I had a interview today and the person taking my interview puzzled me with his statement asking if it possible that
HashSet but not
TreeSet. I said “no” but according to him the answer is “yes”.
How is it even possible?
Your interviewer is right, they do not hold equivalence relation for some specific cases. It is possible that
TreeSet can be equal to
HashSet and not vice-versa. Here is an example:
TreeSet<String> treeSet = new TreeSet<>(String.CASE_INSENSITIVE_ORDER); HashSet<String> hashSet = new HashSet<>(); treeSet.addAll(List.of("A", "b")); hashSet.addAll(List.of("A", "B")); System.out.println(hashSet.equals(treeSet)); // false System.out.println(treeSet.equals(hashSet)); // true
The reason for this is that a
TreeSet uses comparator to determine if an element is duplicate while
Note that the ordering maintained by a set (whether or not an explicit comparator is provided) must be consistent with equals if it is to correctly implement the Set interface.