| layout | title | folder | permalink | categories | tags | ||
|---|---|---|---|---|---|---|---|
pattern |
Flyweight |
flyweight |
/patterns/flyweight/ |
Structural |
|
Use sharing to support large numbers of fine-grained objects efficiently.
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.
Programmatic example
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 {
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));
}
}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();
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;
}
}And it can be used as below:
var 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)Program output:
You become invisible. (Potion=6566818)
You feel healed. (Potion=648129364)
You become invisible. (Potion=6566818)
You feel blessed. (Potion=1104106489)
You feel blessed. (Potion=1104106489)
You feel healed. (Potion=648129364)
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.
- java.lang.Integer#valueOf(int) and similarly for Byte, Character and other wrapped types.