docs: protected void putToFieldCache(Object key, Object value)
      {
          if (key != null)
          {
              if (fkCache == null)
              {
                  fkCache = new ReferenceIdentityMap (ReferenceIdentityMap.WEAK, ReferenceIdentityMap.HARD, true);
              }
              if (value != null)
                   fkCache.put(key, value);
               else
                   fkCache.remove (key);
          }
      }