In Java, sometimes it can be useful to have exactly one instance of a class. This is known as a Singleton pattern. A logger, print spooler, or window manager would be potential uses for this.
In essence, a Singleton ensures that a class has a single instance, and a global point of access to it. The class declaration typically looks like this:
For a much more in-depth guide to this design pattern, check out http://www.javaworld.com/article/2073352/core-java/simply-singleton.html