Double xDistance = deg2rad(x2 - x1);
Double yDistance = deg2rad(y2 - y1);
Double a = Math.sin(xDistance / 2) * Math.sin(xDistance / 2)
		+ Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2))
		* Math.sin(yDistance / 2) * Math.sin(yDistance / 2);
Double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));

double deg2rad(double deg) {
    return (deg * Math.PI / 180.0);
}