Animation self-avoiding random walk

AnimationSelfAvoidingRandomWalk.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class AnimationSelfAvoidingRandomWalk extends JFrame {
    
    private static final int N = 16;
    private Point[][] lattice = new Point[N + 1][N + 1];
    private int i = (N + 1) / 2;
    private int j = (N + 1) / 2;
    private JButton start = new JButton("Start");
    private Timer timer = new Timer(100, new TimerListener());
    
    AnimationSelfAvoidingRandomWalk() {
        add(new P());
        JPanel panel = new JPanel();
        panel.setBorder(BorderFactory.createLineBorder(Color.lightGray));
        panel.add(start);
        add(panel, BorderLayout.SOUTH);
        start.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                timer.stop();
                getPath();
                timer.start();
            }
        });
    }
    
    private void getPath() {
        // Refresh the lattics
        for (int i = 0; i < lattice.length; i++) {
            for (int j = 0; j < lattice[i].length; j++) {
                lattice[i][j] = null;
            }
        }
        
        i = (N + 1) / 2;
        j = (N + 1) / 2;
    }
    
    private class TimerListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            if (i > 0 && i < N && j > 0 && j < N) {
                if (lattice[i - 1][j] != null && lattice[i + 1][j] != null
                        && lattice[i][j - 1] != null && lattice[i][j + 1] != null) {
                    return;
                }

                double r = Math.random();
                if (r < 0.25 && lattice[i][j + 1] == null) {
                    lattice[i][j] = new Point(i, j + 1); // Right
                    j++;
                } else if (r < 0.5 && lattice[i + 1][j] == null) {
                    lattice[i][j] = new Point(i + 1, j); // Down
                    i++;
                } else if (r < .75 && lattice[i][j - 1] == null) {
                    lattice[i][j] = new Point(i, j - 1); // Left
                    j--;
                } else if (r < 1.0 && lattice[i - 1][j] == null) {
                    lattice[i][j] = new Point(i - 1, j); // Up
                    i--;
                }
                
                repaint();
            }
        }
        
    }
    
    private class P extends JPanel {
        
        @Override
        protected void paintComponent(Graphics g) {
            int hGap = getWidth() / N;
            int vGap = getHeight() / N;
            
            // Draw the lattics
            g.setColor(Color.lightGray);
            for (int i = 0; i < lattice.length; i++) {
                g.drawLine(0, i * vGap, getWidth(), i * vGap);
                g.drawLine(i * hGap, 0, i * hGap, getHeight());
            }
            
            int i = (N + 1) / 2;
            int j = (N + 1) / 2;
            
            // Draw the path
            g.setColor(Color.BLACK);
            while (lattice[i][j] != null) {
                Point p = lattice[i][j];
                g.drawLine(i * hGap, j * vGap, p.x * hGap, p.y * vGap);
                i = p.x;
                j = p.y;
            }
        }
    }
    
    public static void main(String[] args) {
        JFrame frame = new AnimationSelfAvoidingRandomWalk();
        frame.setSize(400, 200);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

}