.

Accidental

Worst form


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Example

Print next line

Reverse string of characters in second

argument

Add 7 to 5th argument

Convert 4th argument to float


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Logical Cohesion

Definition: Elements of component are

related logically and not functionally.

Several logically related elements are in

the same component and one of the

elements is selected by the client

component.


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Example-1

A component reads inputs from tape, disk,

and network. All the code for these

functions are in the same component.

Operations are related, but the functions aresignificantly different.


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Example-2

A component reads inputs from tape, disk,and network. All the code for thesefunctions are in the same component.Operations are related, but the functionsare significantly different.

Improvement

A device component has a read operation

that is overridden by sub-classcomponents. The tape sub-class readsfrom tape. The disk sub-class reads fromdisk. The network sub-class reads from thenetwork.


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Temporal Cohesion

Definition: Elements of a component are

related by timing.

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.


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Example-1

A system initialization routine: this routine

contains all of the code for initializing all of the

parts of the system. Lots of different activities

occur, all at init time.


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Example-2

A system initialization routine: this routinecontains all of the code for initializing all ofthe parts of the system. Lots of different

activities occur, all at init time.

Improvement

A system initialization routine sends an

initialization message to each component. Each component initializes itself at

component instantiation time.


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Procedural Cohesion

Definition: Elements of a component are

related only to ensure a particular order of

execution.

Actions are still weakly connected and

unlikely to be reusable


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Example

...

Read part number from

data base

update repair record onmaintenance file.

...

May be useful toabstract the intent ofthis sequence. Makethe data base and

repair recordcomponents handlereading and updating.Make component that

handles moreabstract operation.


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Communicational Cohesion

Definition: Module 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


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Example

Update record in data

base and send it to

the printer.

database.Update

(record).

record.Print().


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Sequential Cohesion

The output of one component is the input to

another.

Occurs naturally in functional programming

languages

Good situation


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Informational Cohesion

Definition: Module 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 single entry and single exit

In logical cohesion, actions of module intertwined ADT and object-oriented paradigm promote


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Functional Cohesion

Definition: Every essential element to a

single computation is contained in the

component.

Every element in the component is

essential to the computation.

Ideal situation.


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Examples of Cohesion-1

Function A

Function

BFunction

D

Function

CFunction

E

Coincidental

Parts unrelated

Function A

Function A

Function A

logic

Logical

Similar functions

Time t0

Time t0 + X

Time t0 + 2X

Temporal

Related by time

Function A

Function B

Function C

Procedural

Related by order of functions


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Examples of Cohesion-2

Function A part 1

Function A part 2

Function A part 3

Functional

Sequential with complete, related functions

Function A

Function B

Function C

Communicational

Access same data

Function A

Function B

Function C

Sequential

Output of one is input to another


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p

q

t

r s

u

12

3 4

5 6

1 Aircraft type Status flag

2 ----- List of aircraft

parts3 Function code -----

4 ----- List of aircraft

parts5 Part number Part manufacturer

6 Part number 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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q r s t u

p

q --- ---

r --- ---

s ---

t

Coupling between pairs of modules


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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


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In Class

P1: What is the effect of cohesion on

maintenance?

P2: What is the effect of coupling on

maintenance?

P3: Produce an example of each type of

cohesion. Justify your answers.

P4: Produce an example of each type of

coupling. Justify your answers.

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CouplingandCohesion CSE VI SEM