code: public void putHeadCache(Short key, String value) {
          if (headerCache == null) {
              synchronized (this) {
                  if (headerCache == null) {
                      headerCache = new TwoWayMap<Short, String>();
                  }
              }
          }
          if (headerCache != null && !headerCache.containsKey(key)) {
              headerCache.put(key, value);
          }
      }
docs: Put header cache
  
  @param key   the key
  @param value the value