// 如果参数值总是等于某个整数,那么结果与该参数相同
Math.floor(1)=1.0
// 如果参数是NaN、无穷、正0、负0,那么结果与参数相同
Math.floor(Double.NaN)=Double.NaN
Math.floor(Double.POSITIVE_INFINITY)=Double.POSITIVE_INFINITY
Math.floor(Double.NEGATIVE_INFINITY)=Double.NEGATIVE_INFINITY
Math.floor(0.0)=0.0