code: public boolean containsKey(Object key) {
          Object k = maskNull(key);
          Object[] tab = table;
          int len = tab.length;
          int i = hash(k, len);
          while (true) {
              Object item = tab[i];
              if (item == k)
                  return true;
              if (item == null)
                  return false;
              i = nextKeyIndex(i, len);
          }
      }
docs: Tests whether the specified object reference is a key in this identity
  hash map.
  
  @param   key   possible key
  @return  <code>true</code> if the specified object reference is a key
  in this map
  @see     #containsValue(Object)