Why is it mandatory to have private Constructor inside a Singleton class

Question

This is my singleton class for obtaining the database connection.

I have a question here: why it compulsory to have a private constructor inside a singleton class (a

Answer 1

For singletons and utilty classes you can use an enum which is a final class and implicitly defines a private constructor.

enum Singleton {
     INSTANCE
}

or

enum Utility {;

}

In your example above you have a utility class because you have no instance fields, methods and don't create an instance.

Answer 2

If there no such a private constructor, Java will provide a default public one for you. Then you are able to call this constructor multiple times to create several instances. Then it is not a singleton class any more.

Answer 3

If you don't need lazy initiation:

public class Singleton {
    private static final Singleton instance = new Singleton();

    // Private constructor prevents instantiation from other classes
    private Singleton() { }

    public static Singleton getInstance() {
            return instance;
    }
}

is the best way, because is thread safe.

Answer 4

For the singleton pattern you use an private constructor to ensure that no other instances can be created, otherwise it wouldn't be a singleton.