For efficiency reasons, Swing components are not designed to be thread safe. This means that Swing components should be manipulated by a single thread at a time. The easiest way to ensure this is to do all your GUI manipulations from the event dispatch thread. Every GUI application has an event dispatch thread: it is the thread that waits for events to occur and then dispatches those events to the appropriate event handlers. All of your event listener methods are invoked by the event dispatch thread, so any GUI manipulations you perform from an event listener are safe.