Abstraction is a technique for dealing with complexity. It works by establishing a level of complexity we are interested in, and suppressing the more complex details below that level.
The guiding principle of abstraction is that only details that are relevant to the current perspective or the task at hand need to be considered. As most programs are written to solve complex problems involving large amounts of intricate details, it is impossible to deal with all these details at the same time. That is where abstraction can help.
Data abstraction: abstracting away the lower level data items and thinking in terms of bigger entities
Within a certain software component, you might deal with a user data type, while ignoring the details contained in the user data item such as name, and date of birth. These details have been ‘abstracted away’ as they do not affect the task of that software component.
Control abstraction: abstracting away details of the actual control flow to focus on tasks at a higher level
print(“Hello”) is an abstraction of the actual output mechanism within the computer.
Abstraction can be applied repeatedly to obtain progressively higher levels of abstraction.
An example of different levels of data abstraction: a
File is a data item that is at a higher level than an array and an array is at a higher level than a bit.
An example of different levels of control abstraction:
execute(Game) is at a higher level than
print(Char) which is at a higher level than an Assembly language instruction
Abstraction is a general concept that is not limited to just data or control abstractions.
Some more general examples of abstraction:
Coupling is a measure of the degree of dependence between components, classes, methods, etc. Low coupling indicates that a component is less dependent on other components. High coupling (aka tight coupling or strong coupling) is discouraged due to the following disadvantages:
In the example below, design
A appears to have more coupling between the components than design
X is coupled to Y if a change to Y can potentially require a change in X.
Foo class calls the method
Foo is coupled to
Bar because a change to
Bar can potentially (but not always) require a change in the
Foo class e.g. if the signature of
Bar#read() is changed,
Foo needs to change as well, but a change to the
Bar#write() method may not require a change in the
Foo class because
Foo does not call
code for the above example
Some examples of coupling:
A is coupled to
Ahas access to the internal structure of
B(this results in a very high level of coupling)
Bdepend on the same global variable
Areceives an object of
Bas a parameter or a return value
Bare required to follow the same data format or communication protocol
Cohesion is a measure of how strongly-related and focused the various responsibilities of a component are. A highly-cohesive component keeps related functionalities together while keeping out all other unrelated things.
Higher cohesion is better. Disadvantages of low cohesion (aka weak cohesion):
Cohesion can be present in many forms. Some examples:
Studentcomponent handles everything related to students.
GameArchivecomponent handles everything related to the storage and retrieval of game sessions.
Suppose a Payroll application contains a class that deals with writing data to the database. If the class includes some code to show an error dialog to the user if the database is unreachable, that class is not cohesive because it seems to be interacting with the user as well as the database.