java_设计模式_策略模式_Strategy pattern（2016-07-15）

感受：将算法从方法级别，提升到类级别。之后利用java多态，来切换不同的算法实现不同的功能。

在阎宏博士的《JAVA与模式》一书中开头是这样描述策略（Strategy）模式的：

　　策略模式属于对象的行为模式。其用意是针对一组算法，将每一个算法封装到具有共同接口的独立的类中，从而使得它们可以相互替换。策略模式使得算法可以在不影响到客户端的情况下发生变化。

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策略模式的结构

　　策略模式是对算法的包装，是把使用算法的责任和算法本身分割开来，委派给不同的对象管理。策略模式通常把一个系列的算法包装到一系列的策略类里面，作为一个抽象策略类的子类。用一句话来说，就是：“准备一组算法，并将每一个算法封装起来，使得它们可以互换”。下面就以一个示意性的实现讲解策略模式实例的结构。

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" alt="" />

　　这个模式涉及到三个角色：

　　●　　环境(Context)角色：持有一个Strategy的引用。

　　●　　抽象策略(Strategy)角色：这是一个抽象角色，通常由一个接口或抽象类实现。此角色给出所有的具体策略类所需的接口。

　　●　　具体策略(ConcreteStrategy)角色：包装了相关的算法或行为。

源代码

　　环境角色类

public class Context {
    //持有一个具体策略的对象
    private Strategy strategy;
    /**
     * 构造函数，传入一个具体策略对象
     * @param strategy    具体策略对象
     */
    public Context(Strategy strategy){
        this.strategy = strategy;
    }
    /**
     * 策略方法
     */
    public void contextInterface(){

        strategy.strategyInterface();
    }

}

　　抽象策略类

public interface Strategy {
    /**
     * 策略方法
     */
    public void strategyInterface();
}

　　具体策略类

public class ConcreteStrategyA implements Strategy {

    @Override
    public void strategyInterface() {
        //相关的业务
    }

}

public class ConcreteStrategyB implements Strategy {

    @Override
    public void strategyInterface() {
        //相关的业务
    }

}

public class ConcreteStrategyC implements Strategy {

    @Override
    public void strategyInterface() {
        //相关的业务
    }

}

使用场景

　　假设现在要设计一个贩卖各类书籍的电子商务网站的购物车系统。一个最简单的情况就是把所有货品的单价乘上数量，但是实际情况肯定比这要复杂。比如，本网站可能对所有的高级会员提供每本20%的促销折扣；对中级会员提供每本10%的促销折扣；对初级会员没有折扣。

　　根据描述，折扣是根据以下的几个算法中的一个进行的：

　　算法一：对初级会员没有折扣。

　　算法二：对中级会员提供10%的促销折扣。

　　算法三：对高级会员提供20%的促销折扣。

　　使用策略模式来实现的结构图如下：

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" alt="" />

源代码

　　抽象折扣类

public interface MemberStrategy {
    /**
     * 计算图书的价格
     * @param booksPrice    图书的原价
     * @return    计算出打折后的价格
     */
    public double calcPrice(double booksPrice);
}

　　初级会员折扣类

public class PrimaryMemberStrategy implements MemberStrategy {

    @Override
    public double calcPrice(double booksPrice) {

        System.out.println("对于初级会员的没有折扣");
        return booksPrice;
    }

}

　　中级会员折扣类

public class IntermediateMemberStrategy implements MemberStrategy {

    @Override
    public double calcPrice(double booksPrice) {

        System.out.println("对于中级会员的折扣为10%");
        return booksPrice * 0.9;
    }

}

　　高级会员折扣类

public class AdvancedMemberStrategy implements MemberStrategy {

    @Override
    public double calcPrice(double booksPrice) {

        System.out.println("对于高级会员的折扣为20%");
        return booksPrice * 0.8;
    }
}

　　价格类

public class Price {
    //持有一个具体的策略对象
    private MemberStrategy strategy;
    /**
     * 构造函数，传入一个具体的策略对象
     * @param strategy    具体的策略对象
     */
    public Price(MemberStrategy strategy){
        this.strategy = strategy;
    }

    /**
     * 计算图书的价格
     * @param booksPrice    图书的原价
     * @return    计算出打折后的价格
     */
    public double quote(double booksPrice){
        return this.strategy.calcPrice(booksPrice);
    }
}

