Software Engineering for Self-Directed Learners »


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

Interface segregation principle

Interface segregation principle (ISP): No client should be forced to depend on methods it does not use.

The Payroll class should not depend on the AdminStaff class because it does not use the arrangeMeeting() method. Instead, it should depend on the SalariedStaff interface.

public class Payroll {
    // violates ISP
    private void adjustSalaries(AdminStaff adminStaff) {
        // ...

public class Payroll {
    // does not violate ISP
    private void adjustSalaries(SalariedStaff staff) {
        // ...

Liskov substitution principle

Liskov substitution principle (LSP): Derived classes must be substitutable for their base classes. -- proposed by Barbara Liskov

LSP sounds the same as substitutability but it goes beyond substitutability; LSP implies that a subclass should not be more restrictive than the behavior specified by the superclass. As you know, Java has language support for substitutability. However, if LSP is not followed, substituting a subclass object for a superclass object can break the functionality of the code.

Suppose the Payroll class depends on the adjustMySalary(int percent) method of the Staff class. Furthermore, the Staff class states that the adjustMySalary method will work for all positive percent values. Both the Admin and Academic classes override the adjustMySalary method.

Now consider the following:

  • The Admin#adjustMySalary method works for both negative and positive percent values.
  • The Academic#adjustMySalary method works for percent values 1..100 only.

In the above scenario,

  • The Admin class follows LSP because it fulfills Payroll’s expectation of Staff objects (i.e. it works for all positive values). Substituting Admin objects for Staff objects will not break the Payroll class functionality.
  • The Academic class violates LSP because it will not work for percent values over 100 as expected by the Payroll class. Substituting Academic objects for Staff objects can potentially break the Payroll class functionality.

Another example

Dependency inversion principle

Dependency inversion principle (DIP):

  1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
  2. Abstractions should not depend on details. Details should depend on abstractions.


In design (a), the higher level class Payroll depends on the lower level class Employee, which is a violation of DIP. In design (b), both Payroll and Employee depend on the Payee interface (note that inheritance is a dependency).

Design (b) is more flexible (and less coupled) because now the Payroll class need not change when the Employee class changes.

Open-closed principle

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>();

SOLID principles

The five OOP principles given below are known as SOLID Principles (an acronym made up of the first letter of each principle):

Single Responsibility Principle (SRP)

Open-Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Interface Segregation Principle (ISP)

Dependency Inversion Principle (DIP)

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.

Law of Demeter

Law of Demeter (LoD):

  • An object should have limited knowledge of another object.
  • An object should only interact with objects that are closely related to it.

Also known as

  • Don’t talk to strangers.
  • Principle of least knowledge

More concretely, a method m of an object O should invoke only the methods of the following kinds of objects:

  • The object O itself
  • Objects passed as parameters of m
  • Objects created/instantiated in m (directly or indirectly)
  • Objects from the objects that are held by instance variables ofdirect association of O

The following code fragment violates LoD due to the following reason: while b is a ‘friend’ of foo (because it receives it as a parameter), g is a ‘friend of a friend’ (which should be considered a ‘stranger’), and g.doSomething() is analogous to ‘talking to a stranger’.

void foo(Bar b) {
    Goo g = b.getGoo();

LoD aims to prevent objects from navigating the internal structures of other objects.

An analogy for LoD can be drawn from Facebook. If Facebook followed LoD, you would not be allowed to see posts of friends of friends, unless they are your friends as well. If Jake is your friend and Adam is Jake’s friend, you should not be allowed to see Adam’s posts unless Adam is a friend of yours as well.

Brooks' law

Brooks' law: Adding people to a late project will make it later. -- Fred Brooks (author of The Mythical Man-Month)

Explanation: The additional communication overhead will outweigh the benefit of adding extra manpower, especially if done near a deadline.

YAGNI principle

YAGNI (You Aren't Gonna Need It!) Principle: Do not add code simply because ‘you might need it in the future’.

The principle says that some capability you presume your software needs in the future should not be built now because chances are "you aren't gonna need it". The rationale is that you do not have perfect information about the future and therefore some of the extra work you do to fulfill a potential future need might go to waste when some of your predictions fail to materialize.

DRY principle

DRY (Don't Repeat Yourself) principle: Every piece of knowledge must have a single, unambiguous, authoritative representation within a system. -- The Pragmatic Programmer, by Andy Hunt and Dave Thomas

This principle guards against the duplication of information.

A functionality being implemented twice is a violation of the DRY principle even if the two implementations are different.

The value of a system-wide timeout being defined in multiple places is a violation of DRY.