Java FocusTraversalPolicy .getDefaultComponent (Container aContainer)

public abstract Component getDefaultComponent(Container aContainer)

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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 Main extends JPanel {

 public Main() {
     setLayout(new GridLayout(6, 1));
     add(new JButton("A"));
     add(new JButton("D"));
     
     add(new JButton("C"));
     
     add(new JButton("E"));
     add(new JButton("B"));
     add(new JButton("F"));
 }

 public static void main(String[] args) {
     JFrame frame = new JFrame();
     frame.setFocusTraversalPolicy(new AlphaButtonPolicy());
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     frame.setContentPane(new Main());
     frame.setSize(400, 300);
     frame.setVisible(true);
 }
}

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());
  }
}