Good hashcode function for 2D coordinates

Question

I would like to use a HashMap to map (x, y) coordinates to values. What is a good hashCode() function definition? In this case, I am only storing integer coordinates of the form (x, y) where y - x = 0, 1, ..., M - 1 for some parameter M.

Answer 1

To calculate a hash code for objects with several properties, often a generic solution is implemented. This implementation uses a constant factor to combine the properties, the value of the factor is a subject of discussions. It seems that a factor of 33 or 397 will often result in a good distribution of hash codes, so they are suited for dictionaries.

This is a small example in C#, though it should be easily adabtable to Java:

public override int GetHashCode()
{
  unchecked // integer overflows are accepted here
  {
    int hashCode = 0;
    hashCode = (hashCode * 397) ^ this.Hue.GetHashCode();
    hashCode = (hashCode * 397) ^ this.Saturation.GetHashCode();
    hashCode = (hashCode * 397) ^ this.Luminance.GetHashCode();
    return hashCode;
  }
}

This scheme should also work for your coordinates, simply replace the properties with the X and Y value. Note that we should prevent integer overflow exceptions, in DotNet this can be achieved by using the unchecked block.