Wizard API updated!
Tim Boudreau has released a new version of the Swing Wizard library (version 0.997) that fixes the WizardException bug reported in JavaWorld's recent Open Source Java Project profile. The article's examples have been reworked to test out the new, improved WizardException. Thanks, Tim, for this helpful fix!
Open Source Java Projects: The Wizard API

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When is a singleton not a singleton?

Avoid multiple singleton instances by keeping these tips in mind

By Joshua Fox, JavaWorld.com, 01/12/01

The singleton is a useful design pattern for allowing only one instance of your class, but common mistakes can inadvertently allow more than one instance to be created. In this article, I'll show you how that can happen and how to avoid it.

The singleton's purpose is to control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes. Since there is only one singleton instance, any instance fields of a singleton will occur only once per class, just like static fields.

Singletons often control access to resources such as database connections or sockets. For example, if you have a license for only one connection for your database or your JDBC driver has trouble with multithreading, the singleton makes sure that only one connection is made or that only one thread can access the connection at a time. If you add database connections or use a JDBC driver that allows multithreading, the singleton can be easily adjusted to allow more connections.

Moreover, singletons can be stateful; in this case, their role is to serve as a unique repository of state. If you are implementing a counter that needs to give out sequential and unique numbers (such as the machine that gives out numbers in the deli), the counter needs to be globally unique. The singleton can hold the number and synchronize access; if later you want to hold counters in a database for persistence, you can change the private implementation of the singleton without changing the interface.

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