How to make a rectangle tangent to a circle

chaimite :

I am trying to make a rectangle object to go around a circle, with the rectangle always tangent to the circle where it is going around. I have the code that will make it go around the circle, but I don't see how to make it tangent to it. This is how it looks so far.

I am using an animation timer, since I don't know the full path that the rectangle will follow as it can change if I it finds something blocking it. I can make the rectangle go around the circle in a smooth way, but I don't know how to make the rectangle tangent to it.

 public void moveInCircle(double radius)
   {

  double newX = getX() + (radius * Math.cos(Math.toDegrees(angle)));
  double newY = getY() + (radius * Math.sin(Math.toDegrees(angle)));
  vehicle.setTranslateX(newX);
  vehicle.setTranslateY(newY);

   }

I know that the tangent will be the adjacent side(x) divided by the opposite side (y), but I don't understand how to incorporate it.

fabian :

I recommend using a Rotate transform. This way you only need to set the initial position and the pivot point and can restrict the updates to the Rotate.angle property.

The following example uses a Timeline to animate the property, but this could easily be done from the moveCircle method by using rotate.setAngle(angleDegrees);:

@Override
public void start(Stage primaryStage) {
    Pane root = new Pane();
    root.setMinSize(500, 500);

    final double radius = 150;
    final double centerX = 250;
    final double centerY = 250;
    final double height = 40;

    Circle circle = new Circle(centerX, centerY, radius, null);
    circle.setStroke(Color.BLACK);

    // rect starts at the rightmost point of the circle touching it with the left midpoint
    Rectangle rect = new Rectangle(centerX + radius, centerY - height / 2, 10, height);
    rect.setFill(Color.RED);

    Rotate rotate = new Rotate(0, centerX, centerY); // pivot point matches center of circle

    rect.getTransforms().add(rotate);

    // animate one rotation per 5 sec 
    Timeline animation = new Timeline(
            new KeyFrame(Duration.ZERO, new KeyValue(rotate.angleProperty(), 0d)),
            new KeyFrame(Duration.seconds(5), new KeyValue(rotate.angleProperty(), 360d)));
    animation.setCycleCount(Animation.INDEFINITE);
    animation.play();

    root.getChildren().addAll(circle, rect);

    Scene scene = new Scene(root);
    primaryStage.setScene(scene);
    primaryStage.show();
}

---

Btw: the following part of your code seems odd

double newX = getX() + (radius * Math.cos(Math.toDegrees(angle)));
double newY = getY() + (radius * Math.sin(Math.toDegrees(angle)));

Math.sin and Math.cos expect radians, not degree. You either need to use toRadians or don't require a conversion...