Flyweight Design Pattern


Intent

Use sharing to support large numbers of fine-grained objects efficiently.

Explanation

Real world example

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

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.

 A Flyweight Pattern says that just "to reuse already existing similar kind of objects by storing them and create new object when no matching object is found".

Advantage of Flyweight Pattern

  • It reduces the number of objects.
  • It reduces the amount of memory and storage devices required if the objects are persisted

Structure


Participants

1. Flyweight
  • declares an interface through which flyweights can receive and action extrinsic state.

2. ConcreteFlyweight

  • implements the Flyweight interface and adds storage for intrinsic state, if any. A ConcreteFlyweight object must be sharable. Any state it stores must be intrinsic; that is, it must be independent of the ConcreteFlyweight object's context.
3. UnsharedConcreteFlyweight
  • not all Flyweight subclasses need to be shared. The Flyweight interface enables sharing; it doesn't enforce it. It's common for UnsharedConcreteFlyweight objects to have ConcreteFlyweight objects as children at some level in the flyweight object structure (as the Row and Column classes have).
4. FlyweightFactory
  • creates and manages flyweight objects.
  • ensures that flyweights are shared properly. When a client requests a flyweight, the FlyweightFactory object supplies an existing instance or creates one, if none exists.
5. Client
  • maintains a reference to flyweight(s).
  • computes or stores the extrinsic state of flyweight(s).

Source Code

Step1 : Let's create different potion types
public interface Potion {
  void drink();
}

public class HealingPotion implements Potion {
  private static final Logger LOGGER = LoggerFactory.getLogger(HealingPotion.class);
  @Override
  public void drink() {
    LOGGER.info("You feel healed. (Potion={})", System.identityHashCode(this));
  }
}

public class HolyWaterPotion implements Potion {
  private static final Logger LOGGER = LoggerFactory.getLogger(HolyWaterPotion.class);
  @Override
  public void drink() {
    LOGGER.info("You feel blessed. (Potion={})", System.identityHashCode(this));
  }
}

public class InvisibilityPotion implements Potion {
  private static final Logger LOGGER = LoggerFactory.getLogger(InvisibilityPotion.class);
  @Override
  public void drink() {
    LOGGER.info("You become invisible. (Potion={})", System.identityHashCode(this));
  }
}
Step 2: Then the actual Flyweight object 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) {
    Potion 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;
  }
}
Step 3 : it can be used as below
PotionFactory factory = new PotionFactory();
factory.createPotion(PotionType.INVISIBILITY).drink(); // You become invisible. (Potion=6566818)
factory.createPotion(PotionType.HEALING).drink(); // You feel healed. (Potion=648129364)
factory.createPotion(PotionType.INVISIBILITY).drink(); // You become invisible. (Potion=6566818)
factory.createPotion(PotionType.HOLY_WATER).drink(); // You feel blessed. (Potion=1104106489)
factory.createPotion(PotionType.HOLY_WATER).drink(); // You feel blessed. (Potion=1104106489)
factory.createPotion(PotionType.HEALING).drink(); // You feel healed. (Potion=648129364)

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 object state can be made extrinsic
  • many groups of objects may be replaced by relatively few shared objects once 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.

Real world examples


Credits