Single responsibility principle is also referred to as SRP in short. SRP says that every class must have only one responsibility. In other words, every class must represent exactly one object. When a class represents multiple objects, it tends to violate SRP and opens up chances for multiple code smells.

For example, let's take a simple Employee class, as follows:

In the preceding class diagram, the Employee class seems to represent three different objects: Employee, Address, and Contact. Hence, it violates the SRP. As per this principle, from the preceding Employee class, two other classes can be extracted, namely Address and Contact, as follows:

For simplicity, the class diagrams used in this section don't show any methods that are supported by the respective classes, as our focus is understanding the SRP with a simple example.

In the preceding refactored design, Employee has one or more addresses (personal and official) and one or more contacts (personal and official). The best part is that after refactoring the design, every class abstracts one and only thing; that is, it has only one responsibility.