code: public Set<String> keys() {
          Set<String> list = new LinkedHashSet<>();
          Enumeration<String> keys = bundle.getKeys();
          while (keys.hasMoreElements()) {
              list.add(keys.nextElement());
          }
          return list;
      }
docs: Gets the keys contained in the bundle.
  
  @return the keys. At least one key is returned as empty bundle
  was rejected.