Sierpinski Triangle and Carpet

Sierpinski.java

package klausur.spierpinski;

import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

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

import static javax.swing.JFrame.EXIT_ON_CLOSE;

public class Sierpinski extends JPanel {

    protected int order;

    public Sierpinski(final int order) {
        this.order = order;
    }

    @NotNull
    private static Point midpoint(@NotNull final Point p1, @NotNull final Point p2) {
        return new Point((p1.x + p2.x) / 2, (p1.y + p2.y) / 2);
    }

    public static void main(@Nullable final String[] args) {
        final JFrame frame = new JFrame("Sierpinkski");
        final Container container = frame.getContentPane();
        container.add(new Sierpinski(5));
        frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        frame.setVisible(true);
        frame.setLocationRelativeTo(null);
    }

    public void drawTriangles(@NotNull final Graphics g, @NotNull final int width, @NotNull final int height) {
        Point p1 = new Point(width / 2, 10);
        Point p2 = new Point(10, height - 10);
        Point p3 = new Point(width - 10, height - 10);
        displayTriangles(g, order, p1, p2, p3);
    }

    private void displayTriangles(@NotNull final Graphics g, @NotNull final int order, final Point p1, @NotNull final Point p2, @NotNull final 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 {
            final Point p12 = midpoint(p1, p2);
            final Point p23 = midpoint(p2, p3);
            final Point p31 = midpoint(p3, p1);

            // Recursively display three triangles
            displayTriangles(g, order - 1, p1, p12, p31);
            displayTriangles(g, order - 1, p12, p2, p23);
            displayTriangles(g, order - 1, p31, p23, p3);
        }
    }

    public void drawSierpinskiCarpet(@NotNull final Graphics g, @NotNull final int xOL, @NotNull final int yOL, @NotNull final int width, int height) {
        if (width > 2 && height > 2) {
            int b = width / 3;
            int h = height / 3;
            g.fillRect(xOL + b, yOL + h, b, h);
            for (int k = 0; k < 9; k++)
                if (k != 4) {
                    int i = k / 3;
                    int j = k % 3;
                    drawSierpinskiCarpet(g, xOL + i * b, yOL + j * h, b, h); // Rekursion
                }
        }
    }

    @Override
    public void paintComponent(final Graphics g) {
        super.paintComponent(g);
        drawTriangles(g, getWidth(), getHeight());
        drawSierpinskiCarpet(g, 0, 0, getWidth(), getHeight());
    }
}