java-design-patterns/strategy/README.md

---
layout: pattern
title: Strategy
folder: strategy
permalink: /patterns/strategy/
categories: Behavioral
tags:
 - 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 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.

```java
@FunctionalInterface
public interface DragonSlayingStrategy {

  void execute();
}

public class MeleeStrategy implements DragonSlayingStrategy {

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

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

public class ProjectileStrategy implements DragonSlayingStrategy {

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

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

public class SpellStrategy implements DragonSlayingStrategy {

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

  @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 is able to pick his fighting strategy based on the 
opponent.

```java
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.

```java
    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 new feature Lambda Expressions in Java 8 provides another approach for the implementation:

```java
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.

```java
    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();
```

Program output is the same as above one.

## Class diagram

![alt text](./etc/strategy_urm.png "Strategy")

## 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 related conditional branches into their own Strategy class

## Tutorial 

* [Strategy Pattern Tutorial](https://www.journaldev.com/1754/strategy-design-pattern-in-java-example-tutorial)

## Credits

* [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)
* [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)
* [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)
* [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)