repl.it
linkrepl.it
link (duplicated)
Single responsibility principle (SRP): A class should have one, and only one, reason to change. -- Robert C. Martin
If a class has only one responsibility, it needs to change only when there is a change to that responsibility.
Consider a TextUi
class that does parsing of the user commands as well as interacting with the user. That class needs to change when the formatting of the UI changes as well as when the syntax of the user command changes. Hence, such a class does not follow the SRP.
Gather together the things that change for the same reasons. Separate those things that change for different reasons. ―- Agile Software Development, Principles, Patterns, and Practices by Robert C. Martin
Resources
Separation of concerns principle (SoC): To achieve better modularity, separate the code into distinct sections, such that each section addresses a separate concern. -- Proposed by Edsger W. Dijkstra
A concern in this context is a set of information that affects the code of a computer program.
Examples for concerns:
add employee
featurepersistence
or security
Employee
entityApplying Separation of ConcernsSoC reduces functional overlaps among code sections and also limits the ripple effect when changes are introduced to a specific part of the system.
If the code related to persistence is separated from the code related to security, a change to how the data are persisted will not need changes to how the security is implemented.
This principle can be applied at the class level, as well as at higher levels.
The n-tier architecture utilizes this principle. Each layer in the architecture has a well-defined functionality that has no functional overlap with each other.
Can identify n-tier architectural style
In the n-tier style, higher layers make use of services provided by lower layers. Lower layers are independent of higher layers. Other names: multi-layered, layered.
Operating systems and network communication software often use n-tier style.
This principle should lead to higher cohesion and lower coupling.
Can explain coupling
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.
Can explain cohesion
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):
Exercises
Correct statements about SoC
“Only the GUI class should interact with the user. The GUI class should only concern itself with user interactions”. This statement follows from,
(a)(b)(c)
Explanation: By making ‘user interaction’ the GUI class’s sole responsibility, we increase its cohesion. This is also in line with the separation of concerns (i.e., we separated the concern of user interaction) and the single responsibility principle (the GUI class has only one responsibility).