this occurs because the fields are empty, so we are actually multiplying nothing by nothing (or empty set by empty set).
之所以会出现这个问题是由于字段是空的,因此实际上进行的计算是零乘零(或者空集乘空集)。
when you invoke a groovy method that has no parameters, you must call it with an empty set of parentheses, which allows groovy to distinguish between a property access from a method call.