Delaying an output

Question

Hi,
I've written a program that acts as a calculator (which accepts RPN expressions).
The stack is displayed at every step of the way:
          try
          Thread.sleep(1000);
          catch(InterruptedException e)
          e.printStackTrace();
          String str = new String(stack).trim();
          System.out.println("STACK: " + str);I'm currently trying to convert into into a GUI. It accepts the RPN expression from a textField, and when a button 'calculate' is pressed, the stack is displayed in a textArea (one line every second).
public void actionPerformed(ActionEvent e)
               textArea.setText("Stack:");
               String rpn = inputLine.getText();
               rpn = (" " + rpn);
                        calculate(temp, stack);
                        inputLine.setText(rpn);The calculate method calls the print stack method much as before:
static void printstack(char[] stack)
          try
          Thread.sleep(1000);
          catch(InterruptedException e)
          e.printStackTrace();
          String str = new String(stack).trim();
          textArea.append(str + LINE);
     }However, when the calculate button is pressed, the program seems to freeze - in fact it is performing the Thread.sleep(1000), but not outputting the stack to the textArea until the whole expression has been worked out.
eg: The expression 3 4 + should output:
3
(wait one second)
4
(wait one second)
+
(wait one second)
7
But what happens is
(wait 3 seconds)
3
4
+
7
Does anyone know what might be the problem and how to fix it?

Answer 1

Calling Thread.sleep(...) in a Swing GUI without care will put the entire GUI to sleep. The problem is that your sleeping the main thread that runs the GUI, the Event Dispatch Thread (or EDT). Better would be to use a Swing Timer. Please search for and check out Sun's excellent tutorial on Concurrency and Swing and on the Swing Timer.