Question

As described in the answer to Double in HashMap, Doubles shouldn’t be used in HashMaps because they are difficult to compare for equality. I believe my case is different, but I thought I’d ask to make sure since I didn’t see anything about this.

I’m going to have a series of double values associated with objects, and I want them to be sorted by the double values. Is TreeMap an appropriate solution? Would there be a better one? The double values are generated a bunch of math, so the likelihood of a duplicate value is extremely low.

EDIT: I should clarify: all I need is to have this list of objects sorted by the doubles they’re associated with. The values of the doubles will be discarded and I’ll never call map.get(key)

Answer 1

Doubles shouldn’t be used in HashMaps because they are difficult to compare for equality.

• Will you ever try to get the values based on certain keys?

  • If yes, then the reasoning about “difficult to compare” applies and you should probably avoid such data structure (or always rely on tailMap / headMap / submap and fetch ranges of the map).

  • If no (i.e. you’ll typically just do for (Double key : map.keySet()) ... or iterate over the entrySet) then I would say you’re fine using Double as keys.

The double values are generated a bunch of math, so the likelihood of a duplicate value is extremely low.

• Is it a bug if you actually do get a duplicate?

  • If yes then it’s not the right data structure to use. You could for instance use a Multimap from Guava instead.

  • If no, (i.e. it doesn’t matter which of the two values it maps to, because they can only differ by a small epsilon anyway) then you should be fine.