Focus Traversal Example
/*
Java Swing, 2nd Edition
By Marc Loy, Robert Eckstein, Dave Wood, James Elliott, Brian Cole
ISBN: 0-596-00408-7
Publisher: O'Reilly
*/
// FocusTraversalExample.java
//Similar to the FocusExample, this class uses the custom AlphaButtonPolicy
//focus traversal policy to navigate another small set of buttons.
//
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.FocusTraversalPolicy;
import java.awt.GridLayout;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class FocusTraversalExample extends JPanel {
public FocusTraversalExample() {
setLayout(new GridLayout(6, 1));
JButton button1 = new JButton("Texas");
JButton button2 = new JButton("Vermont");
JButton button3 = new JButton("Florida");
JButton button4 = new JButton("Alabama");
JButton button5 = new JButton("Minnesota");
JButton button6 = new JButton("California");
setBackground(Color.lightGray);
add(button1);
add(button2);
add(button3);
add(button4);
add(button5);
add(button6);
}
public static void main(String[] args) {
JFrame frame = new JFrame("Alphabetized Button Focus Traversal");
frame.setFocusTraversalPolicy(new AlphaButtonPolicy());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setContentPane(new FocusTraversalExample());
frame.setSize(400, 300);
frame.setVisible(true);
}
}
//AlphaButtonPolicy.java
//A custom focus traversal policy that uses alphabetical ordering of button
//labels to determine "next" and "previous" buttons for keyboard traversal.
//
class AlphaButtonPolicy extends FocusTraversalPolicy {
private SortedMap getSortedButtons(Container focusCycleRoot) {
if (focusCycleRoot == null) {
throw new IllegalArgumentException("focusCycleRoot can't be null");
}
SortedMap result = new TreeMap(); // Will sort all buttons by text.
sortRecursive(result, focusCycleRoot);
return result;
}
private void sortRecursive(Map buttons, Container container) {
for (int i = 0; i < container.getComponentCount(); i++) {
Component c = container.getComponent(i);
if (c instanceof JButton) { // Found another button to sort.
buttons.put(((JButton) c).getText(), c);
}
if (c instanceof Container) { // Found a container to search.
sortRecursive(buttons, (Container) c);
}
}
}
// The rest of the code implements the FocusTraversalPolicy interface.
public Component getFirstComponent(Container focusCycleRoot) {
SortedMap buttons = getSortedButtons(focusCycleRoot);
if (buttons.isEmpty()) {
return null;
}
return (Component) buttons.get(buttons.firstKey());
}
public Component getLastComponent(Container focusCycleRoot) {
SortedMap buttons = getSortedButtons(focusCycleRoot);
if (buttons.isEmpty()) {
return null;
}
return (Component) buttons.get(buttons.lastKey());
}
public Component getDefaultComponent(Container focusCycleRoot) {
return getFirstComponent(focusCycleRoot);
}
public Component getComponentAfter(Container focusCycleRoot,
Component aComponent) {
if (!(aComponent instanceof JButton)) {
return null;
}
SortedMap buttons = getSortedButtons(focusCycleRoot);
// Find all buttons after the current one.
String nextName = ((JButton) aComponent).getText() + "\0";
SortedMap nextButtons = buttons.tailMap(nextName);
if (nextButtons.isEmpty()) { // Wrapped back to beginning
if (!buttons.isEmpty()) {
return (Component) buttons.get(buttons.firstKey());
}
return null; // Degenerate case of no buttons.
}
return (Component) nextButtons.get(nextButtons.firstKey());
}
public Component getComponentBefore(Container focusCycleRoot,
Component aComponent) {
if (!(aComponent instanceof JButton)) {
return null;
}
SortedMap buttons = getSortedButtons(focusCycleRoot);
SortedMap prevButtons = // Find all buttons before this one.
buttons.headMap(((JButton) aComponent).getText());
if (prevButtons.isEmpty()) { // Wrapped back to end.
if (!buttons.isEmpty()) {
return (Component) buttons.get(buttons.lastKey());
}
return null; // Degenerate case of no buttons.
}
return (Component) prevButtons.get(prevButtons.lastKey());
}
}
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