Sierpinksi Triangle using Java brainstorming

programming
2 
package com.example.demo;

import javafx.geometry.Point2D;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Polygon;

public class SierpinskiTrianglePane extends Pane {
    private int order = 0;

    SierpinskiTrianglePane() {

    }

    public void setOrder(int order) {
        this.order = order;
        paint();
    }

    protected void paint() {
        Point2D p1 = new Point2D(getWidth() / 2, 10);
        Point2D p2 = new Point2D(10, getHeight() - 10);
        Point2D p3 = new Point2D(getWidth() - 10, getHeight() - 10);
        this.getChildren().clear();
        displayTriangles(order, p1, p2, p3);

    }

    public void displayTriangles(int order, Point2D p1, Point2D p2, Point2D p3) {

        if (order == 0) {
            Polygon triangle = new Polygon();
            triangle.getPoints().addAll(p1.getX(), p1.getY(), p2.getX(), p2.getY(), p3.getX(), p3.getY());
            triangle.setStroke(Color.BLACK);
            triangle.setFill(Color.WHITE);
            this.getChildren().add(triangle);
        } else {
            Point2D p12 = p1.midpoint(p2);
            Point2D p23 = p2.midpoint(p3);
            Point2D p31 = p3.midpoint(p1);

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

}

And

package com.example.demo;

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class Sierpinski extends Application {
    @Override
    public void start(Stage primaryStage) {
        SierpinskiTrianglePane trianglePane = new SierpinskiTrianglePane();
        TextField tfOrder = new TextField();
        tfOrder.setOnAction(e -> trianglePane.setOrder(Integer.parseInt(tfOrder.getText())));
        tfOrder.setPrefColumnCount(4);
        tfOrder.setAlignment(Pos.BOTTOM_RIGHT);

        HBox hBox = new HBox(10);
        hBox.getChildren().addAll(new Label("Enter an order"), tfOrder);
        hBox.setAlignment(Pos.CENTER);

        BorderPane borderPane = new BorderPane();
        borderPane.setCenter(trianglePane);
        borderPane.setBottom(hBox);

        Scene scene = new Scene(borderPane, 200, 210);
        primaryStage.setTitle("SierpinskiTriangle");
        primaryStage.setScene(scene);
        primaryStage.show();

        scene.widthProperty().addListener(ov -> trianglePane.paint());
        scene.heightProperty().addListener(ov -> trianglePane.paint());
    }
}

I got the solution from Daniel Liang's java textbook. I understand the solution, but I would not be able to create it on my own.