thread-safety or thread-safe code in Java refers to code which can safely be used or shared in concurrent or multi-threading environment and they will behave as expected. any code, class, or object which can behave differently from its contract on the concurrent environment is not thread-safe. thread-safety is one of the risks introduced by using threads in Java and I have seen java programmers and developers struggling to write thread-safe code or just understanding what is thread-safe code and what is not?