EJ-3 Enforce the Singleton Property with a Private Constructor or an Enum Type

Ref: Effective Java by Joshua Bloch

A singleton is simply a class the is instantiated exactly once, although making a class a singleton can make it difficult to test its clients, as it's impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type.
Take a look at Misko Hevery's considerations about Singletons in terms of testability

• Root Cause of Singletons
• Where Have All the Singletons Gone?

In one approach, the member is a final field:

	// Singleton with public final field
	public class Elvis {
	public static final Elvis INSTANCE = new Elvis();
	private Elvis() { ... }
	public void leaveTheBuilding() { ... }
	}

view raw Singleton1.java hosted with ❤ by GitHub

In second approach, the public member is a static factory method:

	// Singleton with static factory
	public class Elvis {
	private static final Elvis INSTANCE = new Elvis();
	private Elvis() { ... }
	public static Elvis getInstance() { return INSTANCE; }
	public void leaveTheBuilding() { ... }
	}

view raw Singleton2.java hosted with ❤ by GitHub

In third and really interesting aproach is to make an Enum type with one element, marked as preferred approach By Mr. Bloch

	// Enum singleton - the preferred approach
	public enum Elvis {
	INSTANCE;
	public void leaveTheBuilding() { ... }
	}

view raw Singleton3.java hosted with ❤ by GitHub

A single-element enum type is the best way to implement singleton.