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

 public class DemoLostFocus extends JApplet {
   JTextField tf1, tf2;
   JLabel label;

   public void init() {
     getContentPane().setLayout(new FlowLayout());
     //
     label = new JLabel("must be 'a' or 'b' ");
     tf1 = new JTextField(5);
     getContentPane().add(label);
     getContentPane().add(tf1);
     tf2 = new JTextField(5);
     getContentPane().add(tf2);

     tf1.addFocusListener(
          new FocusListener() {
             public void focusGained(FocusEvent e) {};

             public void focusLost(FocusEvent e) {
               if (!e.isTemporary() && isEnabled() ) {
                 String content = tf1.getText();
                 if (!content.equals("a") && !content.equals("b")) {
                    Toolkit.getDefaultToolkit().beep();
                    System.out.println("illegal value! " + content );
                    SwingUtilities.invokeLater(new FocusGrabber(tf1));
                 }
             }}
      });
   }
}

class FocusGrabber implements Runnable {
  private JComponent component;

  public FocusGrabber(JComponent component) {
   this.component = component;
   }
  public void run() {
   component.grabFocus();
   }
} 

 
 import java.awt.*;
 import java.util.*;
 import java.awt.event.*;
 import javax.swing.*;

 // the first JTextField expects the string "howto" as input, 
 // and will allow focus to change only if the required string
 // is typed.

 class VerifierTest extends JFrame {
  public VerifierTest () {
    JTextField tf,tf2;
    tf = new JTextField ("howto is required");

    getContentPane().add (tf, BorderLayout.NORTH);
    tf.setInputVerifier(new HowtoVerifier());

    tf2 = new JTextField ("howto come here");
    getContentPane().add (tf2, BorderLayout.SOUTH);

    addWindowListener(new WindowCloser());
  }
 

  public static void main (String [] args) {
    Frame f = new VerifierTest ();
    f.pack();
    f.show();
  }
 }

class WindowCloser extends WindowAdapter {
 public void windowClosing(WindowEvent e) {
   Window win = e.getWindow();
   win.setVisible(false);
   System.exit(0);
 }
}


class HowtoVerifier extends InputVerifier {
  public boolean verify(JComponent input) {
    JTextField tf = (JTextField) input;
    String pass = tf.getText();
    return pass.equals("howto");
  }
}

import javax.swing.border.*;

...

class HowtoVerifier extends InputVerifier {
  public boolean verify(final JComponent input) {
    JTextField tf = (JTextField) input;
    String pass = tf.getText();
    if (!pass.equals("howto")) {
        SwingUtilities.invokeLater(new Runnable(){
          public void run(){
           input.setBorder( new LineBorder(Color.RED) );
          }
        });
        return false;
    }
    SwingUtilities.invokeLater(new Runnable(){
       public void run(){
         input.setBorder( LineBorder.createBlackLineBorder() );
       }
    });
    return true;
    }
  }
  
...