Beginner Swing recursion

Question

This programming assignment has me stumped; the code is as follows:

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

public class HTree extends JPanel {

private Color color = new Color((int)(Math.random() * 256), (int)(Math.random() * 256), (int)(Math.random() * 256));

public void draw(Graphics g, int n, double sz, double x, double y) {
    if (n == 0) return; 
    double x0 = x - sz/2, x1 = x + sz/2;
    double y0 = y - sz/2, y1 = y + sz/2;
    // draw the 3 line segments of the H  
    g.setColor(color);
    g.drawLine((int)x0, (int)y, (int)x1, (int)y);
    g.drawLine((int)x0, (int)y0, (int)x0, (int)y1);
    g.drawLine((int)x1, (int)y0, (int)x1, (int)y1);
    // recursively draw 4 half-size
    // H-trees of order n-1
    g.setColor(color);
    draw(g, n-1, sz/2, x0, y0);
    draw(g, n-1, sz/2, x0, y1);
    draw(g, n-1, sz/2, x1, y0);
    draw(g, n-1, sz/2, x1, y1);
    repaint();
}

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    draw(g, 3, .5, .5, .5);
}

public static void main(String[] args) {
    HTree h = new HTree();
    JFrame application = new JFrame("HTree");
    application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    application.add(h);
    application.setSize(1000, 1000);
    application.setVisible(true);
}
}

It compiles correctly, but when running the program, all I get is an empty JFrame. I am not too familiar with Swing, but I think the problem is that either the HTree constructor is wrong, or I may need to call draw() in main—I'm not sure how to call it with a Graphics object. Any help is appreciated, Thank you!

Answer 1

Some observations:

• Use integer coordinates relative to the panel's current size.

• Override getPreferredSize() to establish the initial geometry.

• Use Color.getHSBColor() to get bright, saturated colors; consider creating a gamut of hues, selecting a different one at each level, for example.

• Construct and manipulate Swing GUI objects only on the event dispatch thread.

• Consider using a JSpinner or JSlider to control recursion depth interactively, as suggested here and here.

• Don't invoke repaint() recursively; use it to schedule a call to your implementation of paintComponent(), as when the recursion depth charges in response to some input.

• Consider using setStroke() and rendering hints, as shown here.

• Consider using a javax.swing.Timer, increasing the recursion depth by one level each time the ActionListener is called.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;

/**
 * @see https://stackoverflow.com/a/37450393/230513
 */
public class HTree {

    private void display() {
        JFrame f = new JFrame("HTree");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new Tree());
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    private static class Tree extends JPanel {

        private final Color color = Color.getHSBColor((float) Math.random(), 1, 1);

        public void draw(Graphics g, int n, double sz, double x, double y) {
            if (n == 0) {
                return;
            }
            double x0 = x - sz / 2, x1 = x + sz / 2;
            double y0 = y - sz / 2, y1 = y + sz / 2;
            // draw the 3 line segments of the H 
            g.setColor(color);
            g.drawLine((int) x0, (int) y, (int) x1, (int) y);
            g.drawLine((int) x0, (int) y0, (int) x0, (int) y1);
            g.drawLine((int) x1, (int) y0, (int) x1, (int) y1);
            // recursively draw 4 half-size
            // H-trees of order n-1
            g.setColor(color);
            draw(g, n - 1, sz / 2, x0, y0);
            draw(g, n - 1, sz / 2, x0, y1);
            draw(g, n - 1, sz / 2, x1, y0);
            draw(g, n - 1, sz / 2, x1, y1);
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            draw(g, 3, getWidth() / 2, getWidth() / 2, getHeight() / 2);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(500, 500);
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new HTree()::display);
    }
}