code: public static File findFile(File rootDir, final String fileName) {
  
          FilenameFilter filter = new FilenameFilter() {
  
              @Override
              public boolean accept(File dir, String name) {
                  return name.equals(fileName);
              }
          };
  
          return findFile(rootDir, filter);
      }
docs: Finds a file with the given name in the given root directory or any subdirectory. The files
  and directories are scanned in alphabetical order, so the result is deterministic.
  <p>
  The method returns the first matching result, if any, and ignores all other matches.
  
  @param rootDir
  root directory
  @param fileName
  exact file name, without any wildcards
  @return matching file, or null