import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.StackPane;
import javafx.scene.control.Button;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
/* Note, it does not have the main method.
   It is not necessary when IDE has JavaFX support. 
*/
public class ControlCircle extends Application {
    private CirclePane circlePane = new CirclePane();

    @Override
    public void start(Stage primaryStage) {
        HBox hBox = new HBox();
        Button btEnlarge = new Button("Enlarge");
        Button btShrink = new Button("Shrink");
        hBox.getChildren().add(btEnlarge);
        hBox.getChildren().add(btShrink);
        btEnlarge.setOnAction(new EnlargeHandler());  // using inner class
        btShrink.setOnAction(e -> circlePane.shrink()); // using Lambda expression
        BorderPane borderPane = new BorderPane();
        borderPane.setCenter(circlePane);
        borderPane.setBottom(hBox);
        hBox.setAlignment(Pos.CENTER);
        Scene scene = new Scene(borderPane, 200, 150);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    class EnlargeHandler implements EventHandler<ActionEvent> {
        @Override
        public void handle(ActionEvent e) {
            circlePane.enlarge();
        }
    }
}

class CirclePane extends StackPane {
    private Circle circle = new Circle(50);

    public CirclePane() {
        getChildren().add(circle);
        circle.setStroke(Color.BLACK);
        circle.setFill(Color.WHITE);
    }
    public void enlarge() {
        circle.setRadius(circle.getRadius() + 2);
    }
    public void shrink() {
        circle.setRadius(circle.getRadius() > 2
                ? circle.getRadius() - 2 : circle.getRadius());
    }
}