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README.md

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pattern Multiton multiton /patterns/multiton/ Creational en
Instantiation

Also known as

Registry

Intent

Ensure a class only has limited number of instances and provide a global point of access to them.

Explanation

Real world example

The Nazgûl, also called ringwraiths or the Nine Riders, are Sauron's most terrible servants. By definition there's always nine of them.

In plain words

Multiton pattern ensures there's predefined amount of instances available globally.

Wikipedia says

In software engineering, the multiton pattern is a design pattern which generalizes the singleton pattern. Whereas the singleton allows only one instance of a class to be created, the multiton pattern allows for the controlled creation of multiple instances, which it manages through the use of a map.

Programmatic Example

Nazgul is the multiton class.

public enum NazgulName {

  KHAMUL, MURAZOR, DWAR, JI_INDUR, AKHORAHIL, HOARMURATH, ADUNAPHEL, REN, UVATHA
}

public final class Nazgul {

  private static final Map<NazgulName, Nazgul> nazguls;

  private final NazgulName name;

  static {
    nazguls = new ConcurrentHashMap<>();
    nazguls.put(NazgulName.KHAMUL, new Nazgul(NazgulName.KHAMUL));
    nazguls.put(NazgulName.MURAZOR, new Nazgul(NazgulName.MURAZOR));
    nazguls.put(NazgulName.DWAR, new Nazgul(NazgulName.DWAR));
    nazguls.put(NazgulName.JI_INDUR, new Nazgul(NazgulName.JI_INDUR));
    nazguls.put(NazgulName.AKHORAHIL, new Nazgul(NazgulName.AKHORAHIL));
    nazguls.put(NazgulName.HOARMURATH, new Nazgul(NazgulName.HOARMURATH));
    nazguls.put(NazgulName.ADUNAPHEL, new Nazgul(NazgulName.ADUNAPHEL));
    nazguls.put(NazgulName.REN, new Nazgul(NazgulName.REN));
    nazguls.put(NazgulName.UVATHA, new Nazgul(NazgulName.UVATHA));
  }

  private Nazgul(NazgulName name) {
    this.name = name;
  }

  public static Nazgul getInstance(NazgulName name) {
    return nazguls.get(name);
  }

  public NazgulName getName() {
    return name;
  }
}

And here's how we access the Nazgul instances.

    LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL));
    LOGGER.info("MURAZOR={}", Nazgul.getInstance(NazgulName.MURAZOR));
    LOGGER.info("DWAR={}", Nazgul.getInstance(NazgulName.DWAR));
    LOGGER.info("JI_INDUR={}", Nazgul.getInstance(NazgulName.JI_INDUR));
    LOGGER.info("AKHORAHIL={}", Nazgul.getInstance(NazgulName.AKHORAHIL));
    LOGGER.info("HOARMURATH={}", Nazgul.getInstance(NazgulName.HOARMURATH));
    LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL));
    LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN));
    LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA));

Program output:

KHAMUL=com.iluwatar.multiton.Nazgul@2b214b94
MURAZOR=com.iluwatar.multiton.Nazgul@17814b1c
DWAR=com.iluwatar.multiton.Nazgul@7ac9af2a
JI_INDUR=com.iluwatar.multiton.Nazgul@7bb004b8
AKHORAHIL=com.iluwatar.multiton.Nazgul@78e89bfe
HOARMURATH=com.iluwatar.multiton.Nazgul@652ce654
ADUNAPHEL=com.iluwatar.multiton.Nazgul@522ba524
REN=com.iluwatar.multiton.Nazgul@29c5ee1d
UVATHA=com.iluwatar.multiton.Nazgul@15cea7b0

Class diagram

alt text

Applicability

Use the Multiton pattern when

  • There must be specific number of instances of a class, and they must be accessible to clients from a well-known access point.