//  AlphaButtonPolicy.java
// A custom focus traversal policy that uses alphabetical ordering of button
// labels to determine "next" and "previous" buttons for keyboard traversal.
//
import java.awt.*;
import java.util.*;
import javax.swing.*;

public 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());
    }
}