Though personally I am not too much in favor of singletons but its a good brain exercise to see different ways in which it could be implemented.

The standard way as we all know is you have a final class (ideally purely for performance optimizations) with a private constructor and a public static getter method. I personally like to create the singleton object eagerly. It saves all the hassles to keep the getter method synchronized and therefore, make it a lot more scalar.

With the advent of generics, their application with different data structures and design patterns evolved and so did a couple of variants of Singleton.

class GenericSingleton {

private static final GenericSingleton instance = new GenericSingleton