Java8 can be executed concurrently, but Stream does not change that the original list, can only return a new list, and assign a value to the reference of the original list. But if the list is RandomAccess , that is, the underlying implementation is an array, such as ArrayList , then you can directly use the traditional for loop to traverse, because for RandomAccess List , the time complexity of accessing the array elements through the subscript is O (1), then the time complexity of traversing is O (N), which is an excellent time complexity, and no extra space is used. The space complexity is O (1);
if not, for example, LinkedList , then the time complexity of the corresponding element in the list is obtained by subscript. If O (N), uses the previous method, then the total time complexity will be O (N ^ 2), then it is recommended to create a new List of the same size, and then traverse the original list, to add the value of + 1 for each element to the new List . At this time, the time complexity is O (N), and the space complexity is also O (N). (of course, you can also use Stream to generate a new List )

so I guess the interviewer is not satisfied with your answer because you don't take into account different List types.