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Copy file name to clipboardExpand all lines: flyweight/README.md
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categories:Structural
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language:en
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tags:
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-GangOf Four
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-Gangof Four
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-Performance
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---
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##Explanation
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Realworld example
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Real-world example
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>Alchemist's shop has shelves full of magic potions. Many of the potions are the same so there is
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>no need to create new object for each of them. Instead one object instance can represent multiple
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>shelf items so memory footprint remains small.
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>no need to createanew object for each of them. Instead, one object instance can represent
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>multipleshelf items so the memory footprint remains small.
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In plain words
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**Programmatic example**
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Translating our alchemist shop example from above. First of all we have different potion types:
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Translating our alchemist shop example from above. First of all, we have different potion types:
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```java
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publicinterfacePotion {
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}
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```
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And it can be used as below:
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`AlchemistShop` contains two shelves of magic potions. The potions are created using the
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aforementioned`PotionFactory`.
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```java
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var factory=newPotionFactory();
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factory.createPotion(PotionType.INVISIBILITY).drink();// You become invisible. (Potion=6566818)
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factory.createPotion(PotionType.HEALING).drink();// You feel healed. (Potion=648129364)
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factory.createPotion(PotionType.INVISIBILITY).drink();// You become invisible. (Potion=6566818)
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factory.createPotion(PotionType.HOLY_WATER).drink();// You feel blessed. (Potion=1104106489)
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factory.createPotion(PotionType.HOLY_WATER).drink();// You feel blessed. (Potion=1104106489)
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factory.createPotion(PotionType.HEALING).drink();// You feel healed. (Potion=648129364)
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@Slf4j
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publicclassAlchemistShop {
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privatefinalList<Potion> topShelf;
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privatefinalList<Potion> bottomShelf;
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publicAlchemistShop() {
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var factory=newPotionFactory();
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topShelf=List.of(
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factory.createPotion(PotionType.INVISIBILITY),
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factory.createPotion(PotionType.INVISIBILITY),
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factory.createPotion(PotionType.STRENGTH),
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factory.createPotion(PotionType.HEALING),
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factory.createPotion(PotionType.INVISIBILITY),
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factory.createPotion(PotionType.STRENGTH),
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factory.createPotion(PotionType.HEALING),
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factory.createPotion(PotionType.HEALING)
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);
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bottomShelf=List.of(
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factory.createPotion(PotionType.POISON),
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factory.createPotion(PotionType.POISON),
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factory.createPotion(PotionType.POISON),
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factory.createPotion(PotionType.HOLY_WATER),
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factory.createPotion(PotionType.HOLY_WATER)
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);
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}
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publicfinalList<Potion>getTopShelf() {
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returnList.copyOf(this.topShelf);
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}
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publicfinalList<Potion>getBottomShelf() {
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returnList.copyOf(this.bottomShelf);
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}
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publicvoiddrinkPotions() {
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LOGGER.info("Drinking top shelf potions\n");
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topShelf.forEach(Potion::drink);
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LOGGER.info("Drinking bottom shelf potions\n");
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bottomShelf.forEach(Potion::drink);
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}
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}
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```
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In our scenario, a brave visitor enters the alchemist shop and drinks all the potions.
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```java
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// create the alchemist shop with the potions
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var alchemistShop=newAlchemistShop();
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// a brave visitor enters the alchemist shop and drinks all the potions
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alchemistShop.drinkPotions();
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```
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Program output:
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```java
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You become invisible. (Potion=6566818)
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You feel healed. (Potion=648129364)
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You become invisible. (Potion=6566818)
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You feel blessed. (Potion=1104106489)
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You feel blessed. (Potion=1104106489)
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You feel healed. (Potion=648129364)
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Drinking top shelf potions
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You become invisible. (Potion=1509514333)
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You become invisible. (Potion=1509514333)
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You feel strong. (Potion=739498517)
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You feel healed. (Potion=125130493)
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You become invisible. (Potion=1509514333)
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You feel strong. (Potion=739498517)
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You feel healed. (Potion=125130493)
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You feel healed. (Potion=125130493)
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Drinking bottom shelf potions
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Urgh!This is poisonous. (Potion=166239592)
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Urgh!This is poisonous. (Potion=166239592)
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Urgh!This is poisonous. (Potion=166239592)
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You feel blessed. (Potion=991505714)
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You feel blessed. (Potion=991505714)
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```
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##Class diagram
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* An application uses a large number of objects.
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* Storage costs are high because of the sheer quantity of objects.
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* Most object state can be made extrinsic.
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* Many groups of objects may be replaced by relatively few shared objects once extrinsic state is
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removed.
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* Mostof theobject state can be made extrinsic.
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* Many groups of objects may be replaced by relatively few shared objects oncetheextrinsic state
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isremoved.
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* The application doesn't depend on object identity. Since flyweight objects may be shared, identity
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tests will return true for conceptually distinct objects.
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##Real world examples
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##Known uses
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*[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.