How to use Focus Events in Swing

Focus events occur when any component gains or loses input focus on a graphical user interface. Focus applies to all components that can recieve input.

To handle a focus event, a class must implement the FocusListener interface.

The following example shows how to use focus events in Swing:

/*
* FocusEventDemo.java is a 1.4 example that requires
* no other files.
*/

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

public class FocusEventDemo extends JPanel
implements FocusListener {
final static String newline = "\n";
JTextArea display;

public FocusEventDemo() {
super(new GridBagLayout());
GridBagLayout gridbag = (GridBagLayout)getLayout();
GridBagConstraints c = new GridBagConstraints();

c.fill = GridBagConstraints.HORIZONTAL;
c.weightx = 1.0; //Make column as wide as possible.
JTextField textField = new JTextField("A TextField");
textField.setMargin(new Insets(0,2,0,2));
textField.addFocusListener(this);
gridbag.setConstraints(textField, c);
add(textField);

c.weightx = 0.1; //Widen every other column a bit, when possible.
c.fill = GridBagConstraints.NONE;
JLabel label = new JLabel("A Label");
label.setBorder(BorderFactory.createEmptyBorder(0,5,0,5));
label.addFocusListener(this);
gridbag.setConstraints(label, c);
add(label);

String comboPrefix = "ComboBox Item #";
final int numItems = 15;
Vector vector = new Vector(numItems);
for (int i = 0; i <>) {
vector.addElement(comboPrefix + i);
}
JComboBox comboBox = new JComboBox(vector);
comboBox.addFocusListener(this);
gridbag.setConstraints(comboBox, c);
add(comboBox);

c.gridwidth = GridBagConstraints.REMAINDER;
JButton button = new JButton("A Button");
button.addFocusListener(this);
gridbag.setConstraints(button, c);
add(button);

c.weightx = 0.0;
c.weighty = 0.1;
c.fill = GridBagConstraints.BOTH;
String listPrefix = "List Item #";
Vector listVector = new Vector(numItems);
for (int i = 0; i <>) {
listVector.addElement(listPrefix + i);
}
JList list = new JList(listVector);
list.setSelectedIndex(1); //It's easier to see the focus change
//if an item is selected.
list.addFocusListener(this);
JScrollPane listScrollPane = new JScrollPane(list);
//We want to prevent the list's scroll bars
//from getting the focus - even with the keyboard.
//Note that in general we prefer setRequestFocusable
//over setFocusable for reasons of accessibility,
//but this is to work around bug #4866958.
listScrollPane.getVerticalScrollBar().setFocusable(false);
listScrollPane.getHorizontalScrollBar().setFocusable(false);
gridbag.setConstraints(listScrollPane, c);
add(listScrollPane);

c.weighty = 1.0; //Make this row as tall as possible.
c.gridheight = GridBagConstraints.REMAINDER;
//Set up the area that reports focus-gained and focus-lost events.
display = new JTextArea();
display.setEditable(false);
//The method setRequestFocusEnabled prevents a
//component from being clickable, but it can still
//get the focus through the keyboard - this ensures
//user accessibility.
display.setRequestFocusEnabled(false);
display.addFocusListener(this);
JScrollPane displayScrollPane = new JScrollPane(display);

//Work around for bug #4866958.
displayScrollPane.getHorizontalScrollBar().setFocusable(false);
displayScrollPane.getVerticalScrollBar().setFocusable(false);
gridbag.setConstraints(displayScrollPane, c);
add(displayScrollPane);

setPreferredSize(new Dimension(450, 450));
setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
}

public void focusGained(FocusEvent e) {
displayMessage("Focus gained", e);
}

public void focusLost(FocusEvent e) {
displayMessage("Focus lost", e);
}

void displayMessage(String prefix, FocusEvent e) {
display.append(prefix
+ (e.isTemporary() ? " (temporary):" : ":")
+ e.getComponent().getClass().getName()
+ "; Opposite component: "
+ (e.getOppositeComponent() != null ?
e.getOppositeComponent().getClass().getName() : "null")
+ newline);
display.setCaretPosition(display.getDocument().getLength());
}

/**
* Create the GUI and show it. For thread safety,
* this method should be invoked from the
* event-dispatching thread.
*/
private static void createAndShowGUI() {
//Make sure we have nice window decorations.
JFrame.setDefaultLookAndFeelDecorated(true);

//Create and set up the window.
JFrame frame = new JFrame("FocusEventDemo");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

//Create and set up the content pane.
JComponent newContentPane = new FocusEventDemo();
newContentPane.setOpaque(true); //content panes must be opaque
frame.setContentPane(newContentPane);

//Display the window.
frame.pack();
frame.setVisible(true);
}

public static void main(String[] args) {
//Schedule a job for the event-dispatching thread:
//creating and showing this application's GUI.
javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
}
}