Author: Real Gagnon
Author's WebSite: http://tactika.com/realhome/realhome.html

The problem is when the lostFocus occurs on a Component, the gainedFocus is already sent for the next component in the SystemEventQueue. We must grab this event, and request the focus for the previous component (if there is a validation error). We can't use Toolkit.getDefaultToolkit().getSystemEventQueue() directly to remove the gainedFocus event because of security restriction in Applet. This can be done with invokeLater method of the SwingUtilities class.

  import java.awt.*; 
  import java.awt.event.*;
  import com.sun.java.swing.*;
  //import javax.swing.*;

  public class tswing extends JApplet {
    JTextField textfield1, textfield2;
    JLabel label1;

    public void init() {
      getContentPane().setLayout(new FlowLayout());
      //
      label1 = new JLabel("must be 'a' or 'b' ");
      textfield1 = new JTextField(5);
      getContentPane().add(label1);
      getContentPane().add(textfield1);
      textfield2 = new JTextField(5);
      getContentPane().add(textfield2);

      textfield1.addFocusListener(new FocusListener() {
         public void focusGained(FocusEvent e) {}; // not needed

         public void focusLost(FocusEvent e) {
           if (!e.isTemporary() && isEnabled() ) {
             String fieldContent = textfield1.getText();
             if (!fieldContent.equals("a") && !fieldContent.equals("b")) {
               Toolkit.getDefaultToolkit().beep();
               System.out.println("illegal value! " + fieldContent );
               SwingUtilities.invokeLater(new FocusGrabber(textfield1));         
               }
           }}
         });
      }
  }

Usage

 import com.sun.java.swing.*;
 //import javax.swing.*;

 public class FocusGrabber implements Runnable {
   private JComponent component;
  
   public FocusGrabber(JComponent component) {
    this.component = component;
    }
   public void run() {
    component.grabFocus();
    }
 }

Posted On: 5-Jul-1999

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