1.HashSet is not repeated and is disordered!
Uniqueness is guaranteed. Repetitive object equals method returns true, the repetitive object returns the same integer hashCode
HashSet is actually a HashMap, but you can only operate this key part of the interface HashMap by the Set,
2.ArrayList is repeatable ordered
Features: high query efficiency, low efficiency deletions lightweight thread safe.
arraylist: data insertion and deletion of poor terms of speed, but in terms of faster random extraction