Java uses the event thread to notify listeners of events by making calls to methods such as actionPerformed() and itemStateChanged(). However, this same thread is also responsible for repainting components. As a result, if your event handling code causes the thread to wait or to perform some long-running operation, your display may not be repainted when it needs to be because the event thread is busy.

To avoid this problem, you should create a separate thread for any lengthy operations associated with your event handling code. Then, instead of using the event thread for those operations, you can use the thread created specifically for that purpose. This will allow the event thread to return from the event handler method and eventually repaint your display. Inner classes can be particularly helpful in defining threads for such a purpose because they usually would not be useful outside of the class in which they’re defined. For example:

 public void doSomethingSlow() {...}  //  public void doSomethingSlow()public void actionPerformed(ActionEvent event) {	// Instead of calling the method directly as in:	// doSomethingSlow();	// Create a separate thread and start it:	Thread t = new WorkerThread();	t.start();}  //  public void actionPerformed()class WorkerThread extends Thread {	public void run() {		doSomethingSlow();	}  //  public void run()}  //  class WorkerThread extends Thread()