Question

If I create a single instance of a Comparator, can that instance be used across multiple threads to sort collections using Collections.sort()? Or, do I need to create a new instance of the Comparator for each call to Collections.sort() to ensure thread safety?

Answer 1

That depends entirely on how you implement the Comparator. If, for example, it has instance variables that are written to or whose contents are changed implicitly during comparison, it would not be threadsafe.

Most Comparator implementations do no such thing, but one scenario that might reasonably occur is using a SimpleDateFormat to compare Strings that represent dates. Unfortunately, SimpleDateFormat itself is not thread safe.