SDSU CS 635 Advanced Object-Oriented Design & Programming
Spring Semester, 2002
Cohesion
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San Diego State University -- This page last updated 28-Mar-02

References

Object Coupling and Object Cohesion, chapter 7 of Essays on Object-Oriented Software Engineering , Vol 1, Berard, Prentice-Hall, 1993,


Doc 11, Cohesion Slide # 2

Cohesion


"Cohesion is the degree to which the tasks performed by a single module are functionally related."

IEEE, 1983


"Cohesion is the "glue" that holds a module together. It can be thought of as the type of association among the component elements of a module. Generally, one wants the highest level of cohesion possible."

Bergland, 1981

"A software component is said to exhibit a high degree of cohesion if the elements in that unit exhibit a high degree of functional relatedness. This means that each element in the program unit should be essential for that unit to achieve its purpose."

Sommerville, 1989


Doc 11, Cohesion Slide # 3
Types of Module CohesionFrom Worst to Best


Coincidental (worst)

Logical

Temporal

Procedural

Communication

Sequential

Functional (best)



Doc 11, Cohesion Slide # 4

Module Cohesion

Coincidental


Little or no constructive relationship among the elements of the module

Common Object Occurrence:

Object does not represent any single object-oriented concept
Collection of commonly used source code as a class inherited via multiple inheritance


class Rous
   {
   public static int findPattern( String text, String pattern)
      { // blah}
   public static int average( Vector numbers )
      { // blah}
   public static OutputStream openFile( String fileName )
      { // blah}
      
   }


Doc 11, Cohesion Slide # 5
Module Cohesion

Logical


Module performs a set of related functions, one of which is selected via function parameter when calling the module

Similar to control coupling

Cure:

Isolate each function into separate operations

public void sample( int flag )
   {
   switch ( flag )
      {
      case ON:
         // bunch of on stuff
         break;
      case OFF:
         // bunch of off stuff
         break;
      case CLOSE:
         // bunch of close stuff
         break;
      case COLOR:
         // bunch of color stuff
         break;
      }
   }


Doc 11, Cohesion Slide # 6
Module Cohesion

Temporal


Elements are grouped into a module because they are all processed within the same limited time period

Common example:

"Initialization" modules that provide default values for objects

"End of Job" modules that clean up

procedure initializeData()
   {
   font = "times";
   windowSize = "200,400";
   foo.name = "Not Set";
   foo.size = 12;
   foo.location = "/usr/local/lib/java";
   }

Cure: Each object should have a constructor and destructor

class foo
   {
   public foo()
      {
      foo.name = "Not Set";
      foo.size = 12;
      foo.location = "/usr/local/lib/java";
      }
   }

Doc 11, Cohesion Slide # 7
Sample Configuration File

[Macintosh]
[General]
EquationWindow=146,171,406,661
Zoom=200
SpacingWindow=0,0,0,0
CustomZoom=150

ShowAll=0

Version=2.01
[Spacing]
OptimalPrinter=1
LineSpacing=150%
MinRect=0
MatrixRowSpacing=150%
ForceOpen=0
MatrixColSpacing=100%
ToolbarDocked=1
SuperscriptHeight=45%
ToolbarShown=1
SubscriptDepth=25%
ToolbarDockPos=1
LimHeight=25%

LimDepth=100%
[Fonts]
LimLineSpacing=100%
Text=Times
NumerHeight=35%
Function=Times
DenomDepth=100%
Variable=Times,I
FractBarOver=1pt
LCGreek=Symbol,I
FractBarThick=0.5pt
UCGreek=Symbol
SubFractBarThick=0.25pt
Symbol=Symbol
FenceOver=1pt
Vector=Times,B
SpacingFactor=100%
Number=Times
MinGap=8%

RadicalGap=2pt
[Sizes]
EmbellGap=1.5pt
Full=12pt
PrimeHeight=45%
Script=7pt

ScriptScript=5pt

Symbol=18pt

SubSymbol=12pt


Call these constructors/destructors from a nonobject-oriented routine that performs a single, cohesive task



Doc 11, Cohesion Slide # 8
Module Cohesion

Procedural


Associates processing elements on the basis of their procedural or algorithmic relationships


Procedural modules are application specific

In context the module seems reasonable

Removed from the context these modules seem strange and very hard to understand

Why is that being done here?

