2Cents

S.O.L.I.D: Single Responsibility Principle

March 25, 2020

What is S.O.L.I.D by the way?

As per Wiki:

In object-oriented computer programming, SOLID is a mnemonic acronym for five design principles intended to make software designs more understandable, flexible and maintainable.

SOLID is an acronym for 5 important design principles when doing OOP. It was first introduced by Robert C. Martin (Uncle Bob), in his 2000 paper Design Principles and Design Patterns.

SOLID stands for -

  • S - Single Responsibility Principle
  • O - Open/Closed Principle
  • L - Liskov’s Substitution Principle
  • I - Implementation Segregation Principle
  • D - Dependency Inversion Principle

In this article, I will be covering S - Single Responsibility Principle. Note - The examples will be in Java, but applies to any OOP language.

S - Single Responsibility Principle (SRP)

A class should have one, and only one reason to change.

In programming, the Single Responsibility Principle states that every module or class should have responsibility for a single part of the functionality provided by the software.

Anti SRP Usage

An action example speaks louder than voice

package app.singleResponsibility;

public class Employee {
  private String name;
  private int perHourRate;

  public Employee(String name) {
    this.name = name;
  }

  public String getName() {
      return name;
  }

  public void setPerHourRate(int rate) {
    this.perHourRate = rate;
  }

  public int getPerDayRate() {
    return perHourRate * 8;
  }


  public String markAttendance() {
    return String.format("%s is present", name);
  }
}

And here goes the UML Diagram for the geeks -

UML diagram of Employee class

So we have a very basic Employee class with two private attributes and a few public methods. At first glance, it looks fine, but it’s actually breaking SRP.

Employee class not only deals with Employee details but it is also concerned with the implementation of the markAttendence method. It has two reasons to change now.

SRP in Action

The previous example violates the SRP law of single responsibility. Let’s dig a little further and try to correct the Employee class.

// Employee.java
package app.singleResponsibility;

public class Employee {
  private String name;
  private int perHourRate;

  public Employee(String name) {
    this.name = name;
  }

  public String getName() {
      return name;
  }

  public void setPerHourRate(int rate) {
    this.perHourRate = rate;
  }

  public int getPerDayRate() {
    return perHourRate * 8;
  }
}

The updated UML diagram for Employee class -

Alt Text

We have removed the markAttendance method from the Employee class to make it compliant with SRP.

Let’s go further and create the AttendanceTracker class to deal with attendance.

// AttendanceTracker.java
package app.singleResponsibility;

public class AttendanceTracker {
  private Employee emp;

  public AttendanceTracker(Employee emp) {
    this.emp = emp;
  }

  public String markAttendance() {
    return String.format("%s is present", emp.getName());
  }
}

UML diagram for AttendanceTracker class

UML diagram for AttendanceTracker class

AttendanceTracker class takes an employee object as a dependency and marks the attendance of that employee.

Now both the classes adhere to the Single Responsibility Principle and it makes the maintenance and testing a breeze.

TL;DR

The more responsibilities (reason to change) a class has, it’s going to get harder to implement some new features and the maintenance will be a growing pain that will burn more time as the project grows, adding more complexity and making the classes responsibilities strongly coupled to each other.

So, always have small classes with a single responsibility.