docs: public static List<File> getMatchingFiles(File root, String filterExpr) {
          if (root == null)
              return Collections.emptyList();
  
          File fileList[];
          FileFilter filter;
  
          if (filterExpr == null)
              filter = new FilesOnly();
          else
              filter = new FilesOnlyFilter(filterExpr);
  
          fileList = root.listFiles(filter);
  
          if (fileList == null)
              return Collections.emptyList();
          else {
              if (fileList.length > 1) {
                  Arrays.sort(fileList, new SortByFileName());
              }
              return Arrays.asList(fileList);
          }
      }