### Question:
SingleValueMapper { public synchronized void put(K k, V v) { Reject.ifNull(k, v); K previousValue = reverse.get(v); if (previousValue != null) { map.remove(previousValue); } map.put(k, v); reverse.put(v, k); }  V get(K k); K getValue(V v); synchronized void put(K k, V v); synchronized V remove(K k);  }### Answer:
@Test (expectedExceptions = NullPointerException.class) public void shouldNotAllowNullToBeStored() { map.put(null, null); }