利用javaFX实现移动一个小球的示例代码

题目：编写一个程序，在面板上移动小球。应该定义一个面板类来显示小球，并提供向上下左右移动小球的方法。请进行边界检查以防止小球移动到视线之外。

问题：我写的程序可以运行但是无法显示小球的移动，如果将移动改为改变小球颜色则可以显示，检查许久也检查不到问题在哪，所以贴上来问问大佬们，问题出在哪里？应该如何改？

代码如下：

public class MoveBall_3 extends Application{
private CirclePane circlePane = new CirclePane(250,250);
  public static void main(String[] args) {
    Application.launch(args);
  }
public void start(Stage primaryStage) throws Exception {
  Button bt1 = new Button("Left");
  Button bt2 = new Button("Right");
  Button bt3 = new Button("Up");
  Button bt4 = new Button("Down");

bt1.setOnAction(new EventHandler<ActionEvent>(){
 @Override
 public void handle(ActionEvent event) {
 // TODO 自动生成的方法存根
 circlePane.moveLeft();
 }
});
bt2.setOnAction(new EventHandler<ActionEvent>() {
 @Override
 public void handle(ActionEvent event) {
 // TODO 自动生成的方法存根
 circlePane.moveRight();
 }
});
bt3.setOnAction(new EventHandler<ActionEvent>() {
 @Override
 public void handle(ActionEvent event) {
 // TODO 自动生成的方法存根
 circlePane.moveUp();
 }  
});
bt4.setOnAction(new EventHandler<ActionEvent>() {
 @Override
 public void handle(ActionEvent event) {
 // TODO 自动生成的方法存根
 circlePane.moveDown();
 }
});

FlowPane pane2 = new FlowPane();
pane2.getChildren().addAll(bt1,bt2,bt3,bt4);
circlePane.getChildren().addAll(pane2);

Scene scene = new Scene(circlePane,500,500);
primaryStage.setTitle("MoveBall");
primaryStage.setScene(scene);
primaryStage.show();
}
}
class CirclePane extends StackPane{
private Circle circle = new Circle(250,250,50);
public CirclePane() {
getChildren().add(circle);
circle.setStroke(Color.BLACK);
circle.setFill(Color.WHITE);
}
public CirclePane(double a,double b) {
getChildren().add(circle);
circle.setCenterX(a);
circle.setCenterY(b);
 circle.setStroke(Color.BLACK);
 circle.setFill(Color.WHITE);
}
public void moveLeft() {
 if(circle.getCenterX()-50-15 >= 0) {
  circle.setCenterX(circle.getCenterX()-15);
  circle.setCenterY(circle.getCenterY());
 }  
 else {
  circle.setCenterX(50);
  circle.setCenterY(circle.getCenterY());
 }
}
public void moveRight() {
 if(circle.getCenterX()+50+15 <= 500) {
  circle.setCenterX(circle.getCenterX()+15);
  circle.setCenterY(circle.getCenterY());
 }
 else {
  circle.setCenterX(450);
  circle.setCenterY(circle.getCenterY());
 }
}
public void moveUp() {
 if(circle.getCenterY()-50-15 >= 0) {
  circle.setCenterY(circle.getCenterY()-15);
  circle.setCenterX(circle.getCenterX());
 }  
 else {
  circle.setCenterY(50);
  circle.setCenterX(circle.getCenterX());
 }
}
public void moveDown() {
 if(circle.getCenterY()+50+15 <= 500) {
  circle.setCenterY(circle.getCenterY()+15);
  circle.setCenterX(circle.getCenterX());
 }
 else {
  circle.setCenterY(450);
  circle.setCenterX(circle.getCenterX());
 }
}
}

来源：https://blog.csdn.net/weixin_43461540/article/details/85631033