Revise Listing 18.9 to develop a program that lets the user use the + and ? buttons to increase or decrease the current order by 1, as shown in Figure 18.12a. The initial order is 0. If the current order is 0, the Decrease button is ignored.

Listing

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(a) 1 import javafx.application. Application; 2 import javafx.geometry.Point2D; 3 import javafx.geometry. Pos; 4 import javafx.scene.Scene; 5 import javafx.scene.control.Label; 6 import javafx.scene.contro1.TextField; 7 import javafx.scene.layout.BorderPane; 8 import javafx.scene.layout. HBox; 9 import javafx.scene.layout.Pane; |10 import javafx.scene.paint.Color; |11 import javafx.scene.shape. Polygon; |12 import javafx.stage.Stage; |13 14 public class SierpinskiTriangle extends Application { 15 @Override // Override the start method in the Application class 16 public void start(Stage primaryStage) { SierpinskiTrianglePane trianglePane = new SierpinskiTrianglePane (); TextField tforder = new TextField(); tfOrder.setOnAction( 17 18 19