java-design-patterns/flyweight/README.md

---
layout: pattern
title: Flyweight
folder: flyweight
permalink: /patterns/flyweight/
categories: Structural
tags:
 - Gang Of Four
 - Performance
---

## 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.

**Programmatic example**

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

```java
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.

```java
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:

```java
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:

```java
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)
```

## Class diagram

![alt text](./etc/flyweight.urm.png "Flyweight pattern class diagram")

## 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

* [java.lang.Integer#valueOf(int)](http://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#valueOf%28int%29) and similarly for Byte, Character and other wrapped types.

## 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)
* [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)