What should I use instead of thread.sleep on a swing applet?

https://docs.oracle.com/javase/tutorial/uiswing/concurrency/initial.html

https://docs.oracle.com/javase/tutorial/uiswing/concurrency/interim.html

The second link has an example similar to yours.

Some code for you to play around ...

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;public class Test
{
  JTextField jTextField1 = null;
  Timer timer = null;
  int hour = 0;
  int min = 0;
  int sec = 0;  public Test()
  {
      //shifted the initialization code to the constructor      JFrame f = new JFrame("Timer Test");
      ActionListener updateListener = new MyUpdateListener();      //somewhat an alternative to thread.sleep()
      //1000 is the 1 sec period for update
      timer = new Timer(1000, updateListener);      jTextField1 = new JTextField();
      jTextField1.setBounds(50, 50, 150, 20);
      JButton b = new JButton("Start");
      b.setBounds(50, 100, 95, 30);
      b.addActionListener(new MyButtonListener());
      f.add(b);
      f.add(jTextField1);
      f.setSize(400, 400);
      f.setLayout(null);
      f.setVisible(true);
  }  private void updateTextbox(String str)  {      System.out.println(str);      jTextField1.setText(str);  }  class MyButtonListener implements ActionListener
  {
      public void actionPerformed(ActionEvent event)
      {
        System.out.println("Entered start button event handler");        // your code for reading the values from combos
        // then set the member variables hour, min and sec        hour = 0;
        min = 0;
        sec = 5;        timer.start();
      }
  }   class MyUpdateListener implements ActionListener
  {
      public void actionPerformed(ActionEvent event)
      {
        if (hour == 0 && min == 0 && sec == 0)
        {
            updateTextbox("The time is up!");
            timer.stop();
        }
        else
        {
            //display - tweak needed for leading zeros
            updateTextbox(hour + ":" + min + ":" + sec);            //reduce by 1 sec
            sec--;            if (sec == -1 && min == 0 && hour > 0)
            {
              hour--;
              min = 59;
              sec = 59;
            }
            else if (sec == -1 && min > 0)
            {
              min--;
              sec = 59;
            }
        }
      }
  }  public static void main(String[] args)
  {
      //for further reading: https://docs.oracle.com/javase/tutorial/uiswing/concurrency/initial.html
      SwingUtilities.invokeLater(
        new Runnable()
        {
            @Override
            public void run()
            {
              new Test();
            }
        }
      );
  }
}

void actionPerformed(ActionEvent e);

https://docs.oracle.com/javase/7/docs/api/java/awt/event/ActionListener.html#actionPerformed(java.awt.event.ActionEvent)

Since you are starting the timer display by clicking the Start button, and not interacting with the textbox, the action listener is attached to the button.

Here, the action is triggered by clicking on the button. So, the actionlistener (ie., MyButtonListener) is set to observe the actions on button click.

      JButton b = new JButton("Start");
      b.setBounds(50, 100, 95, 30);
      b.addActionListener(new MyButtonListener());

Within the button click event handler, you need to read the values from your combo boxes and set them to hour/min/sec member variables.

class MyButtonListener implements ActionListener
{
    public void actionPerformed(ActionEvent event)
    {
      System.out.println("Entered start button event handler");      // your code for reading the values from combos
      // then set the member variables hour, min and sec      hour = 0;
      min = 0;
      sec = 5;      timer.start();
    }
}

And then lastly, the timer starts. This is the equivalent of your TimeUnit.SECONDS.sleep(1); call.

Since the timer is initialized to

      //somewhat an alternative to thread.sleep()
      //1000 is the 1 sec period for update
      timer = new Timer(1000, updateListener);

The timer takes care of invoking the actionPerformed() method of the updateListener at defined interval in this case every 1 sec (1000 ms).

   class MyUpdateListener implements ActionListener   {      public void actionPerformed(ActionEvent event)      {         ...

and that contains the logic to update the textbox and when to stop the timer.

I am not sure about the generated code as I don't use Netbeans. The post contains a working class. You just needed to add your combo boxes and read the values and set them to the variables.