Flyweight Pattern

Real-World Example

Magic Potions Example

Alchemist’s shop has shelves full of magic potions. Many of the potions are the same so there is no need to create a new object for each of them. Instead, one object instance can represent multiple shelf items so the memory footprint remains small

Example Implementation

Magic Potions Example (Simple)

Translating our alchemist shop example from above. First of all, we have different potion types:

public class HealingPotion {
  public void drink() { System.out.println("You feel healed. Potion=" + System.identityHashCode(this)); }
}

Then the actual Flyweight class PotionFactory, which is the factory for creating potions

public class PotionFactory {
  private HealingPotion potion = null;

  public HealingPotion createPotion() {
    if (potion == null) {
      potion = new HealingPotion();
    }
    return potion;
  }
}

AlchemistShop contains two shelves of magic potions. The potions are created using the aforementioned PotionFactory.

public class AlchemistShop {

  private final List<Potion> topShelf;
  private final List<Potion> bottomShelf;

  public AlchemistShop() {
    var factory = new PotionFactory();
    topShelf = List.of(
        factory.createPotion(),
        factory.createPotion(),
        factory.createPotion()
    );
    bottomShelf = List.of(
        factory.createPotion(),
        factory.createPotion()
    );
  }

  public final List<Potion> getTopShelf() {
    return List.copyOf(this.topShelf);
  }
  public final List<Potion> getBottomShelf() {
    return List.copyOf(this.bottomShelf);
  }

  public void drinkPotions() {
    System.out.println("Drinking top shelf potions");
    topShelf.forEach(Potion::drink);
    System.out.println("Drinking bottom shelf potions");
    bottomShelf.forEach(Potion::drink);
  }
}
 

In our scenario, a brave visitor enters the alchemist shop and drinks all the potions.

// create the alchemist shop with the potions
var alchemistShop = new AlchemistShop();
// a brave visitor enters the alchemist shop and drinks all the potions
alchemistShop.drinkPotions();

Program output:

Drinking top shelf potions 
You feel healed. Potion=125130493
You feel healed. Potion=125130493
You feel healed. Potion=125130493
Drinking bottom shelf potions
You feel healed. Potion=125130493
You feel healed. Potion=125130493

Magic Potions Example (Complex)

Translating our alchemist shop example from above. First of all, we have different potion types:

public interface Potion {
  void drink();
}

public class HealingPotion implements Potion {
  public void drink() { System.out.println("You feel healed. Potion=" + System.identityHashCode(this)); }
}
public class HolyWaterPotion implements Potion {
  public void drink() { System.out.println("You feel blessed. Potion=" + System.identityHashCode(this)); }
}
public class InvisibilityPotion implements Potion {
  public void drink() { System.out.println("You become invisible. Potion=", System.identityHashCode(this)); }
}

Then the actual Flyweight class PotionFactory, which is the factory for creating potions

public class PotionFactory {
  private final Map<PotionType, Potion> potions = new EnumMap<>(PotionType.class);

  Potion createPotion(PotionType type) {
    var potion = potions.get(type);
    if (potion == null) {
      switch (type) {
        case HEALING:
          potion = new HealingPotion();
          potions.put(type, potion);
          break;
        case HOLY_WATER:
          potion = new HolyWaterPotion();
          potions.put(type, potion);
          break;
        case INVISIBILITY:
          potion = new InvisibilityPotion();
          potions.put(type, potion);
          break;
        default:
          break;
      }
    }
    return potion;
  }
}

AlchemistShop contains two shelves of magic potions. The potions are created using the aforementioned PotionFactory.

public class AlchemistShop {

  private final List<Potion> topShelf;
  private final List<Potion> bottomShelf;

  public AlchemistShop() {
    var factory = new PotionFactory();
    topShelf = List.of(
        factory.createPotion(PotionType.INVISIBILITY),
        factory.createPotion(PotionType.INVISIBILITY),
        factory.createPotion(PotionType.STRENGTH),
        factory.createPotion(PotionType.HEALING),
        factory.createPotion(PotionType.INVISIBILITY),
        factory.createPotion(PotionType.STRENGTH),
        factory.createPotion(PotionType.HEALING),
        factory.createPotion(PotionType.HEALING)
    );
    bottomShelf = List.of(
        factory.createPotion(PotionType.POISON),
        factory.createPotion(PotionType.POISON),
        factory.createPotion(PotionType.POISON),
        factory.createPotion(PotionType.HOLY_WATER),
        factory.createPotion(PotionType.HOLY_WATER)
    );
  }

  public final List<Potion> getTopShelf() {
    return List.copyOf(this.topShelf);
  }
  public final List<Potion> getBottomShelf() {
    return List.copyOf(this.bottomShelf);
  }

  public void drinkPotions() {
    System.out.println("Drinking top shelf potions");
    topShelf.forEach(Potion::drink);
    System.out.println("Drinking bottom shelf potions");
    bottomShelf.forEach(Potion::drink);
  }
}
 

In our scenario, a brave visitor enters the alchemist shop and drinks all the potions.

// create the alchemist shop with the potions
var alchemistShop = new AlchemistShop();
// a brave visitor enters the alchemist shop and drinks all the potions
alchemistShop.drinkPotions();

Program output:

Drinking top shelf potions 
You become invisible. Potion=1509514333
You become invisible. Potion=1509514333
You feel strong. Potion=739498517
You feel healed. Potion=125130493
You become invisible. Potion=1509514333
You feel strong. Potion=739498517
You feel healed. Potion=125130493
You feel healed. Potion=125130493
Drinking bottom shelf potions
Urgh! This is poisonous. Potion=166239592
Urgh! This is poisonous. Potion=166239592
Urgh! This is poisonous. Potion=166239592
You feel blessed. Potion=991505714
You feel blessed. Potion=991505714

Applicability

The Flyweight pattern’s effectiveness depends heavily on how and where it’s used. Apply the Flyweight pattern when all of the following are true:

• An application uses a large number of objects.
• Storage costs are high because of the sheer quantity of objects.
• Most of the object states can be made extrinsic.
• Many groups of objects may be replaced by relatively few shared objects once the extrinsic state is removed.
• The application doesn’t depend on object identity. Since flyweight objects may be shared, identity tests will return true for conceptually distinct objects.

Comparisons

Click here to expand...

Facade Pattern vs Flyweight Pattern

• Flyweight Pattern shows how to make lots of little objects
• Facade Pattern shows how to make a single object represent an entire subsystem

Link to original

Resources

• https://java-design-patterns.com/patterns/flyweight/