A short and very enjoyable read by Uncle Bob on singletons:

Do you recognize this: ``` public class X { private static X instance = null;

private X() {}

public static X instance() { if (instance == null) instance = new X(); return instance; }

// more methods... }

<em>Of course. That's the Singleton pattern from the GOF book. I've always heard we shouldn't use it.</em>

Why shouldn't we use it?

<em>Because it makes our systems hard to test.</em>

It does? Why is that?

....</blockquote>
You can read the whole conversation on <a href="http://blog.cleancoder.com/uncle-bob/2015/07/01/TheLittleSingleton.html">http://blog.cleancoder.com/uncle-bob/2015/07/01/TheLittleSingleton.html</a>