HashSet vs. ArrayList

When its comes to the behavior of ArrayList and HashSet they are completely different classes.

• ArrayList Does not validate duplicates.
• get() is O(1)
• contains() is O(n) but you have fully control over the order of the entries. get add contains next remove(0) iterator.remove ArrayList O(1) O(1) O(n) O(1) O(1) O(1)
• Not thread safe and to make it thread safe you have to use Collections.synchronizedList(...)

• HashSet ensures there are no duplicates.
• Gives you an O(1) contains() method but doesn’t preserve order. add contains next notes HashSet O(1) O(1) O(h/n) h is the table
• Not thread safe and to make it thread safe you have to use Collections.synchronizedSet(...)