Double in HashMap

I was thinking of using a Double as the key to a HashMap but I know floating point comparisons are unsafe, that got me thinking. Is the equals method on the Double class also unsafe? If it is then that would mean the hashCode method is probably also incorrect. This would mean that using Double as the key to a HashMap would lead to unpredictable behavior.

Can anyone confirm any of my speculation here?

Solution 1:

Short answer: Don't do it

Long answer: Here is how the key is going to be computed:

The actual key will be a java.lang.Double object, since keys must be objects. Here is its hashCode() method:

public int hashCode() {
  long bits = doubleToLongBits(value);
  return (int)(bits ^ (bits >>> 32));
}

The doubleToLongBits() method basically takes the 8 bytes and represent them as long. So it means that small changes in the computation of double can mean a great deal and you will have key misses.

If you can settle for a given number of points after the dot - multiply by 10^(number of digits after the dot) and convert to int (for example - for 2 digits multiply by 100).

It will be much safer.

Solution 2:

I think you are right. Although the hash of the doubles are ints, the double could mess up the hash. That is why, as Josh Bloch mentions in Effective Java, when you use a double as an input to a hash function, you should use doubleToLongBits(). Similarly, use floatToIntBits for floats.

In particular, to use a double as your hash, following Josh Bloch's recipe, you would do:

public int hashCode() {
  int result = 17;
  long temp = Double.doubleToLongBits(the_double_field);
  result = 37 * result + ((int) (temp ^ (temp >>> 32)));
  return result;
}

This is from Item 8 of Effective Java, "Always override hashCode when you override equals". It can be found in this pdf of the chapter from the book.

Hope this helps.