【图解设计模式】Strategy模式

Posted on | In

整体地替换算法的实现部分，能轻松地以不同的算法去解决同一个问题。

示例

让电脑玩“猜拳”游戏。
第一种策略是“如果这局猜拳获胜，那么下一局也出一样的手势”；另外一种策略是“根据上一局的手势从概率上计算出下一局的手势”。

类图

eyh5Qg.png

Hand类

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public class Hand {
    public static final int HANDVALUE_GUU = 0;
    public static final int HANDVALUE_CHO = 1;
    public static final int HANDVALUE_PAA = 2;

    public static final Hand[] hand = {
            new Hand(HANDVALUE_GUU),
            new Hand(HANDVALUE_CHO),
            new Hand(HANDVALUE_PAA)
    };

    public static final String[] name = {
            "石头", "剪刀", "布"
    };

    private int handvalue;

    private Hand(int handvalue) {
        this.handvalue = handvalue;
    }

    public static Hand getHand(int handvalue) {
        return hand[handvalue];
    }

    public boolean isStrongerThan(Hand h) {
        return fight(h) == 1;
    }

    public boolean isWeakerThan(Hand h) {
        return fight(h) == -1;
    }

    private int fight(Hand h) {
        if (this == h)
            return 0;
        else if ((this.handvalue + 1) % 3 == h.handvalue)
            return 1;
        else
            return -1;
    }

    @Override
    public String toString() {
        return name[handvalue];
    }
}

Strategy接口

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public interface Strategy {
    public abstract Hand nextHand();
    public abstract void study(boolean win);
}

WinningStrategy类

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import java.util.Random;

public class WinningStrategy implements Strategy {
    private Random random;
    private boolean won = false;
    private Hand prevHand;

    public WinningStrategy(int seed) {
        random = new Random(seed);
    }

    @Override
    public Hand nextHand() {
        if (!won)
            prevHand = Hand.getHand(random.nextInt(3));

        return prevHand;
    }

    @Override
    public void study(boolean win) {
        won = win;
    }
}

ProbStrategy类

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import java.util.Random;

public class ProbStrategy implements Strategy {
    private Random random;
    private int prevHandValue = 0;
    private int currentHandValue = 0;
    // history[上一局出的手势][这一局出的手势]
    private int[][] history = {
            {1, 1, 1},
            {1, 1, 1},
            {1, 1, 1}
    };

    public ProbStrategy(int seed) {
        random = new Random(seed);
    }

    @Override
    public Hand nextHand() {
        int bet = random.nextInt(getSum(currentHandValue));
        int handvalue;

        if (bet < history[currentHandValue][0])
            handvalue = 0;
        else if (bet < history[currentHandValue][0] + history[currentHandValue][1])
            handvalue = 1;
        else
            handvalue = 2;

        prevHandValue = currentHandValue;
        currentHandValue = handvalue;
        return Hand.getHand(handvalue);
    }

    @Override
    public void study(boolean win) {
        if (win)
            history[prevHandValue][currentHandValue]++;
        else {
            history[prevHandValue][(currentHandValue + 1) % 3]++;
            history[prevHandValue][(currentHandValue + 2) % 3]++;
        }
    }

    private int getSum(int hv) {
        int sum = 0;

        for (int i = 0; i < 3; i++)
            sum += history[hv][i];

        return sum;
    }
}

Player类

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public class Player {
    private String name;
    private Strategy strategy;
    private int wincount;
    private int losecount;
    private int gamecount;

    public Player(String name, Strategy strategy) {
        this.name = name;
        this.strategy = strategy;
    }

    public Hand nextHand() {
        return strategy.nextHand();
    }

    public void win() {
        strategy.study(true);
        wincount++;
        gamecount++;
    }

    public void lose() {
        strategy.study(false);
        losecount++;
        gamecount++;
    }

    public void even() {
        gamecount++;
    }

    @Override
    public String toString() {
        return "[" + name + ":" + gamecount + " games, "
                + wincount + " win, " + losecount + " lose" + "]";
    }
}

Main类

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public class Main {

    public static void main(String[] args) {
        if (args.length != 2) {
            System.out.println("Usage: java Main randomseed1 randomseed2");
            System.out.println("Example: java Main 314 15");
            System.exit(0);
        }

        int seed1 = Integer.parseInt(args[0]);
        int seed2 = Integer.parseInt(args[1]);
        Player player1 = new Player("Taro", new WinningStrategy(seed1));
        Player player2 = new Player("Hana", new ProbStrategy(seed2));

        for (int i = 0; i < 10000; i++) {
            Hand nextHand1 = player1.nextHand();
            Hand nextHand2 = player2.nextHand();

            if (nextHand1.isStrongerThan(nextHand2)) {
                System.out.println("Winner:" + player1);
                player1.win();
                player2.lose();
            }
            else if (nextHand1.isWeakerThan(nextHand2)) {
                System.out.println("Winner:" + player2);
                player1.lose();
                player2.win();
            }
            else {
                System.out.println("Even...");
                player1.even();
                player2.even();
            }
        }

        System.out.println("Total result:");
        System.out.println(player1.toString());
        System.out.println(player2.toString());
    }
}

运行结果

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Even...
Winner:[Hana:1 games, 0 win, 0 lose]
Winner:[Taro:2 games, 0 win, 1 lose]
Even...
Winner:[Hana:4 games, 1 win, 1 lose]
Winner:[Taro:5 games, 1 win, 2 lose]
Even...
Even...
Winner:[Taro:8 games, 2 win, 2 lose]
Winner:[Taro:9 games, 3 win, 2 lose]
Winner:[Taro:10 games, 4 win, 2 lose]
Even...

...

Even...
Winner:[Taro:9992 games, 3164 win, 3488 lose]
Winner:[Hana:9993 games, 3488 win, 3165 lose]
Winner:[Taro:9994 games, 3165 win, 3489 lose]
Winner:[Taro:9995 games, 3166 win, 3489 lose]
Winner:[Hana:9996 games, 3489 win, 3167 lose]
Even...
Even...
Even...
Total result:
[Taro:10000 games, 3167 win, 3490 lose]
[Hana:10000 games, 3490 win, 3167 lose]

登场角色

Strategy（策略）

Strategy角色负责决定实现策略所必需的接口（API）。在示例程序中，由Strategy接口扮演此角色。

ConcreteStrategy（具体的策略）

ConcreteStrategy角色负责实现Strategy角色的接口（API），即负责实现具体的策略（战略、方向、方法和算法）。在示例程序中，由WinningStrategy类和ProbStrategy类扮演此角色。

Context（上下文）

负责使用Strategy角色。Context角色保存了ConcreteStrategy角色的实例，并使用ConcreteStrategy角色去实现需求（总之，还是要调用Strategy角色的接口（API））。在示例程序中，由Player类扮演此角色。

类图

e6hOC6.png