code: private String normalizePath(File root, File file) {
          String relativePath = file.getPath().substring(root.getPath().length() + 1);
          String path = relativePath.replaceAll("\\" + File.separator, "/");
          return path;
      }
docs: Returns the relative path of the given file with respect to the root directory, with all file
  separators replaced by slashes.
  
  Example: For root {@code C:\work} and file {@code C:\work\com\example\Foo.class}, the result
  is {@code com/example/Foo.class}
  
  @param root
  root directory
  @param file
  relative path
  @return normalized file path