从HashSet获得的流过滤器的时间复杂度是多少？

Question

Set<Integer> set = new HashSet<>();
set.add(1);
set.add(2);
...

what is the time complexity of below operation? O(n) or O(1)?

set.stream().filter(e -> e == 1).findFirst();

Answer 1

You can understand it better if you see it from another side, your solution is same like this :

for(Integer i : set){
    if(i == 1){
        break;
    }
}

So it is O(n) because it loop over all the set, and check one by one, if the condition is correct return the value else continue until n elsement

Answer 2

It's O(n) - after you create a Stream instance, you are no longer operating on a HashSet , but on a Stream that goes over all elements of the source Set .

It's a lazy linear sequence where all elements are visited one by one - hence O(n)