code: private Set<String> convertAbsToRelative(Collection<File> files) {
          Set<String> result = new HashSet<String>();
          for (File f : files) {
              String fAbsPath = PathUtils.fixPathString(f); //assumption here that f is a dir, and path will end in File.seperator
              if (fAbsPath.startsWith(rootAbsolutePath)) {
                  String absPath = fAbsPath; // canonicalPath(f);
                  absPath = absPath.substring(rootAbsolutePath.length());
                  if ("\\".equals(File.separator)) {
                      absPath = absPath.replace('\\', '/');
                  }
                  //handle / case. (path will have been the same, so the substring created emptystring)
                  if (absPath.length() == 0) {
                      absPath = "/";
                  }
                  result.add(absPath);
              } else
                  throw new IllegalStateException(fAbsPath + " " + rootAbsolutePath);
          }
          return result;
      }
docs: convert java.io.files into relative paths under the root container.