Can not understand module without understanding the program and the conditions existing when module is called

Makes module hard to modify, understand

Class Builder verse Program writer

Cure:

Redesign the system
If a module is necessary, remove it from objects



Doc 11, Cohesion Slide # 9
Module Cohesion

Communication


Operations of a module all operate upon the same input data set and/or produce the same output data


Cure:

Isolate each element into separate modules


Rarely occurs in object-oriented systems due to polymorphism



Doc 11, Cohesion Slide # 10
Module Cohesion

Sequential


Sequential association the type in which the output data from one processing element serve as input data for the next processing element

A module that performs multiple sequential functions where the sequential relationship among all of the functions is implied by the problems or application statement and where there is a data relationship among all of the functions


Cure:

Decompose into smaller modules



Doc 11, Cohesion Slide # 11
Module Cohesion

Functional



If the operations of a module can be collectively described as a single specific function in a coherent way, the module has functional cohesion

If not, the module has lower type of cohesion



In an object-oriented system:





Doc 11, Cohesion Slide # 12
Module Cohesion

Informational Strength


Myers states:

"The purpose of an informational-strength module is to hide some concept, data structure, or resource within a single module.

An informational-strength module has the following definition:






Doc 11, Cohesion Slide # 13

Object Cohesion


The degree to which components of a class are tied together

Evaluating cohesion requires:






Doc 11, Cohesion Slide # 14
Questions to probe cohesiveness of an object

Does the object represent a complete and coherent concept or does it more closely resemble a partial concept, or a random collection of information?

Does the object directly correspond to a "real world entity," physical or logical?

Is the object characterized in very non-specific terms?

Collection of data, statistics, etc.

Do each of the methods in the public interface for the object perform a single coherent function?

If the object (or system of objects) is removed from the context of the immediate application, does it still represent a coherent and complete object-oriented concept?

For objects that are "system of objects"

Does the system represent an object-oriented concept?

Do all the objects directly support, or directly contribute to the support of, the object-oriented concept that the system represents?

Are there missing objects?



Doc 11, Cohesion Slide # 15
Objects in Isolation

Isolation means without considering any hierarchy that may contain the object or class

Does not discuss non-objects:




Doc 11, Cohesion Slide # 16
Individual Objects

A primitive method is any method that cannot be implemented simply, efficiently, and reliably without knowledge of the underlying implementation of the object

A composite method is any method constructed from two or more primitive methods – sometimes from different objects

A sufficient set of primitive methods for an object is a minimum set of primitive methods to accomplish all necessary work with on the object

A sufficient set of primitive methods has two major problems:



A complete set of primitive methods is a set of primitive methods that both allows us to easily work with the object, and fully captures the abstraction represented by the object.




Doc 11, Cohesion Slide # 17
An object is not as cohesive as it could be if the public interface contains:





Note




Doc 11, Cohesion Slide # 18
Composite Objects

A composite object is an object that is conceptually composed of two, or more, other objects, which are externally discernable.

Component objects are those that make up the composite object.

Component objects are externally discernable if





Doc 11, Cohesion Slide # 19
Ranking of Cohesion of Composite ObjectsIncreasing order of Goodness









Doc 11, Cohesion Slide # 20
Accessing Cohesion of an Individual Object

Accessment of the public methods/public non-methods/component objects

Are all the items appropriate for the given object?

Do we have at least a minimally sufficient set of items?

Do we have extra or application-specific items?



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2002 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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