Calculating Gravity in Java with 2 Bodies

[Echilon]

I'm pulling my hair out with this problem. I have two bodies and I'm trying to calculate gravity between them. This is for a game, and there are actually three bodies currently, each frame, I check the distance each one moves due to gravity and store it in a cache (to simulate gravity acting on all objects simultaneously), then apply each vector to the body. This is the code I have:

public void calculateGravity(GravitationalBody planet) {
	if(isFixed || gravityIndex.contains(planet.hashCode()) || planet.getGravityIndex().contains(this.hashCode())) {
	    return;
	}
	double radiusGravity = this.getCenter().distance(planet.getCenter());
	double force = (Constants.G * this.mass * planet.mass)/(radiusGravity*radiusGravity); // F=GMm/r^2
	double frameRateMs = (Constants.FrameRate/1000.0);
	double gravityAngle = Math.atan2(Math.max(location.y,planet.location.y) - Math.min(location.y,planet.location.y),
	                                 Math.max(location.x,planet.location.x) - Math.min(location.x,planet.location.x)) + (Math.PI/2);
	double gravityDistance = ((force*frameRateMs)/this.mass)*Constants.SpriteGravityMultiplier;
	if(gravityAngle < 0){ // keep angle positive
	    gravityAngle += MaxAngle;
	}
	//        System.out.println("--"+name+"<->"+planet.getName()+"-"+gravityDistance+"px @ "+Math.round(gravityAngle)+"rad");
	if(planet.IsFixed()) { // gravity acts only on this body, pulling it towards the planet
	    gravityCache.add(new Vector2D(gravityAngle, gravityDistance));
	    gravityIndex.add(planet.hashCode());
	} else { // gravity acts on both, pulling them toward each other
	    double resultantGravAngle = (gravityAngle/2),
	           resultantGravDistance = (gravityDistance /2);
	    gravityCache.add(new Vector2D(resultantGravAngle, resultantGravDistance));
	    gravityIndex.add(planet.hashCode());
	    planet.getGravityCache().add(new Vector2D(resultantGravAngle, resultantGravDistance));
	    planet.getGravityIndex().add(planet.hashCode());
	}
}
Assuming Constants.Framerate = 30, Constants.G = 6.667, Constants.SpriteGravityMultiplier = 1e1

The angles are off though, and I'm convinced it has something to do with the coordinate system Java uses for drawing but I can't get it to work properly. Is there anything obvious I've been overlooking?

 

[Coin]

1. What do you mean by "the angles are off"?
2. Why do you do this thing with subtracting the min from the max, when you calculate gravityAngle? Maybe I just don't understand but it seems like that ought to introduce error to which quadrant your angle winds up in.

 

[Echilon]

Gravity doesn't behave in the way you'd expect, it causes things to move round in seemingly random directions which don't appear to follow a pattern.

The subtracting thing was my way of trying to keep the angle positive. Ie: between 0 and 2pi and avoiding -1pi or 7pi, but I've since found it's easier to just do:

gravityAngle = gravityAngle % MaxAngle;

 

[mathmate]

The angles are off

Could you describe what is "off". Upside down, or random, or a constant difference?

Java plots with the x-axis to the right, and y-axis towards the bottom.
You may need to reverse the y-axis if you want to plot it like on a cartesian plane.

 

[Echilon]

They seem to be randomly offset, or at least I can't see a pattern. I've followed suggestions and made some changes to the gravity methods, which I've posted at http://pastebin.com/m5e53fe34 for the syntax highlighting. Maybe if you watch what's happening it would become apparent, so I've uploaded the current game at http://mi6.nu/ml.zip. It's just an executable jar.

Thanks again for any insight. :)