A couple of my readers ask questions, what does it mean by Swing is not thread-safe and How does it affect coding in Swing GUI application? This post is an effort to help those readers and several other programmers to understand Swing and thread safety in a bit more detailed way. To keep it simple, let's revise what does it mean by being thread-safe? We say an object is thread-safe if we can call its a method, which can change its states, from multiple threads at the same time. To give you an example, java.lang.String is a thread-safe class, which means you can call any method e.g. substring(), toUpperCase() or toLowerCase() from multiple threads.