HashSet vs. TreeSet

ORDER

• HashSet stores in random order.
• TreeSet stores in natural order.

NULL VALUES

• HashSet allows null
• TreeSet will throw NullPointerException if you store null object

SPEED

• HashSet is much faster than TreeSet

PERFORMANCE

• HashSet has constant time performance for basic operation
  (add / remove / contains / size)
• TreeSet has log(n) time cost for basic operation

FUNCTIONALITY

• TreeSet is more rich than HashSet - pollFirst(), pollLast(), first(), last(), ceiling(), lower() methods