每個軟體元件應該只有一個且一個職責

軟體元件可以是類別、方法或模組

例如，瑞士軍刀是一種多用途工具，違反了軟體開發的單一責任原則，相反，刀是遵循單一責任的一個很好的例子（因為與瑞士軍刀不同，它只能用於切割可用於切割、打開罐頭、作為萬能鑰匙、剪刀等）

由於無論是現實世界或軟體開發，變化都是不變的，單一職責原則的定義也會隨之改變

每個軟體元件都應該有且只有一個更改的理由

下面的 Employee 類別中可能發生變化的原因有 3 個原因

1. 員工屬性變化
2. 資料庫更改
3. 稅金計算的變化

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;

/**
 * Employee class has details of employee
 * This class is responsible for saving employee details, getting tax of
 * employee and getting
 * details of employee like name, id, address, contact, etc.
 */
public class Employee {
    private String employeeId;
    private String employeeName;
    private String employeeAddress;
    private String contactNumber;
    private String employeeType;

    public void save() {
        String insert = MyUtil.serializeIntoString(this);
        Connection connection = null;
        Statement statement = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection("jdbc://mysql://localhost:8080/MyDb", "root", "password");
            statement = connection.createStatement();
            statement.execute("insert into employee values ("   insert   ")");

        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    public void calculateTax() {
        if (this.getEmployeeType().equals("fulltime")) {
            // tax calculation for full time employee
        } else if (this.getEmployeeType().equals("contract")) {
            // tax calculation for contract type employee
        }
    }

    public String getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(String employeeId) {
        this.employeeId = employeeId;
    }

    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public String getEmployeeAddress() {
        return employeeAddress;
    }

    public void setEmployeeAddress(String employeeAddress) {
        this.employeeAddress = employeeAddress;
    }

    public String getContactNumber() {
        return contactNumber;
    }

    public void setContactNumber(String contactNumber) {
        this.contactNumber = contactNumber;
    }

    public String getEmployeeType() {
        return employeeType;
    }

    public void setEmployeeType(String employeeType) {
        this.employeeType = employeeType;
    }

}

由於SRP（單一職責原則）建議類別中只有一個更改原因，我們將不得不在Employee類別中進行一些修改

SRP 變化

現在，只有一個原因可以導致 Employee 類別發生變化

變更原因：員工屬性變更

/**
 * Employee class has details of employee
 */
public class Employee {
    private String employeeId;
    private String employeeName;
    private String employeeAddress;
    private String contactNumber;
    private String employeeType;

    public void save() {
       new EmployeeRepository().save(this);
    }

    public void calculateTax() {
       new TaxCalculator().calculateTax(this);
    }

    public String getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(String employeeId) {
        this.employeeId = employeeId;
    }

    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public String getEmployeeAddress() {
        return employeeAddress;
    }

    public void setEmployeeAddress(String employeeAddress) {
        this.employeeAddress = employeeAddress;
    }

    public String getContactNumber() {
        return contactNumber;
    }

    public void setContactNumber(String contactNumber) {
        this.contactNumber = contactNumber;
    }

    public String getEmployeeType() {
        return employeeType;
    }

    public void setEmployeeType(String employeeType) {
        this.employeeType = employeeType;
    }

}

此外，EmployeeRepository 類別中發生變更的原因只有一個

更改原因：資料庫變更

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;

public class EmployeeRepository {

    public void save(Employee employee) {
         String insert = MyUtil.serializeIntoString(employee);
        Connection connection = null;
        Statement statement = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection("jdbc://mysql://localhost:8080/MyDb", "root", "password");
            statement = connection.createStatement();
            statement.execute("insert into employee values ("   insert   ")");

        } catch (Exception e) {
            e.printStackTrace();
        }
}

}

最後，TaxCalculator 類別中發生變化的原因只有一個

變更原因：稅務計算變更

public class TaxCalculator {

    public void calculateTax(Employee employee) {
        if (employee.getEmployeeType().equals("fulltime")) {
            // tax calculation for full time employee
        } else if (employee.getEmployeeType().equals("contract")) {
            // tax calculation for contract type employee
        }
    }
}

注意：所有 3 個類別現在都遵循單一職責原則，因此遵循兩個定義

在建立類別或任何軟體元件時請記住：以高內聚和鬆散耦合為目標

每個軟體元件應該只有一個和一個職責和
每個軟體元件都應該有且僅有一個更改的理由