Chapter 18 Exercise 20, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

18.20 (Display circles)
Write a Java program that displays ovals. The circles are centered in the pane.
The gap between two adjacent circles is 10 pixels, and the gap between the
border of the pane and the largest circle is also 10.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class Exercise_20  extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {

        CirclePane pane = new CirclePane();

        Scene scene = new Scene(pane);
        scene.widthProperty().addListener(e-> pane.draw());
        primaryStage.setScene(scene);
        primaryStage.setTitle("Circle pane");
        primaryStage.show();
    }

    class CirclePane extends StackPane {

        CirclePane() {
            setPrefHeight(200);
            setPrefWidth(200);
        }

        public void draw() {
            getChildren().clear();

            double range = Math.min(getWidth(), getHeight());
            createCircle(range / 2 - 10);
        }

        public void createCircle(double radius) {
            if (radius > 10) {
                Circle c = new Circle(radius, Color.TRANSPARENT);
                c.setStroke(Color.BLACK);
                getChildren().add(c);
                createCircle(radius - 10);
            }
        }

    }
}