code: public static Map<String, ?> propsToStringKeyMap(Properties properties) {
      ImmutableMap.Builder<String, Object> mapBuilder = ImmutableMap.builder();
      for (Map.Entry<Object, Object> entry : properties.entrySet()) {
        mapBuilder.put(entry.getKey().toString(), entry.getValue());
      }
      return mapBuilder.build();
    }
docs: Converts a {@link Properties} object to a {@link Map} where each key is a {@link String}.