What does a Singleton class do?

Question

Hello All,
I would like to know what functioanlity can be achieved using Singleton classes?It was an interview question.
can Anyone help me explaining from the basic about a Singleton class?
Thanks in advance,
Vijay

Answer 1

A singleton class only has one instance. I've used it once for databas connection pooling. Permitting only one instance of a class may sometimes be desirable, there isn't any magic about it.
This is a singleton class:
public class MySingleton
    private static MySingleton instance;
    private MySingleton() {}
    public static getInstance()
        if (instance == null) instance = new MySingleton();
        return instance;
}