Double in HashMap

Question

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 als

Answer 1

Short answer: It probably won't work.

Honest answer: It all depends.

Longer answer: The hash code isn't the issue, it's the nature of equal comparisons on floating point. As Nalandial and the commenters on his post point out, ultimately any match against a hash table still ends up using equals to pick the right value.

So the question is, are your doubles generated in such a way that you know that equals really means equals? If you read or compute a value, store it in the hash table, and then later read or compute the value using exactly the same computation, then Double.equals will work. But otherwise it's unreliable: 1.2 + 2.3 does not necessarily equal 3.5, it might equal 3.4999995 or whatever. (Not a real example, I just made that up, but that's the sort of thing that happens.) You can compare floats and doubles reasonably reliably for less or greater, but not for equals.