　　客户端

public class Client {

    public static void main(String[] args) {
        //选择并创建需要使用的策略对象
        MemberStrategy strategy = new AdvancedMemberStrategy();
        //创建环境
        Price price = new Price(strategy);
        //计算价格
        double quote = price.quote(300);
        System.out.println("图书的最终价格为：" + quote);
    }

}

　　从上面的示例可以看出，策略模式仅仅封装算法，提供新的算法插入到已有系统中，以及老算法从系统中“退休”的方法，策略模式并不决定在何时使用何种算法。在什么情况下使用什么算法是由客户端决定的。

认识策略模式

　　策略模式的重心

　　策略模式的重心不是如何实现算法，而是如何组织、调用这些算法，从而让程序结构更灵活，具有更好的维护性和扩展性。

　　算法的平等性

　　策略模式一个很大的特点就是各个策略算法的平等性。对于一系列具体的策略算法，大家的地位是完全一样的，正因为这个平等性，才能实现算法之间可以相互替换。所有的策略算法在实现上也是相互独立的，相互之间是没有依赖的。

　　所以可以这样描述这一系列策略算法：策略算法是相同行为的不同实现。

　　运行时策略的唯一性

　　运行期间，策略模式在每一个时刻只能使用一个具体的策略实现对象，虽然可以动态地在不同的策略实现中切换，但是同时只能使用一个。

　　公有的行为

　　经常见到的是，所有的具体策略类都有一些公有的行为。这时候，就应当把这些公有的行为放到共同的抽象策略角色Strategy类里面。当然这时候抽象策略角色必须要用Java抽象类实现，而不能使用接口。

