设计模式 - 策略模式(Strategy)

Strategy模式定义一系列的算法,把它们一个个封装起来, 并且使它们可相互替换。本模式使得算法可独立于使用它的客户而变化。

类图：



案例： 不同的员工薪资计算不同。

没有实现Strategy模式的例子：

public class Employee {
public static final int ENGINEER = 0;
public static final int SALESMAN = 1;
public static final int MANAGER  = 2;

private int type = 0;
private int monthlySalary;
private int commission;
private int bonus;
public Employee(int type, int monthlySalary) {
this.type = type;
this.monthlySalary = monthlySalary;
}
public int getType() {
return type;
}
public int getMonthlySalary() {
return monthlySalary;
}
public int getCommission() {
return commission;
}
public void setCommission(int commission) {
this.commission = commission;
}
public int getBonus() {
return bonus;
}
public void setBonus(int bonus) {
this.bonus = bonus;
}
public int payAmount() {
int result = 0;
switch(type) {
case ENGINEER:
result = monthlySalary;
break;
case SALESMAN:
result = monthlySalary + commission;
break;
case MANAGER:
result = monthlySalary + bonus;
break;
}
return result;
}
}

测试代码

public class EmployeeTest {
@Test
public void testEngineer() {
Employee engineer = new Employee(Employee.ENGINEER, 8000);
assertEquals(8000, engineer.payAmount());
engineer.setCommission(1000);
engineer.setBonus(2000);
assertEquals(8000, engineer.payAmount());
}

@Test
public void testSalesman() {
Employee salesman = new Employee(Employee.SALESMAN, 8000);
assertEquals(8000, salesman.payAmount());
salesman.setCommission(1000);
salesman.setBonus(2000);
assertEquals(9000, salesman.payAmount());
}

@Test
public void testManager() {
Employee manager = new Employee(Employee.MANAGER, 8000);
assertEquals(8000, manager.payAmount());
manager.setCommission(1000);
manager.setBonus(2000);
assertEquals(10000, manager.payAmount());
}
}

可以看到payAmount()方法有条件语句，不同类型的员工，薪资计算的算法不同，可以使用strategy模式重构：

每个类型码对应一个具体的strategy类， 每个strategy类定义一个payAmount(Employee employee)方法，Employee持有一个strategy类型的引用，它的payAmount委托给这个strategy引用对象，为了多态实现strategy, 提炼strategy接口。

public interface EmployeeType {//strategy interface
public int payAmount(Employee employee);
}

public class Engineer implements EmployeeType {
public int payAmount(Employee employee) {
return employee.getMonthlySalary();
}
}

public class Salesman implements EmployeeType {
public int payAmount(Employee employee) {
return employee.getMonthlySalary() + employee.getCommission();
}
}

public class Manager implements EmployeeType {
public int p
4000
ayAmount(Employee employee) {
return employee.getMonthlySalary() + employee.getBonus();
}
}

修改Employee类，将类型码替换成strategy类型引用，payAmount()委托给strategy类型的引用对象的payAmount(Employee employee)方法：

public class Employee {
private EmployeeType employeeType;
private int monthlySalary;
private int commission;
private int bonus;
public Employee(EmployeeType employeeType, int monthlySalary) {
this.employeeType = employeeType;
this.monthlySalary = monthlySalary;
}
public EmployeeType getType() {
return employeeType;
}
public int getMonthlySalary() {
return monthlySalary;
}
public int getCommission() {
return commission;
}
public void setCommission(int commission) {
this.commission = commission;
}
public int getBonus() {
return bonus;
}
public void setBonus(int bonus) {
this.bonus = bonus;
}
public int payAmount() {
return employeeType.payAmount(this);
}
}