@item_router.get("/")
def get_items(user_id: Optional[str] = None):
    if user_id is None:
        return [{"item_id": "i1", "user_id": "u1"}, {"item_id": "i2", "user_id": "u2"}]
    else:
        return [{"item_id": "i2", "user_id": user_id}]


@item_router.get("/{item_id}")
def get_item(item_id: str, user_id: Optional[str] = None):
    if user_id is None:
        return {"item_id": item_id}
    else:

        List<Restriction> restrictions = new ArrayList<>(r1.size() + r2.size());
        Iterator<Restriction> i1 = r1.iterator();
        Iterator<Restriction> i2 = r2.iterator();
        Restriction res1 = i1.next();
        Restriction res2 = i2.next();

        boolean done = false;
        while (!done) {
            if (res1.getLowerBound() == null
                    || res2.getUpperBound() == null

    // Test & Verify
    Iterator<String> i1 = Iterables.consumingIterable(list).iterator();
    Iterator<String> i2 = Iterables.consumingIterable(list).iterator();

    i1.next();
    assertThrows(ConcurrentModificationException.class, () -> i2.next());
  }

  public void testConsumingIterable_queue_iterator() {
    List<Integer> items = ImmutableList.of(4, 8, 15, 16, 23, 42);