　　这其实也是典型的将代码向继承等级结构的上方集中的标准做法。

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2tKIpx35LDJnnEOSdvXybBke/srNUbE5ILN8u4l/faflGTBibvzcdnlvq+l8bmnMuyzDk3blpt2/q352VZjq/qbx6TBbM2z/fdt8bn96Rv+QPusiyrqvI5ku8+WKs3JiQXbvbzhLJgbR5vyeV3Wcjqupa70cZ9S9Y7f/1DLpL5Ry68gyOAtXk+S72r1UpSmNyXIe9LfIhlWcqpv7Ztq6qSW2/kTcxAvvthrd6YkFy4g/WtrutkXVsul3KBRLbZfK/d9738bFEUs/+Wj7V5Pm+90mmGYej7fjOdsiyXy+UwDFVVSd+S8TC5fEW+M7JWb0xILtyM78flJJKcBvS/3zO+G14Otvz9ZrKcySN+Gznl6C+cyN2Jy+WS+83CzFVv3/cSxPjBSXbjOwz9ffAT5Dsva/XGhOTCzTXu5YLHMAx1XY/fRPu/jlHXtX/fLTcTTp6hrms5szR5vGkarn8Em6Xevu8vi2D8eF3Xvv10XTc5liLffbBWb0xILpy1cU+9caNeaEFy4ayNe+qNG/VCC5ILZ23cU2/cqBdakFw4a+OeeuNGvdCC5MJZG/fUGzfqhRYkF+7s7Cyx5Ozs7Ngv+UGRb9wS+pZaJBfO2rin3rhRL7QguXDWxj31xo16oQXJhbM27qk3btQLLUgunLVxT71xo15oQXLhrI176o0b9UILkgtnbdxTb9yoF1qQXDhr455640a90ILkwlkb99QbN+qFFiQXztq4p964US+0ILlw1sY99caNeqEFyYWzNu6pN27UCy1ILpy1cU+9caNeaEFy4ayNe+qNG/VCC5ILZ23cU2/cqBdakFw4a+OeeuNGvdCC5MJZG/fUGzfqhRYkF87auKfeuFEvtCC5cHwebtzIN24JfUstkgtnbdxTb9yoF1qQXDhr455640a90ILkwlkb99QbN+qFFiQXztq4p964US+0ILlw1sY99caNeqEFyYWzNu6pN27UCy1ILpy1cU+9caNeaEFy4ayNe+qNG/VCC5ILZ23cU2/cqBdakFw4a+OeeuNGvdCC5MJZG/fUGzfqhRYkF87auKfeuFEvtCC5cNbGPfXGjXqhBcmFszbuqTdu1AstSC6ctXFPvXGjXmhBcuGsjXvqjRv1QguSC2dt3FNv3KgXWpBcuPPz810+TnY2N27cOMr/e35+fuyX/KCOle+xWMs3oW+pRXLKfP311x9++OEnn3xy7B0BdKNv6UVymvz999/vvvvuer3+6KOPfv3112PvDqAYfUsvktPk9u3bT548GYbh2bNnb7/99rF3B1CMvqUXyanx8OHDr776yv/zxx9//Oabb464P4Bq9C29SE6HV69enZ+fv3z50j+yXq9v3rz5/PnzI+4VoBd9Sy+S0+Gzzz77+eefJw/++eefH3zwwVH2B9COvqUXySnwxx9/3Llz58Jvff75548ePTrw/gARoG/pRXIKvPPOO0+fPr3wWy9evLh58+arV68OvEuAdvQtvUju1H3//ff379+/YoNHjx598cUXh9odIBL0Lb1I7qQ9f/781q1bVx9Ordfr999//6+//jrYXgERoG/pRXIn7eOPP/7tt9/euFnTNO+9994B9geIBn1LL5I7XY8fP/7000+33Pjbb7/94Ycf9ro/QEzoW3qR3Ona9e+63rhx49i7DKhB39KL5PRhvgHXxzzSi+T0Yb4B18c80ovk9GG+AdfHPNKL5PRhvgHXxzzSi+T0Yb4B18c80ovk9GG+AdfHPNKL5PRhvgHXxzzSi+T0Yb4B18c80ovk9GG+AdfHPNKL5PRhvgHXxzzSK4bkFovFTn8PSbu33nrr2LtwUIvF4thDDBFK6FtqxZAc4y9u5It9YFzpFUNyjL+4kS/2gXGlVwzJMf7iRr7YB8aVXjEkx/iLG/liHxhXesWQHOMvbuSLfWBc6RVDcoy/uJEv9oFxpVcMyTH+4ka+2AfGlV4xJMf4ixv5Yh8YV3rFkBzjL27ki31gXOkVQ3KMv7iRL/aBcaVXDMkx/uJGvtgHxpVeMSTH+Isb+WIfGFd6xZAc4y9u5It9YFzpFUNyjL+4kS/2gXGlVwzJMf7iRr7YB8aVXnMmZ+1zsKx9LhT5IiYJfUutOZOzNg6oN27W6rWGfPWib4Wj3rhZq9ca8