docs: @Override
      public boolean hasNext(int startPosition, int contentLength) {
          char ch = nextGoodChar();
          if (ch == ',') return true;
          if (ch == '}' || ch == ']' || ch == 0) return false;
          backChar(ch); // { [ 交由 readObjectB 或 readMapB 或 readArrayB 读取
          return true;
      }