With his design patttern we allways get the same instance, as you can see next.

1. The Singleton class:

package pt.joaobrito.singleton;

public final class Singleton {

	/**
	 * A private constructor. This is necessary in order to not allow create a new Singleton object
	 * from the outside with the keyword new
	 */
	private Singleton() {}

	private static final class SingletonHolder {
		public static final Singleton INSTANCE = new Singleton();
	}

	public static final Singleton getInstance() {
		return SingletonHolder.INSTANCE;
	}
}

2. The main class:

package pt.joaobrito.singleton;

public class Main {

	public static void main(String[] args) {
		Singleton s1 = Singleton.getInstance();
		Singleton s2 = Singleton.getInstance();

		// test if the reference are the same
		System.out.println(s1 == s2);
	}

}

3. The result:

UML