Introducing the Single Java Object

For those who haven't heard of design patterns before, or who are familiar with the term but not its meaning, a design pattern is a template for software development. The purpose of the template is to define a particular behavior or technique that can be used as a building block for the construction of software - to solve universal problems that commonly face developers. Think of design code as a way of passing on some nifty piece of advice, just like your mother used to give. "Never wear your socks for more than one day" might be an old family adage, passed down from generation to generation. It's common sense solutions that are passed on to others. Consider a design pattern as a useful piece of advice for designing software.

Design patterns out of the way, let's look at the singleton. By now, you're probably wondering what a singleton is - isn't jargon terrible? A singleton is an object that cannot be instantiated. At first, that might seem counterintuitive - after all, we need an instance of an object before we can use it. Well yes a singleton can be created, but it can't be instantiated by developers - meaning that the singleton class has control over how it is created. The restriction on the singleton is that there can be only one instance of a singleton created by the Java Virtual Machine (JVM) - by prevent direct instantiation we can ensure that developers don't create a second copy.

So why would this be useful? Often in designing a system, we want to control how an object is used, and prevent others (ourselves included) from making copies of it or creating new instances. For example, a central configuration object that stores setup information should have one and one only instance - a global copy accessible from any part of the application, including any threads that are running. Creating a new configuration object and using it would be fairly useless, as other parts of the application might be looking at the old configuration object, and changes to application settings wouldn't always be acted upon. I'm sure you can think of a other situations where a singleton would be useful - perhaps you've even used one before without giving it a name. It's a common enough design criteria (not used everyday, but you'll come across it from time to time). The singleton pattern can be applied in any language, but since we're all Java programmers here (if you're not, shame!) let's look at how to implement the pattern using Java.