This week's book giveaways are in the Cloud and AI/ML forums. We're giving away four copies each of Cloud Native Patterns and Natural Language Processing and have the authors on-line! See this thread and this one 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 Javayou 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.