Main differences between a HashSet and TreeSet are:

1. Ordering: In a HashSet elements are stored in a random

order. In a TreeSet, elements are stored according to

natural ordering.

2. Null Value Element: We can store null value object in a

HashSet. A TreeSet does not allow to add a null value

object.

3. Performance: HashSet performs basic operations like

add(), remove(), contains(), size() etc in a constant size

time. A TreeSet performs these operations at the order of

log(n) time.

4. Speed: A HashSet is better than a TreeSet in performance

for most of operations like add(), remove(), contains(),

size() etc .

5. Internal Structure: a HashMap in Java internally backs a

HashSet. A NavigableMap backs a TreeSet internally.

6. Features: A TreeSet has more features compared to a

HashSet. It has methods like pollFirst(), pollLast(), first(),

last(), ceiling(), lower() etc.

7. Element Comparison: A HashSet uses equals() method for

comparison. A TreeSet uses compareTo() method for

comparison to maintain ordering of elements.

https://byjus.com/gate/difference-between-hashset-and-treeset-in-java/#:~:text=Hashset%20is%20the%20execution%20of,and%20is%20backed%20by%20TreeMap.