设计模式读书笔记之策略模式(Strategy) . -

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设计模式读书笔记之策略模式(Strategy) .

博客分类：

DesignPattern

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：

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：

[java] view plain copy print ?

//策略接口
package designpattern.strategy;
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();
}
}

//策略接口
package designpattern.strategy;

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。

[java] view plain copy print ?

class StrategyExample {
//test case.
public static void main(String[] args) {
Context context;
// Three contexts following different strategies
context = new Context(new FirstStrategy());
context.execute();
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();
}
}

class StrategyExample {
	//test case.
    public static void main(String[] args) {
        Context context;
        // Three contexts following different strategies
        context = new Context(new FirstStrategy());
        context.execute();
        
        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();
    }
}

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设计模式读书笔记之装饰器模式(Decorator) ... | Java中的finalize

2012-08-02 13:37
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