tWLvhWOeuNmrV5ryFevE+1bTdP4r1erVd/3F25W1/WuzzYja+P+wPW2bbvlluSLAOSr14n2rSzL/GLUtm2WZZvbVFWVpmlZlmVZFkWxuYF8axiGNE2HYVgul865ufZwsDfu5603TdPitSzLJKnJBvJ+pW3bPM83n4F8cR3kq5fWvlVVlXNutVoNw1AURVVVXddtPo9zruu6NE27riuKYrVazfje3Nq4n71vla855yZHV1VV+U7Wtq18vRkx+SIY+ep1cn1LFiB5M57nuXMuz3P5Z5ZlsrrJO2vnnCx5RVHIF36lkyeRNTHLsjRN8zz3q6R0u+uzNu5n71v+67IsJ31LspbU0jT1TU7ODJMvro989Tq5viVk2arrWlY0Od4anwXyjUpa2qRveXmeL5fLNE2bpsnzPM/zyy6VBbA27vd6nnDct5qm8QfcTdPIe5QLD6TIF8HIV6/T7VuyJF3Wt5qmaZpGDrzk/XWe51VVjTeQxW61WvkfnPcSiLVxP3vfal8rimLct+SwSR5xzsnbl0lw5ItrIl+9Tq5vyXttv6hJA5P35uMzS33fS2NzzmVZ5pwbN622bf3b87Isx1f1N4/Jglkb9/vuW+Pze9K3/AF3WZZVVfkcyRfXR756nVzfEnLiSBaszeMtufwuC1ld13I32rhvyXrnr3/IRTL/yIV3cASwNu5nqXe1WkkKk/sy5H2JD7EsSzn117ZtVVVy6428iRnIF3MgX71U9q2u62RdWy6XcoFEttl8r933vfxsURSz/5aPtXE/b73SaYZh6Pt+M52yLJfL5TAMVVVJ35LxMLl8Rb4IRr56nVzfkvfjchJJTgP63+8Z3w0vB1v+fjNZzuQRv42ccvQXTuTuxOVyyf1mYeaqt+97CWL84CS78R2G/j74CfLFdZCvXifXt4ZhkAsewzDUdT1+E+3/OkZd1/59t9xMOHmGuq7lzNLk8aZpuP4RbJZ6+76/LILx43Vd+/bTdd3kWIp8cX3kq9cp9i0tqDdu1uq1hnz1om+Fo964WavXGvLVi74VjnrjZq1ea8hXL/pWOOqNm7V6rSFfvehb4ag3btbqtYZ89ZozubOzs8SSs7OzGV+900e+iElC31KL461w1Bs3a/VaQ7560bfCUW/crNVrDfnqRd8KR71xs1avNeSrF30rHPXGzVq91pCvXvStcNQbN2v1WkO+etG3wlFv3KzVaw356kXfCke9cbNWrzXkqxd9Kxz1xs1avdaQr170rXDUGzdr9VpDvnrRt8JRb9ys1WsN+epF3wpHvXGzVq815KsXfSsc9cbNWr3WkK9e9K1w1Bs3a/VaQ7560bfCUW/crNVrDfnqRd8KR71xs1avNeSrF30rHPXGzVq91pCvXvStcNQbN2v1WkO+evF5x+GsfR4u+SImCX1LLY63wlFv3KzVaw356kXfCke9cbNWrzXkqxd9Kxz1xs1avdaQr170rXDUGzdr9VpDvnrRt8JRb9ys1WsN+epF3wpHvXGzVq815KsXfSsc9cbNWr3WkK9e9K1w1Bs3a/VaQ7560bfCUW/crNVrDfnqRd8KR71xs1avNeSrF30rHPXGzVq91pCvXvStcNQbN2v1WkO+etG3wlFv3KzVaw356kXfCke9cbNWrzXkqxd9Kxz1xs1avdaQr170rXDUGzdr9VpDvnrRt8JRb9ys1WsN+eo1Z3Ln5+e7fJzsbP7xj38c5f89Pz+f8dU7fcfK91is5WtNQt9SS31yP/3002KxePDgwbF3BIAm9C29dCf34sWLu3fvrtfre/fuPX369Ni7A0AN+pZeupP75z//+eTJk2EYnj17tlgsjr07ANSgb+mlOLk///zzX//6l//nv//978ePHx9xfwAoQt/SS2ty6/X69u3bL1++vOIRALgMfUsvrcldeHQ1OQIDgMvQt/RSmdwVV7P8FS8AuAJ9Sy+VyV1x96C/w/DAuwRAF/qWXvqS++WXX67+ba2ffvrp4cOHB9sfABrRt/RSltzLly/v3r376tWrK7ZZr9eLxeK///3vwfYKgDr0Lb2UJXf//v3//Oc/b9ysaZovv/xy/7sDQCv6ll6aktupG23Z4QDYRN/SS1Ny9+7d2+nvot69e/fYuwzgRNG39IohOcYfgF2xbugVQ3KMPwC7Yt3QK4bkGH8AdsW6oVcMyTH+AOyKdUOvGJJj/AHYFeuGXjEkx/gDsCvWDb1iSI7xB2BXrBt6xZAc4w/Arlg39IohOcYfgF2xbugVQ3KMPwC7Yt3QK4bkGH8AdsW6oVcMyTH+AOyKdUOvGJJj/AHYFeuGXjEkx/gDsCvWDb1iSI7xB2BXrBt6xZAc4w/Arlg39IohOcYfgF2xbugVQ3KMPwC7Yt3QK4bkGH8AdsW6oVcMyTH+AOyKdUOvGJJj/AHYFeuGXjEkx/gDsCvWDb3mTG6xWCSWLBaLGV89AIeU0LfUmjM5a+PAWr1ATJi/etG3wlmrF4gJ81cv+lY4a/UCMWH+6qW4b7Vtu+WWTdPsYwcY94BezF+9TrdvpWlavJZlWVmWmxv0fT8MQ9u2eZ5vPkNZlvJTaZoOw7BcLp1zM+4h4x7Qi/mr10n3rfI159zk6KqqKt/J2raVr7uumzyJc67rujRNu64rimK1Ws147MW4B/Ri/up10n3Lf12W5aRvZVkmh1llWaZp6ptcXdfDMEiXkkeyLEvTVLYUq9Vqlj1k3AN6MX/1Oum+NT5POO5bTdNkWSZHTk3TOOeccxceSOV5vlwu0zRtmibP8zzP5dTiLBj3gF7MX71Oum+1rxVFMe5bctgkjzjn6rqWc4njH5d+1jTNarXy35r3EhfjHtCL+auXpr41Pr8nfUs6llzfqqrK353Rtq0//CrLcnzXxub9HcEY94BezF+9Tq5vrVYrOZya3JdRFEWaplVVyWZlWcqpv7Ztq6oqiqKqqizLpEVJP/PXt5xz4+tbm7dvhGHcA3oxf/U6ub7lSacZhqHv+6IoJpevyrJcLpfDMFRVJX1Ljswml6/6vs+ybBiGzWe4PsY9oBfzV69T7Ft93zvnpC15cleF/+f4DkN/H/yE3L7hN8vzXJ6W+wkBMH/1Orm+1ff9ZZegxo/Xde3bT9d1k2Opuq7lzOHkGZqm4foWgIH5q9nJ9S1FrNULxIT5qxd9K5y1eoGYMH/1om+Fs1YvEBPmr170rXDW6gViwvzVi74Vzlq9QEyYv3rNmdzZ2VliydnZ2YyvHoBDSuhbanG8Fc5avUBMmL960bfCWasXiAnzVy/6Vjhr9QIxYf7qRd8KZ61eICbMX73oW+Gs1QvEhPmrF30rnLV6gZgwf/Wib4WzVi8QE+avXvStcNbqBWLC/NWLvhXOWr1ATJi/etG3wlmrF4gJ81cv+lY4a/UCMWH+6kXfCmetXiAmzF+96FvhrNULxIT5qxd9K5y1eoGYMH/1om+Fs1YvEBPmr170rXDW6gViwvzVi74Vzlq9QEyYv3rxecfh+LxjQK+EvqUWx1vhrNULxIT5qxd9K5y1eoGYMH/1om+Fs1YvEBPmr170rXDW6gViwvzVi74Vzlq9QEyYv3rRt8JZqxeICfNXL/pWOGv1AjFh/upF3wpnrV4gJsxfvehb4azVC8SE+asXfSuctXqBmDB/9aJvhbNWLxAT5q9e9K1w1uoFYsL81Yu+Fc5avUBMmL960bfCWasXiAnzVy/6Vjhr9QIxYf7qRd8KZ61eICbMX73oW+Gs1QvEhPmr15zJnZ+fz/uBwifu/Px8xlcPwCEl9C21SA6ARfQtvUgOgEX0Lb1IDoBF9C29SA6ARbdv3z72LiAQfQsAoAl9CwCgCX0LAKAJfQsAoAl9CwCgCX0LAKAJfQsAoAl9CwCgCX0LAAWi8yQAAAArSURBVKAJfQsAoAl9CwCgCX0LAKAJfQsAoAl9CwCgCX0LAKAJfQsAoMn/AMf9A0W3O98gAAAAAElFTkSuQmCC" 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策略模式的优点

　　（1）策略模式提供了管理相关的算法族的办法。策略类的等级结构定义了一个算法或行为族。恰当使用继承可以把公共的代码移到父类里面，从而避免代码重复。

　　（2）使用策略模式可以避免使用多重条件(if-else)语句。多重条件语句不易维护，它把采取哪一种算法或采取哪一种行为的逻辑与算法或行为的逻辑混合在一起，统统列在一个多重条件语句里面，比使用继承的办法还要原始和落后。

策略模式的缺点

　　（1）客户端必须知道所有的策略类，并自行决定使用哪一个策略类。这就意味着客户端必须理解这些算法的区别，以便适时选择恰当的算法类。换言之，策略模式只适用于客户端知道算法或行为的情况。

　　（2）由于策略模式把每个具体的策略实现都单独封装成为类，如果备选的策略很多的话，那么对象的数目就会很可观。

转自：http://www.cnblogs.com/java-my-life/archive/2012/05/10/2491891.html