This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Avoid ever having a null collection. It's much better to assume you have a valid collection and check the size for something greater than zero. Also, unless you have an old version of Java you should specifiy what is in your collection. Never use Hashtable: use HashMap. You can synchronize access to your HashMap, if needed, and use it consitently everywhere. Finally, always declare the collection interface not the concrete type.