code: public String get(String key) throws EtcdClientException {
          Preconditions.checkNotNull(key, "key can't be null");
  
          URI uri = buildUriWithKeyAndParams(key, null);
          HttpGet httpGet = new HttpGet(uri);
  
          EtcdResult result = syncExecute(httpGet, new int[] {200, 404}, 100);
          if (result.isError()) {
              if (result.errorCode == 100) {
                  return null;
              }
          }
  
          return result.node != null ? result.node.value : null;
      }
docs: Get the value of a key.
  @param key the key
  @return the corresponding value