The last pattern we looked at was the Adapter pattern. It was a good pattern to start with as it maps well to the physical world and is fairly simple. Now we will look at the Singleton pattern. In some respects it is the simplest of patterns, but there are some things to think about.

The Singleton in the Physical World

The idea behind the Singleton is to ensure that there can be only one (insert Highlander jokes here). If you had a factory that made widgets, you would want a widget to be created many many times. But there is only one factory. If you were modeling the US government, you would only want one president. Only one senate.

The Singleton in the Software World

On the software side, the idea is the same. It is usually used when we only want one set of configuration variables, one controller, one data base connection pool, that kind of thing.
Continue reading →