java - Is it possible that TreeSet equals HashSet but not HashSet equals TreeSet

Question

I had a interview today and the person taking my interview puzzled me with his statement asking if it possible that TreeSet equals HashSet but not HashSet equals TreeSet. I said "no" but according to him the answer is "yes".

How is it even possible?

Answer 1

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 HashSet uses equals.

Quoting TreeSet:

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.