116 lines
3.3 KiB
Markdown
116 lines
3.3 KiB
Markdown
---
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title: Multiton
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category: Creational
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language: en
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tags:
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- Instantiation
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---
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## Also known as
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Registry
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## Intent
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Ensure a class only has limited number of instances and provide a global point of access to them.
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## Explanation
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Real world example
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> The Nazgûl, also called ringwraiths or the Nine Riders, are Sauron's most terrible servants. By
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> definition there's always nine of them.
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In plain words
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> Multiton pattern ensures there's predefined amount of instances available globally.
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Wikipedia says
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> In software engineering, the multiton pattern is a design pattern which generalizes the singleton
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> pattern. Whereas the singleton allows only one instance of a class to be created, the multiton
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> pattern allows for the controlled creation of multiple instances, which it manages through the use
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> of a map.
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**Programmatic Example**
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`Nazgul` is the multiton class.
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```java
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public enum NazgulName {
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KHAMUL, MURAZOR, DWAR, JI_INDUR, AKHORAHIL, HOARMURATH, ADUNAPHEL, REN, UVATHA
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}
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public final class Nazgul {
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private static final Map<NazgulName, Nazgul> nazguls;
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private final NazgulName name;
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static {
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nazguls = new ConcurrentHashMap<>();
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nazguls.put(NazgulName.KHAMUL, new Nazgul(NazgulName.KHAMUL));
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nazguls.put(NazgulName.MURAZOR, new Nazgul(NazgulName.MURAZOR));
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nazguls.put(NazgulName.DWAR, new Nazgul(NazgulName.DWAR));
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nazguls.put(NazgulName.JI_INDUR, new Nazgul(NazgulName.JI_INDUR));
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nazguls.put(NazgulName.AKHORAHIL, new Nazgul(NazgulName.AKHORAHIL));
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nazguls.put(NazgulName.HOARMURATH, new Nazgul(NazgulName.HOARMURATH));
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nazguls.put(NazgulName.ADUNAPHEL, new Nazgul(NazgulName.ADUNAPHEL));
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nazguls.put(NazgulName.REN, new Nazgul(NazgulName.REN));
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nazguls.put(NazgulName.UVATHA, new Nazgul(NazgulName.UVATHA));
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}
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private Nazgul(NazgulName name) {
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this.name = name;
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}
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public static Nazgul getInstance(NazgulName name) {
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return nazguls.get(name);
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}
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public NazgulName getName() {
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return name;
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}
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}
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```
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And here's how we access the `Nazgul` instances.
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```java
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LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL));
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LOGGER.info("MURAZOR={}", Nazgul.getInstance(NazgulName.MURAZOR));
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LOGGER.info("DWAR={}", Nazgul.getInstance(NazgulName.DWAR));
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LOGGER.info("JI_INDUR={}", Nazgul.getInstance(NazgulName.JI_INDUR));
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LOGGER.info("AKHORAHIL={}", Nazgul.getInstance(NazgulName.AKHORAHIL));
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LOGGER.info("HOARMURATH={}", Nazgul.getInstance(NazgulName.HOARMURATH));
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LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL));
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LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN));
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LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA));
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```
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Program output:
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```
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KHAMUL=com.iluwatar.multiton.Nazgul@2b214b94
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MURAZOR=com.iluwatar.multiton.Nazgul@17814b1c
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DWAR=com.iluwatar.multiton.Nazgul@7ac9af2a
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JI_INDUR=com.iluwatar.multiton.Nazgul@7bb004b8
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AKHORAHIL=com.iluwatar.multiton.Nazgul@78e89bfe
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HOARMURATH=com.iluwatar.multiton.Nazgul@652ce654
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ADUNAPHEL=com.iluwatar.multiton.Nazgul@522ba524
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REN=com.iluwatar.multiton.Nazgul@29c5ee1d
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UVATHA=com.iluwatar.multiton.Nazgul@15cea7b0
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```
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## Class diagram
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## Applicability
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Use the Multiton pattern when
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* There must be specific number of instances of a class, and they must be accessible to clients from
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a well-known access point.
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