Object-Oriented Programming


Michael L. Collard, Ph.D.

Department of Computer Science, The University of Akron


Degree of connectivity among the elements of a single module and in object-oriented design, a single class/object

Internal measure on an individual class/object/module/method

Why is Cohesion Important?

  • A cohesive class is much easier to comprehend because it has a single purpose
  • Increase reuse since the class is usable in multiple ways in many different projects or the same project
  • Reduce (maintenance) costs for fixes and enhancements
  • Simplify testing
  • Allow for replacement

Cohesion Goal

Maximize internal interaction (intramural) among subelements

  • Methods share data members (fields) with other methods
  • Data members are used in combination
  • The more advanced the features of a class, the more this occurs

Types of Cohesion

  • Informational (best)
  • Functional (best)
  • Sequential (better)
  • Communicational (better)
  • Procedural
  • Temporal
  • Logical
  • Coincidental (worst)


  • system
  • subsystem/directory
  • class/file
  • method/function
  • section of code

Coincidental Cohesion

  • Performs multiple, completely unrelated actions
  • In the example, why are all these methods in this one class? Because John Doe worked on them
  • Often based on factors outside of the design: personnel, company organization, history, avoidance of small modules
  • No reusability, very difficult to maintain or enhance

Logical Cohesion

  • Performs a series of related actions, one of which is selected by the calling module
  • In the example, why are all these methods in this one class? Because they all do output
  • Parts have a logical association, but not the primary logical association
  • Primary logical association is based on the highest level of abstraction

Logical Cohesion (cont)

  • Often includes both high and low-level actions (in terms of abstraction) in the same class
  • Often includes unused parameters for specific uses
  • Difficult to understand interface. Many unrelated actions
  • In OO, we put methods near the abstract concept that they work on

Temporal Cohesion

  • Perform a series of actions that are related by time occurrence
  • In the example, why are all these methods in these classes? Because they all occur at startup or shutdown time
  • Addition of subsystems may require additions to multiple modules
  • Degrades to Logical Cohesion if the time of action changes
  • In OO, we build time occurrence actions into the class and externally control the lifetime

Procedural Cohesion

  • Action based on the ordering of steps, related by usage in ordering
  • In the example, why does this one method contain both reading a part and updating a directory? Because they both always occur in this order
  • Changes to the ordering of steps or purpose of steps require changing the element abstraction, e.g., we add another step
  • Situations where this particular sequence applies are often limited
  • Each element, including methods/operations, should have one cohesive action

Communicational Cohesion

  • Each method performs several operations on its own data
  • Actions are related but still not completely separated
  • Element (e.g., the example methods) cannot be reused
  • Maybe the highest cohesion possible/desirable

Sequential Cohesion

  • We place actions together because they are performed in order each part on the result of the last
  • In the example, why are all these operations in this one method? Because the output of each operation is an input to the next operation
  • Requires a crisp abstraction that then becomes the name of the method

Functional Cohesion

  • Element (methods) that performs a single action or achieves a single goal
  • In the example, why do we have these separate methods? Because each does a well-defined action, indicated by a precise method name
  • Maintenance involves the entire element
  • High reuse because the element is entirely independent (in its actions) of other elements
  • Superior, easily replaced for performance, testing, platform changes, etc.

Informational Cohesion

  • Performs several actions
  • In the example, why are all these methods in each class? Because they are organized according to the primary abstractions of Weather and Financials
  • Each action has a separate entry point and independent code
  • All actions are performed on a shared data structure
  • Superior, truly Object-Oriented

Types of Cohesion

  • Informational (best)
  • Functional (best)
  • Sequential (better)
  • Communicational (better)
  • Procedural
  • Temporal
  • Logical
  • Coincidental (worst)