137 lines
4.7 KiB
Markdown
137 lines
4.7 KiB
Markdown
---
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layout: pattern
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title: Strategy
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folder: strategy
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permalink: /patterns/strategy/
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categories: Behavioral
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tags:
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- Gang of Four
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---
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## 又被称为
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政策（方针）模式
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## 目的
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定义一个家族算法，并封装好其中每一个，使它们可以互相替换。策略模式使算法的变化独立于使用它的客户。
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## 解释
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现实世界例子
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> 屠龙是一项危险的职业。有经验将会使它变得简单。经验丰富的屠龙者对不同类型的龙有不同的战斗策略。
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直白点说
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> 策略模式允许在运行时选择最匹配的算法。
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维基百科上说
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> 在程序编程领域，策略模式（又叫政策模式）是一种启用在运行时选择算法的行为型软件设计模式。
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**编程实例**
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让我们先介绍屠龙的策略模式接口和它的实现。
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```java
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@FunctionalInterface
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public interface DragonSlayingStrategy {
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void execute();
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}
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public class MeleeStrategy implements DragonSlayingStrategy {
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private static final Logger LOGGER = LoggerFactory.getLogger(MeleeStrategy.class);
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@Override
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public void execute() {
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LOGGER.info("With your Excalibur you sever the dragon's head!");
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}
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}
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public class ProjectileStrategy implements DragonSlayingStrategy {
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private static final Logger LOGGER = LoggerFactory.getLogger(ProjectileStrategy.class);
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@Override
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public void execute() {
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LOGGER.info("You shoot the dragon with the magical crossbow and it falls dead on the ground!");
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}
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}
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public class SpellStrategy implements DragonSlayingStrategy {
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private static final Logger LOGGER = LoggerFactory.getLogger(SpellStrategy.class);
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@Override
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public void execute() {
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LOGGER.info("You cast the spell of disintegration and the dragon vaporizes in a pile of dust!");
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}
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}
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```
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现在有一个强力的屠龙者要基于上面的组件来选择他的战斗策略。
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```java
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public class DragonSlayer {
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private DragonSlayingStrategy strategy;
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public DragonSlayer(DragonSlayingStrategy strategy) {
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this.strategy = strategy;
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}
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public void changeStrategy(DragonSlayingStrategy strategy) {
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this.strategy = strategy;
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}
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public void goToBattle() {
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strategy.execute();
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}
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}
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```
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最后是屠龙者的行动。
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```java
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LOGGER.info("Green dragon spotted ahead!");
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var dragonSlayer = new DragonSlayer(new MeleeStrategy());
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dragonSlayer.goToBattle();
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LOGGER.info("Red dragon emerges.");
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dragonSlayer.changeStrategy(new ProjectileStrategy());
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dragonSlayer.goToBattle();
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LOGGER.info("Black dragon lands before you.");
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dragonSlayer.changeStrategy(new SpellStrategy());
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dragonSlayer.goToBattle();
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// Green dragon spotted ahead!
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// With your Excalibur you sever the dragon's head!
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// Red dragon emerges.
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// You shoot the dragon with the magical crossbow and it falls dead on the ground!
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// Black dragon lands before you.
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// You cast the spell of disintegration and the dragon vaporizes in a pile of dust!
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```
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## 类图
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## 应用
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使用策略模式当
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* 许多相关的类只是行为不同。策略模式提供了一种为一种类配置多种行为的能力。
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* 你需要一种算法的不同变体。比如，你可能定义反应不用时间空间权衡的算法。当这些算法的变体使用类的层次结构来实现时就可以使用策略模式。
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* 一个算法使用的数据客户不应该对其知晓。使用策略模式来避免暴露复杂的，特定于算法的数据结构。
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* 一个类定义了许多行为，这些行为在其操作中展现为多个条件语句。移动相关的条件分支到它们分别的策略类中来代替这些条件语句。
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## 教学
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* [Strategy Pattern Tutorial](https://www.journaldev.com/1754/strategy-design-pattern-in-java-example-tutorial)
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## 鸣谢
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions](https://www.amazon.com/gp/product/1937785467/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1937785467&linkCode=as2&tag=javadesignpat-20&linkId=7e4e2fb7a141631491534255252fd08b)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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* [Refactoring to Patterns](https://www.amazon.com/gp/product/0321213351/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321213351&linkCode=as2&tag=javadesignpat-20&linkId=2a76fcb387234bc71b1c61150b3cc3a7)
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