import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class Test {

    public static void main(String[] args) throws Exception {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Test().createAndShowUI();
            }
        });
    }

    private void createAndShowUI() {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        initComponents(frame);
        frame.setPreferredSize(new Dimension(400, 400));
        frame.pack();
        frame.setVisible(true);
    }

    private void initComponents(JFrame frame) {
        JScrollPane jsp = new JScrollPane();
        jsp.setViewportView(new Panel(800, 800, jsp));
        frame.getContentPane().add(jsp);
    }
}

class Panel extends JPanel {

    private int across, down;
    private Panel.Tile[][] tiles;
    private Color color = Color.black;
    private final JScrollPane jScrollPane;

    public Panel(int width, int height, JScrollPane jScrollPane) {
        this.setPreferredSize(new Dimension(width, height));
        this.jScrollPane = jScrollPane;
        createTiles();
        changePanelColorTimer();//just something to do to check if its repaints fine
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (int i = 0; i < across; i++) {
            for (int j = 0; j < down; j++) {
                g.setColor(color);
                for (int k = 0; k < 5; k++) {
                    g.drawRect(tiles[i][j].x + k, tiles[i][j].y + k, tiles[i][j].side - k * 2, tiles[i][j].side - 2 * k);
                }
            }
        }
        updateScrollPane();//refresh the pane after every paint
    }

    //calls repaint on the scrollPane instance
    private void updateScrollPane() {
        jScrollPane.repaint();
    }

    private void createTiles() {
        across = 13;
        down = 9;
        tiles = new Panel.Tile[across][down];

        for (int i = 0; i < across; i++) {
            for (int j = 0; j < down; j++) {
                tiles[i][j] = new Panel.Tile((i * 50), (j * 50), 50);
            }
        }
    }

    //change the color of the grid lines from black to red and vice versa every 2s
    private void changePanelColorTimer() {
        Timer timer = new Timer(2000, new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                if (color == Color.black) {
                    color = Color.red;
                } else {
                    color = Color.black;
                }
            }
        });
        timer.setInitialDelay(2000);
        timer.start();
    }

    private class Tile {

        int x, y, side;

        public Tile(int inX, int inY, int inSide) {
            x = inX;
            y = inY;
            side = inSide;
        }
    }
}