在循环中使用迭代器而不会导致任何异常

Question

I used iterator class to solve a question. I thought that iterator() method of HashSet returns the collection of its iterator. But whenever I ran this code(especially the 5th line) it occurs ConcurrentModificationException, even though I've checked the iterator has next elements.

Sorry for my bad English, but I really want to find out what's wrong with my code and fix the problems.

for (int i = 0; i < N; i++) {
    int input = scan.nextInt();
    iterator = set.iterator();
    while (iterator.hasNext()) {
        int num = iterator.next();
        if (!set.contains(num + input)) set.add(num + input);
    }
}

Answer 1

Store the Adds in a local Set/List.

after iterating overall, just add them all

Set toAdd = new HashSet();
for (int i = 0; i < N; i++) {
    int input = scan.nextInt();
    iterator = set.iterator();
    toAdd.clear();
    while (iterator.hasNext()) {
        int num = iterator.next();
        if (!set.contains(num + input)) toAdd.add(num + input);
    }
    set.addAll(toAdd);
}