docs: public int length(Object obj) {
          if (isArray(obj)) {
              return ((List) obj).size();
          } else if (isMap(obj)){
              return getPropertyKeys(obj).size();
          } else if(obj instanceof String){
              return ((String)obj).length();
          }
          throw new JsonPathException("length operation cannot be applied to " + obj!=null?obj.getClass().getName():"null");
      }