Question

I have a simple Java game where you fire a bullet up at a moving target. Both objects are GRects. I have collision detection that checks when the ufo and bullet intersect each other, but here is the weirdness:

This works:

private void collideWithUFO() { 
    if (bullet != null) {
        GObject collObj = getElementAt(ufo.getX(), ufo.getY()); 
        if (collObj == bullet) {
            remove(ufo); 
            remove(bullet); 
            ufo = null; 
            bullet = null;
        }
     }
}

..but if I change the getElementAt to bullet like below, and check with respect to ufo, it fails to detect collisions:

private void collideWithUFO() { 
    if (bullet != null) {
        GObject collObj = getElementAt(bullet.getX(), bullet.getY()); 
        if (collObj == ufo) {
            remove(ufo); 
            remove(bullet); 
            ufo = null; 
            bullet = null;
        }
     }
 }

It should be irrelevant whether I choose ufo or bullet first, but apparently it isn’t. Now here is something that’s even WEIRDER. If I change the bullet from GRect to GOval, all of a sudden the second form of collision detection works. I am a Java noob so please let me know if this behavior makes any sense.

Answer 1

public GObject getElementAt(double x, double y)

Returns the topmost graphical object that contains the point (x, y), or null if no such object exists.

So getElementAt(bullet.getX(), bullet.getY()) will only give you the ufo if the ufo is the topmost graphical element, otherwise you will get the bullet.