Principles

Can explain single responsibility principle

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}

An explanation of the SRP from www.oodesign.com
Another explanation (more detailed) by Patkos Csaba
A book chapter on SRP written by the father of the principle itself, Robert C Martin

Can explain open-closed principle (OCP)

The Open-Closed Principle aims to make a code entity easy to adapt and reuse without needing to modify the code entity itself.

Open-closed principle (OCP): A module should be open for extension but closed for modification. That is, modules should be written so that they can be extended, without requiring them to be modified. _{-- proposed by Bertrand Meyer}

In object-oriented programming, OCP can be achieved in various ways. This often requires separating the specification (i.e. interface) of a module from its implementation.

In the design given below, the behavior of the CommandQueue class can be altered by adding more concrete Command subclasses. For example, by including a Delete class alongside List, Sort, and Reset, the CommandQueue can now perform delete commands without modifying its code at all. That is, its behavior was extended without having to modify its code. Hence, it is open to extensions, but closed to modification.

The behavior of a Java generic class can be altered by passing it a different class as a parameter. In the code below, the ArrayList class behaves as a container of Students in one instance and as a container of Admin objects in the other instance, without having to change its code. That is, the behavior of the ArrayList class is extended without modifying its code.

ArrayList students = new ArrayList<Student>();
ArrayList admins = new ArrayList<Admin>();

Which of these is closest to the meaning of the open-closed principle?

a. We should be able to change a software module’s behavior without modifying its code.
b. A software module should remain open to modification as long as possible.
c. A software module should be open to modification and closed to extension.
d. Open source software rocks. Closed source software sucks.

(a)

Explanation: Please refer the handout for the definition of OCP.

: Can explain separation of concerns principle

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:

A specific feature, such as the code related to the add employee feature
A specific aspect, such as the code related to persistence or security
A specific entity, such as the code related to the Employee entity

Applying 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.

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:

Maintenance is harder because a change in one module could cause changes in other modules coupled to it (i.e. a ripple effect).
Integration is harder because multiple components coupled with each other have to be integrated at the same time.
Testing and reuse of the module is harder due to its dependence on other modules.

In the example below, design A appears to have more coupling between the components than design B.

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.

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’.

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.

Choose the correct statements.

a. As coupling increases, testability decreases.
b. As coupling increases, the risk of regression increases.
c. As coupling increases, the value of automated regression testing increases.
d. As coupling increases, integration becomes easier as everything is connected together.
e. As coupling increases, maintainability decreases.

(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):

Lowers the understandability of modules as it is difficult to express module functionalities at a higher level.
Lowers maintainability because a module can be modified due to unrelated causes (reason: the module contains code unrelated to each other) or many modules may need to be modified to achieve a small change in behavior (reason: because the code related to that change is not localized to a single module).
Lowers reusability of modules because they do not represent logical units of functionality.

“Only the GUI class should interact with the user. The GUI class should only concern itself with user interactions”. This statement follows from,

a. A software design should promote separation of concerns in a design.
b. A software design should increase cohesion of its components.
c. A software design should follow single responsibility principle.

(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).

Principles

Single Responsibility Principle

Open-Closed Principle

Separation of Concerns Principle