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Ilkka Seppälä 2020-08-29 21:55:51 +03:00
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multiton/README.md
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@ -9,16 +9,19 @@ tags:
---
## 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.
> 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
@ -26,11 +29,14 @@ In plain words
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.
> 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.
`Nazgul` is the multiton class.
```java
public enum NazgulName {
@ -71,7 +77,7 @@ public final class Nazgul {
}
```
And here's how we access the Nazgul instances.
And here's how we access the `Nazgul` instances.
```java
LOGGER.info("KHAMUL={}", Nazgul.getInstance(NazgulName.KHAMUL));
@ -83,22 +89,29 @@ And here's how we access the Nazgul instances.
LOGGER.info("ADUNAPHEL={}", Nazgul.getInstance(NazgulName.ADUNAPHEL));
LOGGER.info("REN={}", Nazgul.getInstance(NazgulName.REN));
LOGGER.info("UVATHA={}", Nazgul.getInstance(NazgulName.UVATHA));
```
// 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
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](./etc/multiton.png "Multiton")
## 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
* There must be specific number of instances of a class, and they must be accessible to clients from
a well-known access point.