使用策略模式，一个类的行为和算法可以在运行时改变。
**
在策略模式中，我们创建提供各种不同策略的对象，和一个执行对象。当它持有不同策略对象的引用时，能够表现出不同的行为。
下面是策略模式的UML图：
策略模式UML.jpg

创建一个屠龙策略接口（DragonSlayingStrategy），使用三种不同的屠龙策略实现该接口（MeleeStrategy、ProjectileStrategy、SpellStrategy）。创建一个屠龙勇士（DragonSlayer），屠龙勇士可以使用不同的屠龙策略去屠龙。

代码实现

创建策略接口

DragonSlayingStrategy.java

  1. /**
  2.  * Strategy interface.
  3.  */
  4. @FunctionalInterface
  5. public interface DragonSlayingStrategy {
  7.   void execute();
  9. }

创建具体的实现类

MeleeStrategy.java

  1. /**
  2.  * Melee strategy.
  3.  */
  4. public class MeleeStrategy implements DragonSlayingStrategy {
  6.   private static final Logger LOGGER = LoggerFactory.getLogger(MeleeStrategy.class);
  8.   @Override
  9.   public void execute() {
  10.     LOGGER.info("With your Excalibur you sever the dragon's head!");
  11.   }
  12. }

ProjectileStrategy.java

  1. /**
  2.  * Projectile strategy.
  3.  */
  4. public class ProjectileStrategy implements DragonSlayingStrategy {
  6.   private static final Logger LOGGER = LoggerFactory.getLogger(ProjectileStrategy.class);
  8.   @Override
  9.   public void execute() {
  10.     LOGGER.info("You shoot the dragon with the magical crossbow and it falls dead on the ground!");
  11.   }
  12. }

SpellStrategy.java

  1. /**
  2.  * 
  3.  * Spell strategy.
  4.  *
  5.  */
  6. public class SpellStrategy implements DragonSlayingStrategy {
  8.   private static final Logger LOGGER = LoggerFactory.getLogger(SpellStrategy.class);
  10.   @Override
  11.   public void execute() {
  12.     LOGGER.info("You cast the spell of disintegration and the dragon vaporizes in a pile of dust!");
  13.   }
  15. }

创建执行类

DragonSlayer.java

  1. /**
  2.  * 
  3.  * DragonSlayer uses different strategies to slay the dragon.
  4.  * 
  5.  */
  6. public class DragonSlayer {
  8.   private DragonSlayingStrategy strategy;
  10.   public DragonSlayer(DragonSlayingStrategy strategy) {
  11.     this.strategy = strategy;
  12.   }
  14.   public void changeStrategy(DragonSlayingStrategy strategy) {
  15.     this.strategy = strategy;
  16.   }
  18.   public void goToBattle() {
  19.     strategy.execute();
  20.   }
  21. }

编写Main方法

App.java

  1. public class App {
  3.   private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
  5.   public static void main(String[] args) {
  6.     // 改变不同的策略来屠龙
  7.     LOGGER.info("Green dragon spotted ahead!");
  8.     DragonSlayer dragonSlayer = new DragonSlayer(new MeleeStrategy());
  9.     dragonSlayer.goToBattle();
  10.     LOGGER.info("Red dragon emerges.");
  11.     dragonSlayer.changeStrategy(new ProjectileStrategy());
  12.     dragonSlayer.goToBattle();
  13.     LOGGER.info("Black dragon lands before you.");
  14.     dragonSlayer.changeStrategy(new SpellStrategy());
  15.     dragonSlayer.goToBattle();
  17.     // Java 8 函数式接口方式
  18.     LOGGER.info("Green dragon spotted ahead!");
  19.     dragonSlayer = new DragonSlayer(
  20.         () -> LOGGER.info("With your Excalibur you severe the dragon's head!"));
  21.     dragonSlayer.goToBattle();
  22.     LOGGER.info("Red dragon emerges.");
  23.     dragonSlayer.changeStrategy(() -> LOGGER.info(
  24.         "You shoot the dragon with the magical crossbow and it falls dead on the ground!"));
  25.     dragonSlayer.goToBattle();
  26.     LOGGER.info("Black dragon lands before you.");
  27.     dragonSlayer.changeStrategy(() -> LOGGER.info(
  28.         "You cast the spell of disintegration and the dragon vaporizes in a pile of dust!"));
  29.     dragonSlayer.goToBattle();
  30.   }
  31. }

运行结果

  1. Green dragon spotted ahead!
  2. With your Excalibur you sever the dragon's head!
  3. Red dragon emerges.
  4. You shoot the dragon with the magical crossbow and it falls dead on the ground!
  5. Black dragon lands before you.
  6. You cast the spell of disintegration and the dragon vaporizes in a pile of dust!
  7. Green dragon spotted ahead!
  8. With your Excalibur you severe the dragon's head!
  9. Red dragon emerges.
  10. You shoot the dragon with the magical crossbow and it falls dead on the ground!
  11. Black dragon lands before you.
  12. You cast the spell of disintegration and the dragon vaporizes in a pile of dust!