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Java设计模式:策略模式

23 Sep 2014

注:本文为译文,原文出处java-design-patterns-in-stories

策略模式也被称为政策模式.

接下来是一个关于策略模式的故事. 设想迈克有时会超速行使, 但是他不是经常这么干. 他可能会被一个警察拦下来. 可能警察很和善, 会不开罚单让他走也或许只是简单警告一下(让我们将这种警察成为”NicePolice”). 也可能他会被一个严厉的警察拦住并且开了罚单(我们可以称呼这种警察为“HardPolice”). 直到他被拦下来, 他才知道是哪种警察. 这就是策略模式的整个场景.

策略模式类图

strategy-pattern-class-diagram

策略模式Java代码

定义一个接口Strategy, 其中包含一个方法processSpeeding()

public interface Strategy {
  //defind a method for police to process speeding case.
  public void processSpeeding(int speed);
}

现在我们定义两种类型的警察.

public class NicePolice implements Strategy{
  @Override
  public void processSpeeding(int speed) {
    System.out.println("This is your first time, be sure don't do it again!");
  }
}

public class HardPolice implements Strategy{
  @Override
  public void processSpeeding(int speed) {
    System.out.println("Your speed is "+ speed+ ", and should get a ticket!");
  }
}

定义警察介入处理超速的场景.

public class Situation {
  private Strategy strategy;

  public Situation(Strategy strategy){
    this.strategy = strategy;
  }

  public void handleByPolice(int speed){
    this.strategy.processSpeeding(speed);
  }
}

最后, 尝试结果.

public class Main {
  public static void main(String args[]){
    HardPolice hp = new HardPolice();
    NicePolice ep = new NicePolice();

    // In situation 1, a hard officer is met
                // In situation 2, a nice officer is met
    Situation s1 = new Situation(hp);
    Situation s2 = new Situation(ep);

    //the result based on the kind of police officer.
    s1.handleByPolice(10);
    s2.handleByPolice(10);
  }
}

输出为:

Your speed is 10, and should get a ticket!
This is your first time, be sure don't do it again!

你可以与状态模式进行比较, 两者非常相似. 主要区别是:状态模式当对象状态发生变化时修改对象的行为, 而策略模式主要是在不同场景应用不同算法.

JDK中的策略模式

  • Java.util.Collections#sort(List list, Comparator < ? super T > c)
  • java.util.Arrays#sort(T[], Comparator < ? super T > c)

排序方法在不同场景中使用不同的比较器. 如果想要了解关于此方法的更多内容, 参见Deep Understanding of Arrays.sort.

也可以参考The Difference between Comparator and Comparable.

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