JsonUtil.getObject() etc behaves differently between explicit null value and absence of the key

[dapengzhang0]

(Noticed by @sergiitk in #8871 (comment)) The current implementation of JsonUtil.getObject() will throw ClassCastException if the input contains the key with an explicit null value.

  /**
   * Gets an object from an object for the given key.  If the key is not present, this returns null.
   * If the value is not a Map, throws an exception.
   */
  @SuppressWarnings("unchecked")
  @Nullable
  public static Map<String, ?> getObject(Map<String, ?> obj, String key) {
    assert key != null;
    if (!obj.containsKey(key)) {
      return null;
    }
    Object value = obj.get(key);
    if (!(value instanceof Map)) {
      throw new ClassCastException(
          String.format("value '%s' for key '%s' in '%s' is not object", value, key, obj));
    }
    return (Map<String, ?>) value;
  }

[sergiitk]

A unit test that confirms current behavior: #8881

[ejona86]

What is the problem here? Failing seems appropriate. If a key is set it needs to have the right type and null isn't the right type. There seems to be no purpose to allowing the values to be set to null and to treat that as not present.