iterators

----------------------
| fail fast iterator |
----------------------
* The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
* Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
* wird über ein internes status feld realisiert, bspw ArrayList mit int modCount

    void failFastIterator() {
        List<String> l = new ArrayList<>();
        l.add("a");
        for (String s : l) { // ConcurrentModificationException beim zweiten Aufruf
            l.add(s);
        }
    }

----------------------
| fail safe iterator |
----------------------
* es fliegt keine exception
* iteratoren arbeiten auf einer kopie
* diese kopie muss nicht aktuell sein

klappt: gibt [a,a] aus
    void failSafeIterator() {
        List<String> l = new CopyOnWriteArrayList<>();
        l.add("a");
        for (String s : l) {
            l.add(s);
        }
        System.out.println(l);
    }


------------------------------
| weakly consistent iterator |
------------------------------
Iterators returned by the collection views are weakly consistent: 
they will never throw ConcurrentModificationException 
and they may or may not show the effects of any modifications to the map 
that occur while the iteration is in progress.

funktioniert, ausgabe [a, aa]
    void weaklyConsistentIterator() {
        ConcurrentSkipListSet<String> set = new ConcurrentSkipListSet<>();
        set.add("a");
        for (String s : set) {
            set.add(s + s);
        }
        System.out.println(set);
    }