SDSU CS 535 Object-Oriented Programming & Design
Spring Semester, 1999
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Contents of Doc 3, Cohesion


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

Doc 3, Cohesion Slide # 2


"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 3, Cohesion Slide # 3

Module Cohesion

From Worst to Best

Coincidental (worst)






Functional (best)

Doc 3, Cohesion Slide # 4

Coincidental Cohesion

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 3, Cohesion Slide # 5

Logical Cohesion

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

Similar to control coupling


Isolate each function into separate operations

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

Doc 3, Cohesion Slide # 6

Temporal Cohesion

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"; = "Not Set";
   foo.size = 12;
   foo.location = "/usr/local/lib/java";

Cure: Each object should have a constructor and destructor

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

Doc 3, Cohesion Slide # 7

Procedural Cohesion

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. Can not understand module without understanding the program and the conditions existing when module is called. Makes modules hard to modify, understand. The following example is a rather poor example. One could argue that it really is an example of coincidental cohesion – partitioning into modules based on size. It is the result of the mindset that creates procedural cohesion: writing programs. In the application I am writing now, first I do A then I do B. Therefore I will put A and B together in one method. It may seem reasonable at the time, but removed from the context it seems very strange. The example below is bizarre. I have seen it and many equally bizarre examples in student code.

class Stack {
   public void pop() {
      int next = elements[ top-- ];
      System.out.println( next );
   public void push() {
      int addMe = Console.readInt( "Type an integer" );
      elements[++top] = addMe;
Class Builder verse Program writer


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

Doc 3, Cohesion Slide # 8

Communication Cohesion

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

Cure: Isolate each element into a separate modules

Rarely occurs in object-oriented systems due to polymorphism

Doc 3, Cohesion Slide # 9

Sequential Cohesion

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 3, Cohesion Slide # 10

Functional Cohesion

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 3, Cohesion Slide # 11

Informational Strength Cohesion

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 3, Cohesion Slide # 12

Object Cohesion

The degree to which components of a class are tied together

Evaluating cohesion requires:

Doc 3, Cohesion Slide # 13
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 3, Cohesion Slide # 14
Objects in Isolation

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

Does not discuss non-objects:

object with only functions
objects with only data

Doc 3, Cohesion Slide # 15

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 3, Cohesion Slide # 16
An object is not as cohesive as it could be if the public interface contains:


Doc 3, Cohesion Slide # 17

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 3, Cohesion Slide # 18
Ranking of Cohesion of Composite Objects
Increasing order of Goodness

Doc 3, Cohesion Slide # 19
Assessing Cohesion of an Individual Object

Assessment 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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