当 getChildren() 返回一个以 Circle 作为第一个元素的可观察列表时，如何访问 Circle 中的值？

Question

我必须检测两个“球”何时在 javaFX 程序中发生碰撞。每次单击按钮时，都会将一个新球添加到窗格中。我知道 getChildren() 返回一个包含每个球节点的可观察列表，当我用两个圆圈打印列表时，它将打印，例如， 圆[centerX=30.0, centerY=30.0, radius=20.0, fill=0x9ac26780], Circle[centerX=224.0, centerY=92.0, radius=20.0, fill=0x9ac26780]

我的想法是使用嵌套循环将每个球的 (x,y) 坐标与其他球进行比较。如何从每个 Circle 访问 centerX 和 centerY 以便比较它们？ 我试过 getChildren().sublist(0,0);以为我会得到第一个元素的 centerX 值，但这不起作用。我也尝试了 getCenterX，因为 Ball 扩展了 Circle，但这也失败了。感谢您的宝贵时间。

public class Exercise20_05 extends Application {
  @Override // Override the start method in the Application class
   public void start(Stage primaryStage) {

MultipleBallPane ballPane = new MultipleBallPane();
ballPane.setStyle("-fx-border-color: yellow");

Button btAdd = new Button("+");
Button btSubtract = new Button("-");
HBox hBox = new HBox(10);
hBox.getChildren().addAll(btAdd, btSubtract);
hBox.setAlignment(Pos.CENTER);

// Add or remove a ball
btAdd.setOnAction(e -> ballPane.add());
btSubtract.setOnAction(e -> ballPane.subtract());

// Pause and resume animation
ballPane.setOnMousePressed(e -> ballPane.pause());
ballPane.setOnMouseReleased(e -> ballPane.play());

// Use a scroll bar to control animation speed
ScrollBar sbSpeed = new ScrollBar();
sbSpeed.setMax(20);
sbSpeed.setValue(10);
ballPane.rateProperty().bind(sbSpeed.valueProperty());

BorderPane pane = new BorderPane();
pane.setCenter(ballPane);
pane.setTop(sbSpeed);
pane.setBottom(hBox);

// Create a scene and place the pane in the stage
Scene scene = new Scene(pane, 250, 150);
primaryStage.setTitle("MultipleBounceBall"); // Set the stage title
primaryStage.setScene(scene); // Place the scene in the stage
primaryStage.show(); // Display the stage
}

private class MultipleBallPane extends Pane {
private Timeline animation;

public MultipleBallPane() {
  // Create an animation for moving the ball
  animation = new Timeline(
    new KeyFrame(Duration.millis(50), e -> moveBall()));
  animation.setCycleCount(Timeline.INDEFINITE); //animation will play indefinitely
  animation.play(); // Start animation
}

public void add() {
  Color color = new Color(Math.random(), 
    Math.random(), Math.random(), 0.5);
  //creates a new Ball at (30, 30) with a radius of 20
  getChildren().add(new Ball(30, 30, 20, color)); 
  ballCollision();
}

public void subtract() {
  if (getChildren().size() > 0) {
    getChildren().remove(getChildren().size() - 1); 
  }
}

public void play() {
  animation.play();
}

public void pause() {
  animation.pause();
}

public void increaseSpeed() {
  animation.setRate(animation.getRate() + 0.1);
}

public void decreaseSpeed() {
  animation.setRate(
    animation.getRate() > 0 ? animation.getRate() - 0.1 : 0);
}

public DoubleProperty rateProperty() {
    return animation.rateProperty();
}

protected void moveBall() {
  for (Node node: this.getChildren()) {
    Ball ball = (Ball)node;
    // Check boundaries
    if (ball.getCenterX() < ball.getRadius() || 
        ball.getCenterX() > getWidth() - ball.getRadius()) {
      ball.dx *= -1; // Change ball move direction
    }
    if (ball.getCenterY() < ball.getRadius() || 
        ball.getCenterY() > getHeight() - ball.getRadius()) {
      ball.dy *= -1; // Change ball move direction
    }

    // Adjust ball position
    ball.setCenterX(ball.dx + ball.getCenterX());
    ball.setCenterY(ball.dy + ball.getCenterY());

    ballCollision();
  }
}

//check for ball collisions
protected void ballCollision() {
    /*System.out.println(getChildren().size());
    getChildren returns an observableList; this observableList is what
    keeps track of the balls (specifically, the nodes added to ballPane)
    added each time the + button is clicked
    */
    ObservableList ballList = getChildren();
    System.out.println(ballList.get(0));
    //if there are 2 or more balls, check for collision
        if (ballList.size() > 1) {
           //compare each (x,y) coordinate value to every other (x,y) value
            for (int i = 0; i < ballList.size(); i++) {
                for (int k = 0; k < ballList.size(); k++) {
//                    if (ballList.sublist(i,i) < 1) {
//                        
//                    }
                }
            }
        }
    }
}

class Ball extends Circle {
 private double dx = 1, dy = 1;

Ball(double x, double y, double radius, Color color) {
  super(x, y, radius);
  setFill(color); // Set ball color
  }
 }

 /**
  * The main method is only needed for the IDE with limited
  * JavaFX support. Not needed for running from the command line.
  */
 public static void main(String[] args) {
   launch(args);
 }
}

编辑：多亏了几个人，我才能够进行碰撞检查。一个球将被移除，但我得到 ConcurrentModificationException。这是更新的方法：

protected void ballCollision() {
    ObservableList ballList = getChildren();
    //if there are 2 or more balls, check for collision
        if (ballList.size() > 1) {
           //compare each (x,y) coordinate value to every other (x,y) value
            for (int i = 0; i < ballList.size(); i++) {
                for (int k = i + 1; k < ballList.size(); k++) {
                    Circle c1 = (Circle) ballList.get(i);
                    Circle c2 = (Circle) ballList.get(k);

                    if ((c1.getCenterX() <= c2.getCenterX() * 1.10 &&
                        (c1.getCenterX() >= c2.getCenterX()*.90)) &&
                       ((c1.getCenterY() <= c2.getCenterY() * 1.10) && 
                         c1.getCenterY() >= c2.getCenterY() * .90)){

                            ballList.remove(c2);

                    }
                }
            }
        }
    }

最终编辑：感谢 David Wallace 抽出宝贵时间帮助我。问题是我在 moveBall 方法的 for-each 循环中调用了 ballCollision。一旦我把它移到循环之外，它就完美地工作了。

Answer 1

您可以像对待任何其他List 一样对待ObservableList。您可能希望将元素转换为正确的类，如此处所示。使用Math 类的hypot 方法计算中心之间的距离。

for (int first = 0; first < ballList.size(); first++) {
    Ball firstBall = (Ball) ballList.get(first);
    for (int second = first + 1; second < ballList.size(); second++) {
        Ball secondBall = (Ball) ballList.get(second);

        double distanceBetweenCentres = Math.hypot(
            firstBall.getCenterX() - secondBall.getCenterX(), 
            firstBall.getCenterY() - secondBall.getCenterY());

        if (distanceBetweenCentres <= firstBall.getRadius() + secondBall.getRadius()) {
            System.out.println("Collision between ball " + first + " and ball " + second);
        }

    }
}