What is it

Class should have only one job.

• therefore, have only one reason for change.

Why use it

Because many different teams can work on the same project and edit the same class for different reasons, conflicting definition/changes could lead to incompatible modules.

Fewer merge conflicts and easier version control by design.

Illustration

Suppose you have a class called Car and another class called Invoice at a car dealership.

class Invoice takes in a Car class and has methods calculateTotal, printInvoice, and saveToDatabase.

printInvoice changes the printing logic.

• Having printInvoice as a method of Invoice class is considered a violation of single responsibility principle because the Invoice class should only contain methods related to business logic, not printing logic.

The printInvoice method should be extracted out to a separate InvoicePrinter class.

saveToDatabase is another violation.

• saveToDatabase method is a persistence logic, not a business logic on Invoice class.

saveToDatabase method should be extracted out to a separate InvoicePersistence class.