### Question:
ColumnIterator implements Iterator<String> { @Override public boolean hasNext() { if (_setIterator != null) { return _setIterator.hasNext(); } else if (_listIterator != null) { return _listIterator.hasNext(); } else { return false; } } @SuppressWarnings("unchecked")  ColumnIterator(Collection<?> columnCollection); @Override boolean hasNext(); @Override String next(); @Override void remove();  }### Answer:
@Test public void test_firewalling() { ColumnIterator iterator = new ColumnIterator(null); assertFalse(iterator.hasNext()); }