I have often read articles saying "If an object is immutable, it is thread safe". But I have never found an article that convinces me that 'immutable' means thread safety. Even the book by Brian Goetz Java Concurrency in Practice with its chapter on immutability did not fully satisfiy me. In this book we can read word for word, in a frame : Immutable objects are always thread-safe. I think this sentence deserves more explanation. So I am going to try to define immutability and its relation to thread safety.