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Ilkka Seppälä 2020-09-13 17:53:50 +03:00
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@ -9,14 +9,16 @@ tags:
--- ---
## Intent ## Intent
Ensure a class only has one instance, and provide a global point of
access to it. Ensure a class only has one instance, and provide a global point of access to it.
## Explanation ## Explanation
Real world example Real world example
> There can only be one ivory tower where the wizards study their magic. The same enchanted ivory tower is always used by the wizards. Ivory tower here is singleton. > There can only be one ivory tower where the wizards study their magic. The same enchanted ivory
> tower is always used by the wizards. Ivory tower here is singleton.
In plain words In plain words
@ -24,7 +26,9 @@ In plain words
Wikipedia says Wikipedia says
> In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. > In software engineering, the singleton pattern is a software design pattern that restricts the
> instantiation of a class to one object. This is useful when exactly one object is needed to
> coordinate actions across the system.
**Programmatic Example** **Programmatic Example**
@ -38,7 +42,7 @@ public enum EnumIvoryTower {
} }
``` ```
Then in order to use Then in order to use:
```java ```java
var enumIvoryTower1 = EnumIvoryTower.INSTANCE; var enumIvoryTower1 = EnumIvoryTower.INSTANCE;
@ -47,9 +51,11 @@ assertEquals(enumIvoryTower1, enumIvoryTower2); // true
``` ```
## Class diagram ## Class diagram
![alt text](./etc/singleton.urm.png "Singleton pattern class diagram") ![alt text](./etc/singleton.urm.png "Singleton pattern class diagram")
## Applicability ## Applicability
Use the Singleton pattern when Use the Singleton pattern when
* There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point * There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point