Display dialog in InputVerifier from JTextField in Java

import java.awt.FlowLayout;
/* w  w  w  . j  a v  a2 s . c  om*/
import javax.swing.InputVerifier;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.text.JTextComponent;

public class Main {
  public static void main(String[] args) {
    JFrame f = new JFrame("Text Field Elements");

    f.setLayout(new FlowLayout());


    JTextField textfield = new JTextField(10);
    InputVerifier verifier = new InputVerifier() {
      public boolean verify(JComponent input) {
        final JTextComponent source = (JTextComponent) input;
        String text = source.getText();
        if ((text.length() != 0) && !(text.equals("Exit"))) {
          Runnable runnable = new Runnable() {
            public void run() {
              JOptionPane.showMessageDialog(source,
                  "Can't leave.", "Error Dialog",
                  JOptionPane.ERROR_MESSAGE);
            }
          };
          SwingUtilities.invokeLater(runnable);
          return false;
        } else {
          return true;
        }
      }
    };
    textfield.setInputVerifier(verifier);
    f.add(textfield);
    f.pack();
    f.setVisible(true);

  }


}