• Object-Oriented Design

Cohesion in a module should be high.

The cohesion of a module is a measure for how well the internal parts of a module (e.g. the methods and attributes of a class) belong together. Having a high cohesion means, that a module should only comprise responsibilities which belong together.

Several kinds of cohesion can be distinguished some of which are strong and some of which are loose. So strong forms of coupling should be preferred: add explanation of cohesion types

Not adhering to this principle, i.e. having a low cohesion, means that one module has several unrelated or only loosely related responsibilities. A change in the requirements for one of these may thus also affect the others which would not be the case in a highly cohesive module.

• Examined
• Accepted

• Tell don't Ask/Information Expert (TdA/IE): Adhering to information expert means that a module only has responsibilities which belong together. So this increases cohesion.
• Constantine's Law: Constantine's Law is just the combination of HC and LC.
• Single Responsibility Principle (SRP): SRP is a stronger version of HC.
• Interface Segregation Principle (ISP): ISP is the application of HC to interfaces.

• More Is More Complex (MIMC): Making a module highly cohesive often results in additional modules. Sometimes it is simpler to assign a minor unrelated responsibility to a module, which lowers the cohesion.
• Model Principle (MP): Adhering to HC sometimes means to split up a class into several smaller ones which might correspond to the model less well.

• Low Coupling (LC): A system consisting of one single module has a very low coupling as there are no dependencies on other modules. But such a system also has low cohesion. The other extreme, very many highly cohesive modules, naturally has a higher coupling between the modules. So here a compromise has to be found.
• Encapsulate the Concept that Varies (ECV): Adhering to HC often results in modules to be split up into several more cohesive ones. ECV gives further advice on how to do that.

Stub

• Albert Endres and Dieter Rombach: A Handbook of Software and Systems Engineering. p. 43pp.
• Cohesion (computer science)
• CouplingAndCohesion