Cohesion refers to the degree to which elements inside a module belong together.
Coupling pertains to the degree to which different modules in an application are interdependent.
A highly cohesive system is desirable since it is more readable and easier to maintain.
A loosely coupled system is desirable to make the system more modular and easier to extend.
We increase cohesion when
1. The functionalities embedded in a class, accessed via methods, have much in common.
2. Methods can carry out a small number of activities. They do one thing.
3. Related methods are grouped together.

Levels of Cohesion

Coincidental Cohesion - when parts of a module are grouped arbitrarily, with the only relationship they have is that they are grouped together.
Logical Cohesion - when parts of a module are grouped together because they are logically categorized into the same thing even though they are different by nature
- Example: Grouping all input handlers together because they handle input.)
Temporal Cohesion - when parts of a module are grouped by when they are processed.
Procedural Cohesion - when parts of a module are grouped because they always follow a certain sequence of execution. That is, one part follows the next in a well defined order.
- Refactor this by putting operations in their own routines.
Informational Cohesion - when parts of a module are grouped because they operate on the same data. but are otherwise unrelated.
Sequential Cohesion - when parts of a module are grouped because the output from one part is the input to another part like an assembly line. There is a well defined sequence for the dependencies of each component.
Functional Cohesion - when parts of a module are grouped together because they all contribute to a single well defined task of the module

Criteria for Coupling

Consider the following criteria for coupling
- Size - the number of connections between modules. Minimize size.
- Visibility - the prominence of the connection between two modules. Maximize visibility.
- Flexibility - how easy can you change the connections between modules. Maximize flexibility.
The most common kinds of coupling are as follows.
- Simple Data Parameter Coupling - all data passed between modules are primitive data types. All data is passed via parameter lists.
- Simple Object Coupling - one object instantiates the other object.
- Object Parameter Coupling - requires to pass it a . This is a tight coupling.
- Semantic Coupling - one module makes use of some semantic knowledge of another module’s inner workings. This should be avoided.