Java HashSet vs HashMap

Question

I understand that HashSet is based on HashMap implementation but is used when you need unique set of elements. So why in the next code when putting

Answer 1

I just wanted to add to these great answers, the answer to your last dilemma. You wanted to know what is the difference between these two collections, if they are returning the same size after your insertion. Well, you can't really see the difference here, because you are inserting two values in the map with the same key, and hence changing the first value with the second. You would see the real difference (among the others) should you have inserted the same value in the map, but with the different key. Then, you would see that you can have duplicate values in the map, but you can't have duplicate keys, and in the set you can't have duplicate values. This is the main difference here.