java-design-patterns/strategy

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layout	title	folder	permalink	categories	language	tags
pattern	Strategy	strategy	/patterns/strategy/	Behavioral	en	Gang of Four

Also known as

Policy

Intent

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from the clients that use it.

Explanation

Real-world example

Slaying dragons is a dangerous job. With experience, it becomes easier. Veteran dragonslayers have developed different fighting strategies against different types of dragons.

In plain words

Strategy pattern allows choosing the best-suited algorithm at runtime.

Wikipedia says

In computer programming, the strategy pattern (also known as the policy pattern) is a behavioral software design pattern that enables selecting an algorithm at runtime.

Programmatic Example

Let's first introduce the dragon-slaying strategy interface and its implementations.

@FunctionalInterface
public interface DragonSlayingStrategy {

  void execute();
}

@Slf4j
public class MeleeStrategy implements DragonSlayingStrategy {

  @Override
  public void execute() {
    LOGGER.info("With your Excalibur you sever the dragon's head!");
  }
}

@Slf4j
public class ProjectileStrategy implements DragonSlayingStrategy {

  @Override
  public void execute() {
    LOGGER.info("You shoot the dragon with the magical crossbow and it falls dead on the ground!");
  }
}

@Slf4j
public class SpellStrategy implements DragonSlayingStrategy {

  @Override
  public void execute() {
    LOGGER.info("You cast the spell of disintegration and the dragon vaporizes in a pile of dust!");
  }
}

And here is the mighty dragonslayer, who can pick his fighting strategy based on the opponent.

public class DragonSlayer {

  private DragonSlayingStrategy strategy;

  public DragonSlayer(DragonSlayingStrategy strategy) {
    this.strategy = strategy;
  }

  public void changeStrategy(DragonSlayingStrategy strategy) {
    this.strategy = strategy;
  }

  public void goToBattle() {
    strategy.execute();
  }
}

Finally, here's the dragonslayer in action.

    LOGGER.info("Green dragon spotted ahead!");
    var dragonSlayer = new DragonSlayer(new MeleeStrategy());
    dragonSlayer.goToBattle();
    LOGGER.info("Red dragon emerges.");
    dragonSlayer.changeStrategy(new ProjectileStrategy());
    dragonSlayer.goToBattle();
    LOGGER.info("Black dragon lands before you.");
    dragonSlayer.changeStrategy(new SpellStrategy());
    dragonSlayer.goToBattle();

Program output:

    Green dragon spotted ahead!
    With your Excalibur you sever the dragon's head!
    Red dragon emerges.
    You shoot the dragon with the magical crossbow and it falls dead on the ground!
    Black dragon lands before you.
    You cast the spell of disintegration and the dragon vaporizes in a pile of dust!    

What's more, the lambda expressions in Java 8 provides another approach for the implementation:

public class LambdaStrategy {

  private static final Logger LOGGER = LoggerFactory.getLogger(LambdaStrategy.class);

  public enum Strategy implements DragonSlayingStrategy {
    MeleeStrategy(() -> LOGGER.info(
        "With your Excalibur you severe the dragon's head!")),
    ProjectileStrategy(() -> LOGGER.info(
        "You shoot the dragon with the magical crossbow and it falls dead on the ground!")),
    SpellStrategy(() -> LOGGER.info(
        "You cast the spell of disintegration and the dragon vaporizes in a pile of dust!"));

    private final DragonSlayingStrategy dragonSlayingStrategy;

    Strategy(DragonSlayingStrategy dragonSlayingStrategy) {
      this.dragonSlayingStrategy = dragonSlayingStrategy;
    }

    @Override
    public void execute() {
      dragonSlayingStrategy.execute();
    }
  }
}

And here's the dragonslayer in action.

    LOGGER.info("Green dragon spotted ahead!");
    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.MeleeStrategy);
    dragonSlayer.goToBattle();
    LOGGER.info("Red dragon emerges.");
    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.ProjectileStrategy);
    dragonSlayer.goToBattle();
    LOGGER.info("Black dragon lands before you.");
    dragonSlayer.changeStrategy(LambdaStrategy.Strategy.SpellStrategy);
    dragonSlayer.goToBattle();

The program output is the same as the above one.

Class diagram

Applicability

Use the Strategy pattern when

• Many related classes differ only in their behavior. Strategies provide a way to configure a class either one of many behaviors
• You need different variants of an algorithm. for example, you might define algorithms reflecting different space/time trade-offs. Strategies can be used when these variants are implemented as a class hierarchy of algorithms
• An algorithm uses data that clients shouldn't know about. Use the Strategy pattern to avoid exposing complex algorithm-specific data structures
• A class defines many behaviors, and these appear as multiple conditional statements in its operations. Instead of many conditionals, move the related conditional branches into their own Strategy class

Tutorial

• Strategy Pattern Tutorial

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software
• Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions
• Head First Design Patterns: A Brain-Friendly Guide
• Refactoring to Patterns