Validate a value on the lostFocus event for JTextField in Java

//from  w ww.jav a 2  s .  c o m
import java.awt.FlowLayout;
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[] args) {
    JFrame f = new JFrame("Text Field Elements");

    final JTextField tf1 = new JTextField(5);
    f.setLayout(new FlowLayout());
    f.add(tf1);
    f.add(new JTextField(5));
    f.pack();
    f.setVisible(true);

    
    tf1.addFocusListener(new FocusListener() {
      public void focusGained(FocusEvent e) {
      };
      public void focusLost(FocusEvent e) {
        if (!e.isTemporary()) {
          String content = tf1.getText();
          if (!content.equals("a") ) {
            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();
  }
}