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JVM / Java / SOLID / Single Responsibility

1. Single Responsibility Principle (SRP)

A class should have only one reason to change. This does not mean that a class must do only one thing. SRP talks about reason to change, not number of methods, not amount of logic.

2. Bad Implementation (violates SRP)

This class has multiple reasons to change: Change / Reason: - Salary rules change (HR / business policy) - Report format changes (Reporting / presentation) - Persistence changes (Infrastructure / database) These reasons come from different actors (HR, finance, DBA, UI team). That’s the real SRP violation.

 
/**
 * SRP - Non complient example
 * ===========================
 * Employee with multiple legitimate beharviors
 */
package solid.srp.non_compliant;

public class BadPayrollApplication {
    public static void main(String[] args) {

        Employee employee = new Employee("John", 1000);
        System.out.println(employee.generatePayrollReport());
        employee.saveToDatabase();

        /*
            Employee: John, Salary: 1200.0
            Saving employee to DB
        */
    }
}

class Employee {
    private final String name;
    private final double baseSalary;

    Employee(String name, double baseSalary) {
        this.name = name;
        this.baseSalary = baseSalary;
    }

    double calculateSalary() {
        return baseSalary * 1.2;
    }

    String generatePayrollReport() {
        return "Employee: " + name + ", Salary: " + calculateSalary();
    }

    void saveToDatabase() {
        System.out.println("Saving employee to DB");
       
    }
}

3. SRP-compliant Implementation

In this example, Employee: - Has behavior - Has multiple methods - Still SRP-compliant Because: - All changes to Employee come from one reason: changes in employee-related business rules

 
/**
 * SRP - Compliant design
 * ======================
 * All changes to Employee come from one reason:
 * changes in employee-related business rules
 */
package solid.srp;

public class PayrollApplication {
    public static void main(String[] args) {

        Employee employee = new Employee("John", 1000);
        PayrollReportService service = new PayrollReportService();
        EmployeeRepository repository = new EmployeeRepository();
        service.generate(employee);
        repository.save(employee);

        /*
            Employee: John / Salary: 1200.0
            Saving employee to DB: John
        */
    }
}

class Employee {
    private final String name;
    private final double baseSalary;

    Employee(String name, double baseSalary) {
        this.name = name;
        this.baseSalary = baseSalary;
    }

    double calculateSalary() {
        return baseSalary * 1.2;
    }

    String getName() {
        return name;
    }
}

class PayrollReportService {
    void generate(Employee employee) {
        System.out.println(
            "Reports employee: " + employee.getName() + " / " + 
            "Salary: " + employee.calculateSalary()
        );
    }
}

class EmployeeRepository {
    void save(Employee employee) {
        System.out.println("Saving employee to DB: " + employee.getName());
    }
}

4. Key Insights (SRP)

SRP is about actors, not code size. A class should be responsible to only one actor. Ask these questions: - Who will ask for this change? - Are these changes requested by different people/teams? - Whould these changes happen for different business reasons? If the answer is yes, SRP is beaing violeted.