code: public static String isAnyFileMissing(File... _files) {
          if (_files == null) {
              return "null";
          }
          for (File obj : _files) {
              if (obj != null && !obj.exists()) {
                  return obj.toString();
              } else if (obj == null) {
                  return null;
              }
          }
          return null;
      }
docs: Checks if any of the passed in files are non-existing.
  @param _files array of files
  @return the filename of the missing file, otherwise returns null.