Singleton in Java
Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code.
Singleton has almost the same pros and cons as global variables. Although they're super-handy, they break the modularity of your code.
You can't just use a class that depends on Singleton in some other context. You'll have to carry the Singleton class as well. Most of the time, this limitation comes up during the creation of unit tests.
Usage of the pattern in Java
Usage examples: A lot of developers consider the Singleton pattern an antipattern. That's why its usage is on the decline in Java code.
Despite this, there are quite a lot of Singleton examples in Java core libraries:
Identification: Singleton can be recognized by a static creation method, which returns the same cached object.