This singleton pattern is not really a big deal, but overused because it gives us the coolness of knowing about design patters without learning too much stuff for that… It does have its uses in some programming languages, while more or less implicit or obsolete in others, depending on the point of view.
I have already discussed the generalization of the singleton.
Actually it can be seen that the enum of Java and of languages who do something like Java’s enum in a similar way can be used as a way to build a singleton by just limiting it to one instance. Having been skeptical about this as an abuse of enums that might confuse people who read this I would now suggest to embrace this as a possible and useful way to write singletons and to be familiar with it when reading other people’s code or even to use it when writing code.
The enum way to write a singleton is the most elegant way in Java, because it uses least boiler plate code and includes some benefits that are usually ignored, especially when it comes to serialization that has to be addressed in singletons quite often, but is quite often ignored, resulting in multiple instances of the „singleton“ to float around…
So here we go:
enum Singleton {
INSTANCE;
private long count;
private Singleton() {
count = 0;
}
public synchronized long nextValue() {
return count++;
}
}
Remarks:
- For real programs it would be important to check if long is sufficient or BigInteger is needed.
- For real programs using an AtomicLong could be a better way than using synchronized.
Some links:
- Design Patterns: Singleton
- Why Enum Singletons are better in Java
- The Java Enum: A Singleton Pattern
- Wikipedia
- Singleton
- Enum für Singleton nutzen (German)
- Enforce the singleton property with a private constructor or an enum type
- Three ways of implementing singleton
- Effective Java
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