I wanted to use `Collection` for only single threaded environment and I am using a `HashMap` that is synchronized. 

However, I still doubt if it is thread safe to have it synchronized or not.
['https://Stackoverflow.com/questions/15152058', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/891556/']
1. the standard java HashMap is not synchronized.
2. If you are in a single threaded environment you don't need to worry about synchronization.
As its a single-threaded environment you can safely use HashMap.