import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

/**
 * Constraints:
 * <ul>
 * <li>Event 2 should be performed before Event 3, otherwise, Event 3 cannot be
 * performed.</li>
 * <li>Any other combinations are all allowed.</li>
 * </ul>
 * 
 * Implementation for the constraints:
 * <ul>
 * <li>Button "Event 3" is disabled at startup, and a click on button "Event 2"
 * enables button "Event 3". After that, button "Event 3" is enabled all the time.
 * </li>
 * </ul>
 * 
 * @author Si Huang
 */
public class SimpleFrameRequires extends JFrame {
    private static final long serialVersionUID = 1L;

    private JButton event1;
    private JButton event2;
    private JButton event3;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                SimpleFrameRequires frame = new SimpleFrameRequires();
                frame.setVisible(true);
            }
        });
    }

    public SimpleFrameRequires() {
        event1 = new JButton("Event 1");
        event2 = new JButton("Event 2");
        event2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                event3.setEnabled(true);
            }
        });
        event3 = new JButton("Event 3");
        event3.setEnabled(false);

        setLayout(new FlowLayout());
        add(event1);
        add(event2);
        add(event3);

        setTitle("SimpleFrame");
        setResizable(false);
        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        pack();

        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        setLocation((screenSize.width - getWidth()) / 2, (screenSize.height - getHeight()) / 2);
    }
}