Java JComponent grab focus

import java.awt.BorderLayout;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class Main {
   public static void main(String[] a) {
      final JTextField textField = new JTextField(5);

      textField.addFocusListener(new FocusListener() {
         public void focusGained(FocusEvent e) {
         }/*w w w.  java 2s  . com*/

         public void focusLost(FocusEvent e) {
            if (!e.isTemporary()) {
               String content = textField.getText();
               if (!content.equals("a")) {
                  System.out.println("illegal value! " + content);
                  SwingUtilities.invokeLater(new FocusGrabber(textField));
               }
            }
         }
      });
      JFrame frame = new JFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.add(textField, BorderLayout.NORTH);
      frame.add(new JTextField(), BorderLayout.SOUTH);
      frame.setSize(300, 300);
      frame.setVisible(true);

   }
}

class FocusGrabber implements Runnable {
   private JComponent component;

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

   public void run() {
      component.grabFocus();
   }
}