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link (duplicated)
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 MOV
.
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 B
.
Exercises
Coupling levels of alternative designs
Discuss the coupling levels of alternative designs x and y.
Overall coupling levels in x and y seem to be similar (neither has more dependencies than the other). (Note that the number of dependency links is not a definitive measure of the level of coupling. Some links may be stronger than the others.). However, in x, A
is highly-coupled to the rest of the system while B
, C
, D
, and E
are standalone (do not depend on anything else). In y, no component is as highly-coupled as A
of x. However, only D
and E
are standalone.
Regressions and coupling
Explain the link (if any) between regressions and coupling.
When the system is highly-coupled, the risk of regressions is higher too e.g. when component A
is modified, all components ‘coupled’ to component A
risk ‘unintended behavioral changes’.
Coupling and testability
Discuss the relationship between coupling and a measure of how easily a given component can be testedtestability.
Coupling decreases testability because if the Software Under TestSUT is coupled to many other components, it becomes difficult to test the SUT in isolation of its dependencies.
Statements about coupling
Choose the correct statements.
(a)(b)(c)(d)(e)
Explanation: High coupling means either more components are required to be integrated at once in a big-bang fashion (increasing the risk of things going wrong) or more drivers and stubs are required when integrating incrementally.
X is coupled to Y if a change to Y can potentially require a change in X.
If the Foo
class calls the method Bar#read()
, 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 Bar#write()
.
code for the above example
class Foo {
...
new Bar().read();
...
}
class Bar {
void read() {
...
}
void write() {
...
}
}
Some examples of coupling: A
is coupled to B
if,
A
has access to the internal structure of B
(this results in a very high level of coupling)A
and B
depend on the same global variableA
calls B
A
receives an object of B
as a parameter or a return valueA
inherits from B
A
and B
are required to follow the same data format or communication protocolExercises
Which indicate coupling?
Which of these indicate coupling between components A and B?
(a)(b)(c)(d)(e)(f)
Explanation: Being written by the same developer does not imply coupling.
Some examples of different coupling types:
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:
Student
component handles everything related to students.GameArchive
component 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.
Exercises
Which class is more cohesive?
Compare the cohesion of the following two versions of the EmailMessage
class. Which one is more cohesive and why?
// version-1
class EmailMessage {
private String sendTo;
private String subject;
private String message;
public EmailMessage(String sendTo, String subject, String message) {
this.sendTo = sendTo;
this.subject = subject;
this.message = message;
}
public void sendMessage() {
// sends message using sendTo, subject and message
}
}
// version-2
class EmailMessage {
private String sendTo;
private String subject;
private String message;
private String username;
public EmailMessage(String sendTo, String subject, String message) {
this.sendTo = sendTo;
this.subject = subject;
this.message = message;
}
public void sendMessage() {
// sends message using sendTo, subject and message
}
public void login(String username, String password) {
this.username = username;
// code to login
}
}
Version 2 is less cohesive.
Explanation: Version 2 is handling functionality related to login, which is not directly related to the concept of ‘email message’ that the class is supposed to represent. On a related note, you can improve the cohesion of both versions by removing the sendMessage functionality. Although sending messages is related to emails, this class is supposed to represent an email message, not an email server.