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

It depends on how you would be using it.

If you're happy with only being able to find the value based on the exact same bit pattern (or potentially an equivalent one, such as +/- 0 and various NaNs) then it might be okay.

In particular, all NaNs would end up being considered equal, but +0 and -0 would be considered different. From the docs for Double.equals:

Note that in most cases, for two instances of class Double, d1 and d2, the value of d1.equals(d2) is true if and only if

d1.doubleValue() == d2.doubleValue() also has the value true. However, there are two exceptions:

• If d1 and d2 both represent Double.NaN, then the equals method returns true, even though Double.NaN==Double.NaN has the value false.
• If d1 represents +0.0 while d2 represents -0.0, or vice versa, the equal test has the value false, even though +0.0==-0.0 has the value true.

This definition allows hash tables to operate properly.

Most likely you're interested in "numbers very close to the key" though, which makes it a lot less viable. In particular if you're going to do one set of calculations to get the key once, then a different set of calculations to get the key the second time, you'll have problems.