Flyweight

Intent

The Flyweight pattern's primary intent is to reduce the number of objects created, decrease memory footprint and increase performance by sharing as much data as possible with similar objects.

Explanation

Real-world example

A real-world example of the Flyweight design pattern is in a document editor like Microsoft Word or Google Docs. In such applications, each character in a document could potentially be a separate object, which would be highly inefficient in terms of memory usage. Instead, the Flyweight pattern can be used to share character objects. For instance, all instances of the letter 'A' can share a single 'A' object with its intrinsic state (e.g., the shape of the character). The extrinsic state, such as the position, font, and color, can be stored separately and applied as needed. This way, the application efficiently manages memory by reusing existing objects for characters that appear multiple times.

In plain words

It is used to minimize memory usage or computational expenses by sharing as much as possible with similar objects.

Wikipedia says

In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory.

Programmatic 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.

First of all, we have different Potion types:

public interface Potion {
  void drink();
}

@Slf4j
public class HealingPotion implements Potion {
  @Override
  public void drink() {
    LOGGER.info("You feel healed. (Potion={})", System.identityHashCode(this));
  }
}

@Slf4j
public class HolyWaterPotion implements Potion {
  @Override
  public void drink() {
    LOGGER.info("You feel blessed. (Potion={})", System.identityHashCode(this));
  }
}

@Slf4j
public class InvisibilityPotion implements Potion {
  @Override
  public void drink() {
    LOGGER.info("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;

  public PotionFactory() {
    potions = new EnumMap<>(PotionType.class);
  }

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

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

@Slf4j
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() {
    LOGGER.info("Drinking top shelf potions\n");
    topShelf.forEach(Potion::drink);
    LOGGER.info("Drinking bottom shelf potions\n");
    bottomShelf.forEach(Potion::drink);
  }
}

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

public static void main(String[] args) {
    // 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:

09:02:52.731 [main] INFO com.iluwatar.flyweight.AlchemistShop -- Drinking top shelf potions
09:02:52.733 [main] INFO com.iluwatar.flyweight.InvisibilityPotion -- You become invisible. (Potion=1395089624)
09:02:52.733 [main] INFO com.iluwatar.flyweight.InvisibilityPotion -- You become invisible. (Potion=1395089624)
09:02:52.733 [main] INFO com.iluwatar.flyweight.StrengthPotion -- You feel strong. (Potion=1450821318)
09:02:52.733 [main] INFO com.iluwatar.flyweight.HealingPotion -- You feel healed. (Potion=668849042)
09:02:52.733 [main] INFO com.iluwatar.flyweight.InvisibilityPotion -- You become invisible. (Potion=1395089624)
09:02:52.733 [main] INFO com.iluwatar.flyweight.StrengthPotion -- You feel strong. (Potion=1450821318)
09:02:52.733 [main] INFO com.iluwatar.flyweight.HealingPotion -- You feel healed. (Potion=668849042)
09:02:52.733 [main] INFO com.iluwatar.flyweight.HealingPotion -- You feel healed. (Potion=668849042)
09:02:52.733 [main] INFO com.iluwatar.flyweight.AlchemistShop -- Drinking bottom shelf potions
09:02:52.734 [main] INFO com.iluwatar.flyweight.PoisonPotion -- Urgh! This is poisonous. (Potion=2096057945)
09:02:52.734 [main] INFO com.iluwatar.flyweight.PoisonPotion -- Urgh! This is poisonous. (Potion=2096057945)
09:02:52.734 [main] INFO com.iluwatar.flyweight.PoisonPotion -- Urgh! This is poisonous. (Potion=2096057945)
09:02:52.734 [main] INFO com.iluwatar.flyweight.HolyWaterPotion -- You feel blessed. (Potion=1689843956)
09:02:52.734 [main] INFO com.iluwatar.flyweight.HolyWaterPotion -- You feel blessed. (Potion=1689843956)

Applicability

The Flyweight pattern's effectiveness depends heavily on how and where it's used. Apply the Flyweight pattern when all 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 state 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.

Known uses

• java.lang.Integer#valueOf(int) and similarly for Byte, Character and other wrapped types.
• Java’s String class, which uses the Flyweight pattern internally to manage string literals.
• GUI applications, where objects like fonts or graphical components are shared rather than duplicated.

Consequences

Benefits:

• Reduces the number of instances of an object, conserving memory.
• Centralizes state management, reducing the risk of inconsistent state.

Trade-offs:

• Increases complexity by adding the management layer for shared objects.
• Potential overhead in accessing shared objects if not well implemented.

Related Patterns

• Composite: Often combined with Flyweight when the composites are shareable. Both are used to manage hierarchies and structures of objects.
• State: Can be used to manage state in a shared Flyweight object, distinguishing internal state (invariant) from external state (context-specific).

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software
• Head First Design Patterns: Building Extensible and Maintainable Object-Oriented Software