Change an image, according to the situation

Question

I am working on a class project it's a flight radar simulator.

The situation is that when 2 planes are dangerously close the image of both flights changes, and when they are in a safe distance they change again.

The problem is that most of the time only one of those images is changed to red and I have no idea where the problem could be.

This is the method I am using to check the closeness condition, very simple and only checking for distance between them.

private void checkConflicts(ArrayList<Flight> flightsInArea) {


    for (int i = 0; i < flightsInArea.size(); i++) {
        for (int j = i + 1; j < flightsInArea.size(); j++) {

            Coordinate currentFlight1 = flightsInArea.get(i).getCurrentPosition();
            Coordinate currentFlight2 = flightsInArea.get(j).getCurrentPosition();

            double cathetusX = Math.pow((currentFlight1.getPositionX() - currentFlight2.getPositionX()), 2);
            double cathetusY = Math.pow((currentFlight1.getPositionY() - currentFlight2.getPositionY()), 2);

            double distance = Math.sqrt(cathetusX + cathetusY);

            if (distance < 100) {
                flightsInArea.get(i).establishImage(true);
                flightsInArea.get(j).establishImage(true);
            } else {
                flightsInArea.get(i).establishImage(false);
                flightsInArea.get(j).establishImage(false);
            }
        }
    }
}

This is the method use to establish the new image.

public void establishImage(boolean conflict) {
    try {
        if (conflict) {
            image = ImageIO.read(this.getClass().getResource("red_plane.png"));
        } else {
            image = ImageIO.read(this.getClass().getResource("blue_plane.png"));
        }
    }
    catch (IOException ex) {
        Logger.getLogger(Vuelo.class.getName()).log(Level.SEVERE, null, ex);
    }
}

Answer 1

Your algorithm is not sound. Consider the following simple example with 3 planes, where planes 1 and 3 are close, but plane 2 is far from both of them. Your loop does this:

- Check 1 & 2.  Not close, mark both blue
- Check 1 & 3.  Close - mark both red
- Check 2 & 3.  Not close, mark both blue

Now, at the end, planes 2 & 3 will be marked blue, and plane 1 will be marked red, even though plane 3 should be marked red. This is because your algorithm is strictly iterative, and doesn't persist red markings. Here's a potential solution:

private void checkConflicts(ArrayList<Flight> flightsInArea) {
  HashSet<Integer> redFlights = new HashSet<Integer>();

  // Check for red flights
  for (int i = 0; i < flightsInArea.size(); i++) {
    for (int j = i + 1; j < flightsInArea.size(); j++) {
      Coordinate currentFlight1 = flightsInArea.get(i).getCurrentPosition();
      Coordinate currentFlight2 = flightsInArea.get(j).getCurrentPosition();

      double cathetusX = Math.pow((currentFlight1.getPositionX() - currentFlight2.getPositionX()), 2);
      double cathetusY = Math.pow((currentFlight1.getPositionY() - currentFlight2.getPositionY()), 2);

      double distance = Math.sqrt(cathetusX + cathetusY);

      if (distance < 100) {
        redFlights.add(i);
        redFlights.add(j);
      }
    }
  }

  // Mark flights
  for (int i = 0; i < flightsInArea.size(); i++) {
    flightsInArea.get(i).establishImage(redFlights.contains(i));
  }
}