code: protected Object calculateItem(Object obj){
          if(obj == null){
              return null;
          }
          if(obj instanceof Number){
              return obj;
          }
          if(obj instanceof Boolean){
              return obj;
          }
          if(obj instanceof String){
              return obj;
          }
          if(obj instanceof Elobj){
              return ((Elobj) obj).fetchVal();
          }
          if(obj instanceof Operator){
              return ((Operator) obj).calculate();
          }
          throw new ElException("未知计算类型!" + obj);
          
      }
docs: 计算子项