S is for the Single Responsibility Principle

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The "S" in SOLID stands for Single Responsibility Principle, and it’s probably the principle that gets the most bad press.

It states that each class should have just one responsibility.

My take on this is that this principle is another way of saying Keep It Simple Stupid (KISS).


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

One of Uncle Bob's favourite examples is "an “Employee” class that calculates the pay and saves the employee data to a persistence store. If there's a change required in how pay is calculated, the class will have to be changed. If the DBA makes some changes to the database schema, the class will probably have to change as well."

Here the "Employee" class has two responsibilities which violates the SRP. One is business logic related and the other data infrastructure related. According to the Single Responsibility Principle, these concerns shouldn’t reside in the same object.

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