Draw Sierpinski iTriangle in GUI

SierpinskiFrame.java

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

public class SierpinskiFrame extends JFrame {

    public static void main(String[] args) {
        SierpinskiFrame frame = new SierpinskiFrame();
        frame.setSize(600, 600);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    SierpinskiPanel sierpinskiPanel = new SierpinskiPanel();
    JLabel label = new JLabel("N = " + sierpinskiPanel.getN());

    public SierpinskiFrame() {
        super("Sierpinski Triangle");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JButton dw = new JButton("-");
        dw.addActionListener(e -> changeN(-1));
        JButton up = new JButton("+");
        up.addActionListener(e -> changeN(+1));

        JPanel buttonPanel = new JPanel();
        buttonPanel.add(dw);
        buttonPanel.add(up);
        add(buttonPanel, BorderLayout.SOUTH);

        add(label, BorderLayout.NORTH);

        add(sierpinskiPanel, BorderLayout.CENTER);
    }

    private void changeN(int d) {
        sierpinskiPanel.setN(sierpinskiPanel.getN() + d);
        label.setText("N = " + sierpinskiPanel.getN());
    }

    static class SierpinskiPanel extends JPanel {
        private int n = 1;

        public int getN() {
            return n;
        }

        public void setN(int n) {
            if (n < 1) return;
            this.n = n;
            repaint();
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Point p1 = new Point(getWidth() / 2, 0);
            Point p2 = new Point(0, getHeight());
            Point p3 = new Point(getWidth(), getHeight());
            drawTriangles(g, n, p1, p2, p3);
        }

        void drawTriangles(Graphics g, int order, Point p1, Point p2, Point p3) {
            if (order == 1) {
                g.drawLine(p1.x, p1.y, p2.x, p2.y);
                g.drawLine(p2.x, p2.y, p3.x, p3.y);
                g.drawLine(p3.x, p3.y, p1.x, p1.y);
            } else {
                Point p12 = new Point((p1.x + p2.x) / 2, (p1.y + p2.y) / 2);
                Point p23 = new Point((p2.x + p3.x) / 2, (p2.y + p3.y) / 2);
                Point p31 = new Point((p3.x + p1.x) / 2, (p3.y + p1.y) / 2);

                drawTriangles(g, order - 1, p1, p12, p31);
                drawTriangles(g, order - 1, p12, p2, p23);
                drawTriangles(g, order - 1, p31, p23, p3);
            }
        }
    }
}