This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
If the Map shouldn't change after it's initialized, you can add some safety to ensure it doesn't change:
Since there is no reference to the actual HashMap, the only way to access its contents is through the unmodifiable (i.e. read-only) map field. It's also made final to prevent it from being overwritten completely.