code: private JsonToken startParsing() throws IOException {
      JsonToken currentToken = getCurrentToken();
      // token is null at start, so get next token
      if (currentToken == null) {
        currentToken = nextToken();
      }
      Preconditions.checkArgument(currentToken != null, "no JSON input found");
      return currentToken;
    }
docs: Starts parsing that handles start of input by calling {@link #nextToken()}.