SierpinskiTriangle drawing sierpinski triangles with java swing

SierpinskiTriangle.java

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

public class Test {
    
    public static void main(String[] args){
        JFrame f = new SierpinskiTriangle();
        f.setSize(300, 300);
        f.setLocationRelativeTo(null);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
}

class SierpinskiTriangle extends JFrame {
    
    private JButton b1 = new JButton("-");
    private JButton b2 = new JButton("+");
    private SierpinskiPanel sie = new SierpinskiPanel();
    private int order = 0;
    
    public SierpinskiTriangle() {
        JPanel panel = new JPanel();
        panel.add(b1);
        panel.add(b2);
        
        add(panel, BorderLayout.SOUTH);
        add(sie);
        
        b1.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (order != 0) {
                    order--;
                    sie.setOrder(order);
                }
            }
        });
        
        b2.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (order != 10) {
                    order++;
                    sie.setOrder(order);
                }
            }
        });
    }
    
    private static class SierpinskiPanel extends JPanel {
        private int order = 0;
        
        public void setOrder(int order) {
            this.order = order;
            repaint();
        }
        
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            
            Point p1 = new Point(getWidth() / 2, 10);
            Point p2 = new Point(getWidth() - 10, getHeight() - 10);
            Point p3 = new Point(10, getHeight() - 10);
            
            displayTriangles(g, order, p1, p2, p3);
        }
        
        private static void displayTriangles(Graphics g, int order, 
                Point p1, Point p2, Point p3) {
            if (order == 0) {
                g.drawLine(p1.x, p1.y, p2.x, p2.y);
                g.drawLine(p1.x, p1.y, p3.x, p3.y);
                g.drawLine(p2.x, p2.y, p3.x, p3.y);
            } else {
                Point p12 = midPoint(p1, p2);
                Point p23 = midPoint(p2, p3);
                Point p31 = midPoint(p3, p1);
                
                displayTriangles(g, order - 1, p1, p12, p31);
                displayTriangles(g, order - 1, p12, p2, p23);
                displayTriangles(g, order - 1, p31, p23, p3);
            }
        }
        
        private static Point midPoint(Point p1, Point p2) {
            return new Point((p1.x + p2.x) / 2, (p1.y + p2.y) / 2);
        }
    }
}