So I've found this class called Vecter2f in Slick2D and inside of it there is a method called getTheta. I don't know what this does but I got the source to the method if that will help. And what use is this method?
public strictfp double getTheta() {
double theta = StrictMath.toDegrees(StrictMath.atan2(this.y, this.x));
if ((theta < -360.0D) || (theta > 360.0D)) {
theta %= 360.0D;
}
if (theta < 0.0D) {
theta += 360.0D;
}
return theta;
}