Java: Halting Program Without Input Prompt

Question

As many of you may know, when you have a while loop (or any loop for that matter) when an input method is called, the program stops and waits for input.

e.g.

  while {
      String input = in.readLine();
      int x = 55;  //This will not execute until input has been given a value
      System.out.println (x + x);
      }

Now I am using buttons for input. Is there any way I can do the same thing (halt the program in a loop) with the use of JButton, JPanel, JFrame etc. ?

NOTE: I am also open to use the Runnable () interface if required.

UPDATE: I am using listeners for the button. This is the exact problem.

 public void actionPerformed (ActionEvent x){ string = x.getActionCommand}

 public void someOtherMethod ()
 {

 while (true){
 start ();
 if (string.equals ("exit") break;  // This line will never execute because start() 
    //is always repeating itself.
     }
 }

EDIT: I found a solution (FINALLY!)

this is all that needs to be done....

 string = "";
 while (true){
 if (string.equals ("");
 start ();
 if (string.equals ("exit") break; // I guess I didn't explain the problem too well...
     }

Thank you for everybody's help!

Answer 1

I think the problem you're having is how to change what the button does depending on what has been entered into the GUI. Remember that with a GUI, the user can interact with any enabled GUI component at any time and in any order. The key is to check the state of the GUI in your button's ActionListener and then altering the behavior of this method depending on this GUI's state. For example if your GUI had three JTextFields, field1, field2, and sumField and a JButton addButton:

   private JTextField field1 = new JTextField(5);
   private JTextField field2 = new JTextField(5);
   private JTextField sumField = new JTextField(5);
   private JButton addButton = new JButton("Add");

And you wanted the addButton to add the numbers in field1 and field2 together and place the results into the sumField, you're obviously not going to want to do any addition if either field is left blank, and so you test for it in the JButton's ActionListener:

  addButton.addActionListener(new ActionListener() {
     public void actionPerformed(ActionEvent e) {
        String text1 = field1.getText().trim();
        String text2 = field2.getText().trim();

        if (text1.isEmpty() || text2.isEmpty()) {
           // data not entered... so return the method and do nothing
           return;
        }

        // if we've reached this point, the user has entered in text and so we handle it

Here's the whole thing:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class WaitForInput extends JPanel {
   private JTextField field1 = new JTextField(5);
   private JTextField field2 = new JTextField(5);
   private JTextField sumField = new JTextField(5);
   private JButton addButton = new JButton("Add");

   public WaitForInput() {
      addButton.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent e) {
            String text1 = field1.getText().trim();
            String text2 = field2.getText().trim();

            if (text1.isEmpty() || text2.isEmpty()) {
               // data not entered... so return the method and do nothing
               return;
            }

            try {
               int number1 = Integer.parseInt(field1.getText());
               int number2 = Integer.parseInt(field2.getText());
               int sum = number1 + number2;

               sumField.setText("" + sum);
            } catch (NumberFormatException e1) {
               // TODO: use JOptionPane to send error message

               // clear the fields
               field1.setText("");
               field2.setText("");
            }
         }
      });

      add(field1);
      add(new JLabel("+"));
      add(field2);
      add(new JLabel("="));
      add(sumField);
      add(addButton);
   }

   private static void createAndShowUI() {
      JFrame frame = new JFrame("WaitForInput");
      frame.getContentPane().add(new WaitForInput());
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      java.awt.EventQueue.invokeLater(new Runnable() {
         public void run() {
            createAndShowUI();
         }
      });
   }
}

EDIT 1
Otherwise if you absolutely have to use a loop, then yes, do it in a Runnable and do that in a background Thread. Remember to call Thread.sleep(...) inside of the loop even for a short bit so it doesn't hog the CPU. For example

import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

public class HaltingProblem extends JPanel {
   private static final int PANEL_HEIGHT = 400;
   private static final int PANEL_WIDTH = 600;
   private static final long SLEEP_DELAY = 100;
   private Color[] colors = {Color.red, Color.orange, Color.yellow,
      Color.green, Color.blue, Color.cyan};
   private boolean halt = false;
   private JButton haltButton = new JButton("Halt");
   private int colorIndex = 0;

   public HaltingProblem() {
      setBackground(colors[colorIndex]);
      haltButton.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent e) {
            halt = !halt; // toggle it!
         }
      });
      add(haltButton);

      new Thread(new Runnable() {
         public void run() {
            while (true) {
               keepDoingThis();
            }
         }
      }).start();
   }

   private void keepDoingThis() {
      try {
         Thread.sleep(SLEEP_DELAY);
      } catch (InterruptedException e) {}

      if (halt) {
         return;
      }
      colorIndex++;
      colorIndex %= colors.length;
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            setBackground(colors[colorIndex]);
         }
      });
   }

   @Override
   public Dimension getPreferredSize() {
      return new Dimension(PANEL_WIDTH, PANEL_HEIGHT);
   }

   private static void createAndShowUI() {
      JFrame frame = new JFrame("HaltingProblem");
      frame.getContentPane().add(new HaltingProblem());
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      java.awt.EventQueue.invokeLater(new Runnable() {
         public void run() {
            createAndShowUI();
         }
      });
   }
}

Answer 2

The programming paradigm is a bit different when you're using a GUI.

By registering listeners on the buttons you get notified when something happens - if nothing happens the Swing event loop (EDT) is already doing "nothing".

Perhaps I misunderstood the question though.

Answer 3

If you have a GUI, you do not generally have a central main loop that executes indefinitely. The canonical way to respond to events is with event listeners:

mybutton.addListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) { System.out.println("Clicked"); }
    });

Answer 4

Make your JFrame implement ActionListener and call button.addActionListener(this). Then the code inside the actionPerformed(ActionEvent event) method will execute when the button is clicked :)

Or you could make the ActionListener anonymous with

button.addActionListener(new ActionListener() {
    void actionPerformed(ActionEvent event) {
        // Insert code to execute when button is clicked
    }
}