FocusListener trong Java Swing

public interface FocusListener
extends EventListener

package vn.viettuts.swing;

import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.WindowConstants;

public class FocusListenerExam1 {
    private JFrame mainFrame;
    private JLabel headerLabel;
    private JLabel statusLabel;
    private JPanel controlPanel;

    public FocusListenerExam1(){
       prepareGUI();
    }

    public static void main(String[] args) {
        FocusListenerExam1 demo = new FocusListenerExam1();
        demo.showFocusListenerDemo();
    }

    private void prepareGUI() {
        mainFrame = new JFrame("Vi du Java Swing");
        mainFrame.setSize(500, 300);
        mainFrame.setLayout(new GridLayout(3, 1));
        headerLabel = new JLabel("", JLabel.CENTER);
        statusLabel = new JLabel("", JLabel.CENTER);
        statusLabel.setSize(350, 100);
        controlPanel = new JPanel();
        controlPanel.setLayout(new FlowLayout());
        mainFrame.add(headerLabel);
        mainFrame.add(controlPanel);
        mainFrame.add(statusLabel);
        mainFrame.setVisible(true);
        mainFrame.setTitle("Ví dụ FocusListener trong Java Swing");
        mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    }

    private void showFocusListenerDemo() {
        headerLabel.setText("Listener in action: FocusListener");
        JButton okButton = new JButton("OK");
        JButton cancelButton = new JButton("Cancel");

        okButton.addFocusListener(new CustomFocusListener());
        cancelButton.addFocusListener(new CustomFocusListener());

        controlPanel.add(okButton);
        controlPanel.add(cancelButton);
        mainFrame.setVisible(true);
    }

    class CustomFocusListener implements FocusListener {
        public void focusGained(FocusEvent e) {
            statusLabel.setText(statusLabel.getText() 
                    + e.getComponent().getClass().getSimpleName() 
                    + " gained focus. ");
        }

        public void focusLost(FocusEvent e) {
            statusLabel.setText(statusLabel.getText() 
                    + e.getComponent().getClass().getSimpleName() 
                    + " lost focus. ");
        }
    }
}