code: @Override
    public void setOwner(Path p, String username, String groupname
        ) throws IOException {
      if (username == null && groupname == null) {
        throw new IOException("username == null && groupname == null");
      }
  
      if (username == null) {
        execCommand(pathToFile(p), Shell.SET_GROUP_COMMAND, groupname); 
      } else {
        //OWNER[:[GROUP]]
        String s = username + (groupname == null? "": ":" + groupname);
        execCommand(pathToFile(p), Shell.SET_OWNER_COMMAND, s);
      }
    }
docs: Use the command chown to set owner.