If the x and y coordinates are available, you can get a more "exact" arctangent through a function like this. It might be helpful to implement as a built-in function so others don't have to code it.

I think the atan2() function is the same thing, but why have a different name?

I'd like to put in a request for acos(x, y) and asin(x, y), too. What about sec(), csc(), and cot()? I think it would be best to define these if for nothing other than code readability. (Java's math library probably does these already...)

Code:

float atan(float tx, float ty) {
  float theta = abs(atan( ty / tx ));
  
  if(tx < 0 && ty > 0) // Quadrant II
    theta = PI - theta;
  else if(tx < 0 && ty < 0) // Quadrant III
    theta = PI + theta;
  else if(tx > 0 && ty < 0) // Quadrant IV
    theta = TWO_PI - theta;
    
  if(ty == 0 && tx > 0)
    theta = 0;
  else if(tx == 0 && ty > 0)
    theta = PI / 2;
  else if(tx == 0 && ty < 0)
    theta = (3.0 / 2.0) * PI;
  else if(ty == 0 && tx < 0)
    theta = PI;
    
  return theta;
}