Answer may be:

Set believes in solitary. It does not allow duplicate values. The existing value inserted again will display an error. There are three types of sets that exist in Java: 

• Hash set: It is unsorted and unordered. It makes use of hash code to insert the values. 
• Linked Hash set: It is similar to the hash set but with ordered property. It keeps the doubly linked list of the elements.
• Tree set: It contains elements either in ascending and descending order.