Singleton pattern question in OCP Oracle Certified Professional Java SE 8 Programmer II (Sybex)

[Alexandru Dragoi]

In the OCP Oracle Certified Professional Java SE 8 Programmer II, chapter 2 test answer to this question:

What are some of the properties of using the singleton pattern? (Choose all that apply.)

This answer is marked as being true:

F. Requires a public static method to retrieve the instance of the singleton.

You do not need a public static method to retrieve the instance.
This is also a valid singleton:

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

    private Singleton() {}
}

[Paul Clapham]

The Wikipedia article Singleton pattern agrees with you. It says

Wikipedia wrote:Typically, this is done by:

   declaring all constructors of the class to be private; and
   providing a static method that returns a reference to the instance.

And of course "typically" is not equivalent to a requirement, as you observe.

[rajeswari kannan]

An implementation of the singleton pattern must:

   ensure that only one instance of the singleton class ever exists; and
   provide global access to that instance.

So we must provide a static method that returns a reference to the instance.

[Alexandru Dragoi]

I did provided a global access to my instance, although it does not respect the encapsulation principle (also another subject of the exam).

[Piet Souris]

I agree. What is the difference between: Singleton.INSTANCE and Singleton.getInstance()?

[Jeanne Boyarsky]

This is question #5 in the Sybex book. As taught in the book, the static method is used. (And I've always seen it that way on the exam too.)

The real exam tells you how many answers are valid though. So if you weren't sure whether to include it is an answer, that would be your clue.

[Alexandru Dragoi]

Thanks Jeanne, good to know this!