策略模式定义了一系列的算法,并将每一个算法封装起来,而且使它们还可以相互替换。策略模式让算法独立于使用它的客户而独立变化.
(原文:The Strategy Pattern defines a family of algorithms,encapsulates each one,and makes them interchangeable.
Strategy lets the algorithm vary independently from clients that use it.)
策略模式的特点:
1 Context(应用实例或client, 即使用Strategy的类)持有一个Strategy实例;
2 Context可以动态灵活的设置Strategy的具体实例,即切换算法;
3 Context通过调用strategy的方法来执行具体的行为。
示例代码:
context = new Context(new SecondStrategy());
context.execute();
context = new Context(new ThirdStrategy());
context.execute();
}
}
// Strategy interface
// The context class uses this to call the concrete strategy
interface Strategy {
void execute();
}
// Implements the algorithm using the strategy interface
class FirstStrategy implements Strategy {
public void execute() {
System.out.println("Called FirstStrategy.execute()");
}
}
class SecondStrategy implements Strategy {
public void execute() {
System.out.println("Called SecondStrategy.execute()");
}
}
class ThirdStrategy implements Strategy {
public void execute() {
System.out.println("Called ThirdStrategy.execute()");
}
}
// Configured with a ConcreteStrategy object and maintains a reference to a Strategy object
class Context {
Strategy strategy;
// Constructor
public Context(Strategy strategy) {
this.strategy = strategy;
}
public void execute() {
this.strategy.execute();
}
}
这个例子很抽象,大多数的设计模式书中都会给出这么个例子,这个例子不具备任何生产价值,于是我想到了一个更好的例子:
笔记本电脑中都有一个电源管理器,提供多种电源管理方案。每个方案都是一种策略,操作系统(或使用电脑的人)应该能灵活切换供电方案。下面用代码模拟:
public interface PowerStrategy {
public void apply();
}
//引用实例,持有策略者
package designpattern.strategy;
public class Computer {
private PowerStrategy ps;
public PowerStrategy getPs() {
return ps;
}
public void setPs(PowerStrategy ps) {
this.ps = ps;
}
public void applyPowerStrategy(){
ps.apply();
}
}
//策略1 高表现性能策略
package designpattern.strategy;
public class HighPerformanceStrategy implements PowerStrategy{
public void apply() {
System.out.println("*** High performance power strategy ***");
System.out.println("Turn off monitor: Never;");
System.out.println("Turn off hard disk: Never;");
System.out.println("System standby: Never;");
System.out.println("System hibernates: Never./n");
}
}
//策略2 中等策略
package designpattern.strategy;
public class MediumStragegy implements PowerStrategy{
public void apply() {
System.out.println("*** Medium power strategy ***");
System.out.println("Turn off monitor: 1 hour;");
System.out.println("Turn off hard disk: 2 hours;");
System.out.println("System standby: 3 hours;");
System.out.println("System hibernates: 4 hours./n");
}
}
//策略3 节能策略
package designpattern.strategy;
public class EnergySavingStrategy implements PowerStrategy{
public void apply() {
System.out.println("*** Energy-saving strategy ***");
System.out.println("Turn off monitor: 5 minutes;");
System.out.println("Turn off hard disk: 15 minutes;");
System.out.println("System standby: 30 minutes;");
System.out.println("System hibernates: 1 hour./n");
}
}
//测试用例
package designpattern.strategy;
public class Test {
public static void main(String[] args){
Computer c = new Computer();
c.setPs(new HighPerformanceStrategy());
c.applyPowerStrategy();
c.setPs(new MediumStragegy());
c.applyPowerStrategy();
c.setPs(new EnergySavingStrategy());
c.applyPowerStrategy();
}
}
经典案例:
1 jdk中的类File有一个方法:public String[] list(FilenameFilter filter),列出当前目录下的所有文件,FilenameFilter就是一个策略接口,使用者可以根据自己的需求自由变换文件筛选策略.
2 jdk中工具类Arrays有一个可以对集合排序的方法: public static <T> void sort(T[] a, Comparator<? super T> c),传入一个数组和一个比较器, 这个比较器就是一个策略.(此案有争议,有人认为是template method模式).
应用场景:
1 多个类只区别在表现行为不同,可以使用Strategy模式,在运行时动态选择具体要执行的行为;
2 需要在不同情况下使用不同的策略(算法)。
优点:
把容易发生变化的算法独立出来,易于扩展。
缺点:
类增多了,使系统难度加大。