Single Responsibility Principle

• A class should always have one responsibility and there should be only a single reason to change it.

Bad Implementation :

• Below Employee class contains personal details, business logic to perform a few calculations, and DB logic to save/update.
• Our class is tightly coupled, hard to maintain, multiple reasons to modify this class.

public class Employee {
    private String fullName;
    private String dateOfJoining;
    private String annualSalaryPackage;

    // standard getters and setters methods

    // business logic
    public long calculateEmployeeSalary(Employee emp) {...}
    public long calculateEmployeeLeaves(Employee emp) {...}
    public long calculateTaxOnSalary(Employee emp) {...}

    // data persistence logic
    public Employee saveEmployee(Employee emp) {...}
    public Employee updateEmployee(Employee emp) ...}
}

Good Implementation :

• We can split a single Employee class into multiple classes as per their specific responsibility.
• It made our class loosely coupled, easy to maintain, and only single reason to modify.

public class Employee {
    private String fullName;
    private String dateOfJoining;
    private String annualSalaryPackage;

    // standard getters and setters methods
}

public class EmployeeService {
    // ...

    public long calculateEmployeeSalary(Employee emp) {...}
    public long calculateEmployeeLeaves(Employee emp) {...}
    public long calculateTaxOnSalary(Employee emp) {...}
}

public class EmployeeDAO {
    // ...

    public Employee saveEmployee(Employee emp) {...}
    public Employee updateEmployee(Employee emp) {...}
}