Java Swing Tutorial - Java FocusTraversalPolicy .getComponentBefore (Container aContainer, Component aComponent)

Syntax

FocusTraversalPolicy.getComponentBefore(Container aContainer, Component aComponent) has the following syntax.

public abstract Component getComponentBefore(Container aContainer,    Component aComponent)

Example

In the following code shows how to use FocusTraversalPolicy.getComponentBefore(Container aContainer, Component aComponent) method.

/*from ww w  . j  av  a  2s  .  c  o  m*/

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());
  }
}