HashSet => 是一種collection，但是只存放唯一值，是把搜尋時間看的很重要的set，用hash方式實作的set，故access time complexity = O(1)
TreeSet => 同上，但是存入的元素都會經過排列，所以速度比HashSet 慢一點
LinkedHashSet => Performance is likely to be just slightly below that of HashSet, due to the added eXPense of maintaining the linked list, with one exception: Iteration over a LinkedHashSet requires time PRoportional to the size of the set, regardless of its capacity. Iteration over a HashSet is likely to be more expensive, requiring time proportional to its capacity.
LinkedHashMap => Performance is likely to be just slightly below that of HashMap, due to the added expense of maintaining the linked list, with one exception: Iteration over the collection-views of a LinkedHashMap requires time proportional to the size of the map, regardless of its capacity. Iteration over a HashMap is likely to be more expensive, requiring time proportional to its capacity.
IdentityHashMap => This has better locality for large tables than does using separate arrays.) For many JRE implementations and Operation mixes, this class will yield better performance than HashMap (which uses chaining rather than linear-probing
