code: public void concat(String trg, String[] srcs, boolean restricted)
        throws IOException {
      checkOpen();
      clearFileStatusCache();
  
      try {
        if (namenodeProtocolProxy != null
            && namenodeProtocolProxy.isMethodSupported("concat", String.class,
                String[].class, boolean.class)) {
          namenode.concat(trg, srcs, restricted);
        } else if (!restricted){
          throw new UnsupportedOperationException(
              "Namenode does not support variable length blocks");
        } else {
          namenode.concat(trg, srcs);
        }
      } catch(RemoteException re) {
        throw re.unwrapRemoteException(AccessControlException.class,
                                       NSQuotaExceededException.class,
                                       DSQuotaExceededException.class);
      }
    }
docs: Move blocks from src to trg and delete src
  See {@link ClientProtocol#concat(String, String [])}.