Mariusz Kumorek thinks this is interesting: If the Date values are not modified after they are placed in the Map, then the synchronization in the synchronizedMap implementation is sufficient to publish the Date values safely, and no additional synchronization is needed when accessing them. From Chapter 3. Sharing Objects from Java Concurrency in Practice by Doug Lea, David Holmes, Joseph Bowbeer, Joshua Bloch, Tim Peierls, Brian Goetz Publisher: Addison-Wesley Professional Released: May 2006 Note why? Share this highlight http://learning.oreilly.com/a/java-concurrency-in/14435738/ Twitter Facebook Google Plus Email Get Instant Access Now Start a Free Trial Have an account? Sign in.