HashMap与ArrayList插入性能的混淆

Question

根据我的理解，哈希表插入为O（1），对于数组列表，插入为O（n），因为对于哈希表，哈希函数会计算哈希码和索引并插入条目，并且每次输入新数组时，数组列表都会进行比较元件。

Answer 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.

Answer 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.