Think Pair Share Think Pair Share ActivityActivity

Think Pair Share Think Pair Share ActivityActivity

Course InstructorPreeti Mishra

Cohesion and CouplingCohesion and Coupling Cohesion and CouplingCohesion and Coupling

About TPS

• For a given topic “The class needs to” :1. Think/ explore/find2. Pair up with similar topic people and

discuss your ideas3. Share the conclusions in class

• This exercise is worth 10 points and will be accordingly counted in your term work!!

So lets Begin…So lets Begin…So lets Begin…So lets Begin…

Some Design Goals

• Extension– facilitate adding features

• Change– facilitate changing requirements

• Simplicity – make easy to understand – make easy to implement

• Efficiency– attain high speed: execution and/or compilation – attain low size: runtime and/or code base

Coupling

• The degree of interdependence between two modules”

• We aim to minimise coupling - to make modules as independent as possible Low coupling can be achieve by:– eliminating unnecessary relationships– reducing the number of necessary relationships– easeing the ‘tightness’ of necessary relationships

Definition of Coupling

• The fewer the connection, the less chance there is for the ripple effect.

– Ideally, changing one component should not require a change in another.

– Should not have to understand details of one component in order to change another (c.f.principle of locality).

Coupling Principles

• Create narrow connections• Create direct Connections• Create local connections• Create obvious connections• Create flexible connections

Narrow (vs. Broad) Connections

• The breadth of connections is number of connections that link two components.

– A connection that passes 15 pieces of information from one component to another is broader than a connection in which only two are passed.

– The greater the number of parameters in a method call the broader the connection

Direct vs. Indirect Connections

• A direct connection is one in which the interface between two components can be understood without having to refer to other pieces of information.– A method definition including pre- and postconditions has a

more direct interface than one that does not.

Local vs. Remote Connections

• Local Connection– All information required to understand the connection is

present in the connection itself.• Remote Connection

– Coupled through global data.– May be hundreds of lines removed from a callpoint.– May be remote in time.

Flexible Connections

• Systems evolve over time.

– information being passed changes.– May want to switch to a different logical orphysical devices

Types of Coupling

1. Data coupling       (Most Required)                          

2. Stamp coupling 

3. Control coupling 

4. Hybrid coupling 

5. Common coupling 

6. Content coupling  (Least Required)

Normal Coupling

• Two components, A and B, are normally coupled if– A calls B– B returns to A– All information passed between them is by parameters

in the call (or a return).• Types of Normal Coupling

– Data Coupling– Stamp Coupling– Control Coupling

Data Coupling

• Keep interfaces as narrow as possible.

• Avoid tramp data:– Data passed from component to component thatis unwanted and meaningless to most.

Stamp coupling

• A composite data is passed between modules 

• Internal structure contains data not used 

• Bundling - grouping of unrelated data into an artificial structure

Stamp coupling warnings

Do not pass aggregate structures containing many fields when only one or two are needed by callee.– You do not want to couple caller to the structure of the

aggregate.– Creates dependencies between otherwise unrelated components.

• Do not bundle unrelated data.

Control coupling

• A module controls the logic of another module through the parameter 

• Controlling module needs to know how the other module works - not flexible

Common coupling

Use of global data as communication between modules  Common (Global) Coupling

• Two components are common coupled if they refer to the same global data area.

• This is an undesirable form of coupling. • Defect in one component using common global data can affect

others.• Introduces remoteness (with respect to time).Dangers of:

ripple effectinflexibility

 difficult to understand the use of data

Content coupling

Two components exhibit content coupling ifone refers to the internals of the other in any

way.• This form of coupling is unacceptable. It forces you

to understand the semantics implemented in another component

• Hybrid coupling• A subset of data used as control

 • Example: account numbers 00001 to 99999

• If 90000 - 90999, send mail to area code of last 3 digit

(000 - 999)

Determine Coupling Type

• Ask:– Can programmers work independently? To what

degree?– What may change that may cause changes to

multiple components?• A pair of components may be coupled in more than

one way:– Their coupling is determined by their worst type that they

exhibit.

“Good” vs. “bad” coupling

• Modules that are loosely coupled (or uncoupled) are better than those that are tightly coupled

• Why? Because of the objective of modules to help with human limitations

• The more tightly coupled are two modules, the harder it is to think about them separately, and thus the benefits become more limited

Effects of Object Technology on Coupling

• Object Technology adds additional relationships that often result in increased coupling.

• Proper application of design principles can reduce the complexity often found in procedural programs.– E.g. effective use of access control can eliminate common

coupling.

Cohesion

• “The measure of the strength of functional relatedness of elements within a module” 

• Elements: instructions, groups of instructions, data definition, call of another module                                                         

• We aim for strongly cohesive modules• Everything in module should be related to one

another - focus on the task  • Strong cohesion will reduce relations between

modules - minimise coupling

Seven levels of cohesion1. • Coincidental (Worst Maintainability)2. • Logical3. • Temporal4. • Procedural5. • Communicational6. • Sequential7. • Functional (Best Maintainability)

Type of cohesion no.

Assigned to roll no.

1 12,39

2 15,38

3 17,37

4 21,36,40

5 2434,

6 26,31

7 27,30

Functional cohesion

• All elements contribute to the execution of one and only one problem-related task 

• Focussed - strong, single-minded purpose 

• No elements doing unrelated activities

• Examples of functional cohesive modules:– Compute cosine of angle

 – Read transaction record

 – Assign seat to airline passenger

Sequential cohesion

• Elements are involved in activities such that output data from one activity becomes input data to the next 

• Usually has good coupling and is easily maintained • Not so readily reusable because activities that will not

in general be useful together • Example of Sequential Cohesion 

– module format and cross-validate record –    use raw record –     format raw record –    cross-validate fields in raw record –    return formatted cross-validated record

•                  

Communicational Cohesion

• Elements contribute to activities that use the same input or output data 

• Not flexible, for example, if we need to focus on some activities and not the others 

• Possible links that cause activities to affect each other 

• Better to split to functional cohesive ones 

Procedural cohesion

• Elements are related only by sequence, otherwise the activities are unrelated 

• Similar to sequential cohesion, except for the fact that elements are unrelated 

• Commonly found at the top of hierarchy, such as the main program module 

Temporal cohesion

• Elements are involved in activities that are related in time 

• Commonly found in initialisation and termination modules 

• Elements are basically unrelated, so the module will be difficult to reuse 

• Good practice is to initialise as late as possible and terminate as early as possible 

Logical cohesion

• Elements contribute to activities of the same general category (type) 

• For example, a report module, display module or I/O module 

• Usually have control coupling, since one of the activities will be selected 

Coincidental cohesion

• Elements contribute to activities with no meaningful relationship to one another 

• Similar to logical cohesion, except the activities may not even be the same type 

• Mixture of activities - like ‘rojak’! 

• Difficult to understand and maintain, with strong possibilities of causing ‘side effects’ every time the module is modified

“Good” vs. “bad” cohesion

• Bergland talks about different levels of cohesion– Best: functional, where the elements collectively

provide a specific behavior or related behaviors– Worst: coincidental, where the elements are

collected for no reason at all

• Many other levels in between• Cohesion is not measurable quantitatively

Cohesion and Coupling

1

2 3 4

5 6High cohesion Low couplingBridge

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Cohesion and Coupling

1

2 3 4

5 6High cohesion Low coupling

High coupling

Bridge

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