Spring Semester, 1999
Cohesion
© 1999, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 09-Feb-99
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CS 535 Object-Oriented Programming & Design Spring Semester, 1999 Cohesion |
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© 1999, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 09-Feb-99 |
Cohesion
"Cohesion is the degree to which the tasks performed by a single module are functionally related."
Module Cohesion
From Worst to Best
Coincidental (worst)
Logical
Temporal
Procedural
Communication
Sequential
Functional (best)
Coincidental Cohesion
Little or no constructive relationship among the elements of the module
Common Object Occurrence:
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}
}
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
Cure:
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;
}
}
Temporal
Cohesion
Elements
are grouped into a module because they are all processed within the same
limited time period
Common
example:
procedure initializeData(){
font = "times";
windowSize = "200,400";
foo.name = "Not Set";
foo.size = 12;
foo.location = "/usr/local/lib/java";
}
class foo {
public foo() {
foo.name = "Not Set";
foo.size = 12;
foo.location = "/usr/local/lib/java";
}
}
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
Cure:
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
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
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:
Informational Strength Cohesion
Myers states:
Object Cohesion
The degree to which components of a class are tied together
Evaluating cohesion requires:
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?
Objects in Isolation
Isolation means without considering any hierarchy that may contain the object or class
Does not discuss non-objects:
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:
An object is not as cohesive as it could be if the public interface contains:
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
Ranking of Cohesion of Composite Objects Increasing order of Goodness
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?
Copyright © 1999 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
All rights reserved.
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