Issue
I want to use a custom comparator to filter duplicate objects from a HashSet
.
In the Mobile
class, I have defined a non-static nested Comparator
class for comparing. The criterium which is used by this comparator is the id
field of the Mobile
object.
Set<Mobile> treeSet = new TreeSet<>(new Mobile().new Comparator()); // works
Set<Mobile> hashSet = new HashSet<>(new Mobile().new Comparator()); // does not work
Even though it works perfectly for a TreeSet
, Eclipse shows a syntax error when I try this with a HashSet
,
It appears that I need to override the equals
and hashcode
methods in the Mobile
class if I want to use a HashSet
. However, I would prefer to use a comparator instead.
Why is this the case?
Solution
HashSet
is backed by a hashtable, which is an unordered data structure. This is why it has no need for a comparator. On the other hand, TreeSet
is backed by a binary search tree (an ordered structure), and so it needs a comparator.
The correctness of a hashtable relies on the equals
method, while its performance relies on a well-chosen hashCode
. You can read more about it in the Javadoc of HashSet
and the Object.hashCode()
method itself.
Answered By - Marko Topolnik
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