问题
From my understanding a hashmap insertion is O(1) and for an arraylist the insertion is O(n) since for the hashmap the hashfunction computes the hashcode and index and inserts the entry and an array list does a comparison every time it enters a new element.
回答1:
Firstly, an operation of complexity O(1) does not always take lesser time than an operation of complexity O(n). O(1) only means that the operation takes a constant time (which could be any value), regardless of the size of the input. O(n) means that the time required for the operation increases linearly with the size of the input. This means that O(1) is theoretically guaranteed to take lesser time than O(n) only when n is infinity.
Now coming to your examples, ArrayList.add() operation runs in amortized constant time, which means although it could take up to O(n) time for a particular iteration, the average complexity spread out over time is O(1). For more information on amortized constant time, refer to this question.
回答2:
ArrayList is faster than the HashMap when you add item at the last of the ArrayList because there is no need to shift the elements in the ArrayList to the right side, you can see the efficiency of the HashMap if you add an item at the front of the ArrayList like this arrayList.add(0, str).
when check this use 1000 as outer loop instead of 100000 otherwise it may hang.
来源：https://stackoverflow.com/questions/30066806/hashmap-vs-arraylist-insertion-performance-confusion