code: public String relativePath(File base, File nested) throws IOException {
          String basePath = base.getCanonicalPath();
          String nestedPath = nested.getCanonicalPath();
          if (nestedPath.equals(basePath)) {
              return "";
          } else if (nestedPath.startsWith(basePath)) {
              return nestedPath.substring(basePath.length() + 1);
          } else {
              return nestedPath;
          }
      }
docs: Returns relative path (without leading '/') if <code>nested</code> is inside <code>base</code>.
  Returns <code>nested</code> (as absolute path) otherwise.
  
  @param base
  @param nested
  @return
  @throws IOException