I am learning about containers in Java and I've read recently that HashSet doesn't give the elements in order. What's interesting for Integer my randomly made HashSet is sorted. When I changed its type to Double the printed HashSet was sorted no more. My question is: does HashSet work differently for various types then?
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1It depends on the hash codes. – SLaks Nov 29 '16 at 15:52
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Do not rely on the order of a hashset. Anything you find is purely coincidental by dint of whatever code & hashing function is implemented under the hood. – CollinD Nov 29 '16 at 15:52
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@CollinD I just made a for loop and put 10 000 elements in the HashSet using random.nextInt(); – shurrok Nov 29 '16 at 15:55
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I'm sorry, I don't understand how that is related to what I said. – CollinD Nov 29 '16 at 15:56
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I mean, i don't understand why hashing function works differently for Integer and Double? – shurrok Nov 29 '16 at 15:58
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The docs for `HashSet` indicates it is backed by a `HashMap`. The entire public contract of the `HashMap` can be read at https://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html – CollinD Nov 29 '16 at 16:01
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HashSet uses a HashMap internally. HashMap stores its elements in a hash table using each Object's hashCode() method.
For int and double, these are auto-boxed into the Integer and Double classes. When you make a HashSet of ints, it uses Integer's hashCode() method, which just returns the int. So if you add ints, they get stored sorted. But for double, Double's hashCode() method is much more complicated, because of the way doubles are represented in memory.
Adam Millerchip
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When hashing double, Java converts it to long since they both are simply 64-bit values in memory.
You can actually experiment the hashcode to see how it works for integer and double.
Check this for more details: Hash a double in Java
