import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import java.awt.Container;
import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

/**
Packs a section of the frame window with circles, one at a time.
*/
public class FillDemo extends JFrame implements ActionListener
{
    public static final int WIDTH = 300;
    public static final int HEIGHT = 200;
    public static final int FILL_WIDTH = 300;
    public static final int FILL_HEIGHT = 100;
    public static final int CIRCLE_SIZE = 10;
    public static final int PAUSE = 100; //milliseconds

    private JPanel box;

    public static void main(String[] args)
    {
        FillDemo gui = new FillDemo( );
        gui.setVisible(true);
    }

    public FillDemo( )
    {
        setSize(WIDTH, HEIGHT);
        setTitle("FillDemo");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Container contentPane = getContentPane( );
        contentPane.setLayout(new BorderLayout( ));

        box = new JPanel( );
        contentPane.add(box, BorderLayout.CENTER);

        JPanel buttonPanel = new JPanel( );
        buttonPanel.setLayout(new FlowLayout( ));
        JButton startButton = new JButton("Start");
        startButton.addActionListener(this);
        buttonPanel.add(startButton);
        contentPane.add(buttonPanel, BorderLayout.SOUTH);
    }

    public void actionPerformed(ActionEvent e)
    {
        fill( );
    }

    public void fill( )
    {
        Graphics g = box.getGraphics( );

        for (int y = 0; y < FILL_HEIGHT; y = y + CIRCLE_SIZE)
            for (int x = 0; x < FILL_WIDTH; x = x + CIRCLE_SIZE)
            {
                g.fillOval(x, y, CIRCLE_SIZE, CIRCLE_SIZE);
                doNothing(PAUSE);
            }
    }

    public void doNothing(int milliseconds)
    {
        try
        {
            Thread.sleep(milliseconds);
        }
        catch(InterruptedException e)
        {
            System.out.println("Unexpected interrupt");
            System.exit(0);
        }
    }
}