Should I use Double as keys in a TreeMap?

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.

Answer 2

The problem with doubles in tree maps is exactly the same as it is with doubles in hash map - comparing for equality. If you avoid calls of treeMap.get(myDouble) and stay with range queries instead (e.g. by using submap) you should be fine.

TreeMap<Double,String> tm = new TreeMap<Double,String>();
tm.put(1.203, "quick");
tm.put(1.231, "brown");
tm.put(1.233, "fox");
tm.put(1.213, "jumps");
tm.put(1.243, "over");
tm.put(1.2301, "the");
tm.put(1.2203, "lazy");
tm.put(1.2003, "dog");
for (Map.Entry<Double,String> e : tm.subMap(1.230, 1.232).entrySet()) {
    System.out.println(e);
}

This prints

1.2301=the
1.231=brown

See this snippet on ideone.