コード匂い111 -横断している間、コレクションを修正すること

TL;DR: Do not modify collections while traversing them

Collection<Integer> people = new ArrayList<>();
//here we add elements to the collection...

for (Object person : people) {
    if (condition(person)) {
        people.remove(person);
    }
}
//We iterate AND remove elements 

Collection<Integer> people = new ArrayList<>();
//here we add elements to the collection...

List<Object> iterationPeople = ImmutableList.copyOf(people);

for (Object person : iterationPeople) {
    if (condition(person)) {
        people.remove(person);
    }
}
//We iterate a copy and remove from original

coll.removeIf(currentIndex -> currentIndex == 5);
//Or use language tools (if available)