每个软件组件都应该只有一个且一个职责

软件组件可以是类、方法或模块

例如，瑞士军刀是一种多用途工具，违反了软件开发的单一责任原则，相反，刀是遵循单一责任的一个很好的例子（因为它只能用于切割，而瑞士军刀可以用于切割、打开罐头、作为万能钥匙、剪刀等）

由于无论是在现实世界还是在软件开发中，变化都是不断变化的，单一职责原则的定义也会随之变化

每个软件组件都应该有一个且仅有一个更改的理由

---

下面的 employee 类会发生变化的原因有以下三个

1. 员工属性变化
2. 数据库更改
3. 税收计算方式的变化

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;
    }

}

由于srp（单一职责原则）建议在类中只有一个更改原因，因此我们必须在employee类中进行一些修改

---

srp 的更改

现在，employee 类中发生变化的原因只有一个

变更原因：员工属性变更

/**
 * 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 类中发生更改的原因只有一个

变更原因：数据库变更

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();
        }
}

}

---

最后，taxcalculator 类中发生变化的原因只有一个

变更原因：税金计算方式变更

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 个类现在都遵循单一职责原则，因此遵循这两个定义

创建类或任何软件组件时请记住：以高内聚和松散耦合为目标

每个软件组件应该只有一个和一个职责和
每个软件组件都应该有且只有一个更改的理由

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