ceil 方法上有这么一段注释:If the argument value is less than zero but
greater than -1.0, then the result is negative zero
如果参数小于0且大于-1.0,结果为 -0
Math.floor(d1)
ceil 和 floor 方法 上都有一句话:If the argument is NaN or an infinity
or positive zero or negative zero, then the result is the same as the argument,意思为:如果参数是 NaN、无穷、正 0、负 0,那么结果与参数相同,
至于为什么是-0.0,而不是0.0,因为源码注释:“If the argument value is less than
zero but greater than -1.0, then the result is negative zero”,翻译过来就是:如果参数是-1.0到0.0之间的数,结果是-0.0。