Importance of Open-Closed Principle

The O in SOLID

Imagine you’re a software developer working on project building a feature for calculating different types of salaries. We had three main categories of employees: Full-time, Part-time, and Contractors. It seemed totally reasonable to handle this scenario in one single class that had a method like calculateSalary()—just a few conditionals, right?

The Innocent-Looking Logic

public class SalaryCalculator {
    
    public double calculateSalary(String employeeType, double baseSalary, double hoursWorked) {
        double totalSalary = 0;

        if ("FULL_TIME".equalsIgnoreCase(employeeType)) {
            // Full-time has some bonus
            totalSalary = baseSalary + 0.2 * baseSalary;
        } else if ("PART_TIME".equalsIgnoreCase(employeeType)) {
            // Part-time gets half the base plus extra for hours
            totalSalary = (baseSalary / 2) + (10 * hoursWorked);
        } else if ("CONTRACTOR".equalsIgnoreCase(employeeType)) {
            // Contractors are paid purely by the hour
            totalSalary = 50 * hoursWorked;
        }

        return totalSalary;
    }
}

At a glance, this might look acceptable, especially for a smaller codebase:

• One class, no big fuss.
• The method is short, easy to…