code: private void setStartClientFields(HandshakerReq.Builder req) {
      // Sets the default values.
      StartClientHandshakeReq.Builder startClientReq =
          StartClientHandshakeReq.newBuilder()
              .setHandshakeSecurityProtocol(HandshakeProtocol.ALTS)
              .addApplicationProtocols(APPLICATION_PROTOCOL)
              .addRecordProtocols(RECORD_PROTOCOL);
      // Sets handshaker options.
      if (handshakerOptions.getRpcProtocolVersions() != null) {
        startClientReq.setRpcVersions(handshakerOptions.getRpcProtocolVersions());
      }
      if (handshakerOptions instanceof AltsClientOptions) {
        AltsClientOptions clientOptions = (AltsClientOptions) handshakerOptions;
        if (!Strings.isNullOrEmpty(clientOptions.getTargetName())) {
          startClientReq.setTargetName(clientOptions.getTargetName());
        }
        for (String serviceAccount : clientOptions.getTargetServiceAccounts()) {
          startClientReq.addTargetIdentitiesBuilder().setServiceAccount(serviceAccount);
        }
      }
      req.setClientStart(startClientReq);
    }
docs: Sets the start client fields for the passed handshake request.