1.Math.ceil() 向上取整
System.out.println(Math.ceil(3.4));
//输出4
System.out.println(Math.ceil(3.7));
//输出4
System.out.println(Math.ceil(-3.4));
//输出-3
System.out.println(Math.ceil(-3.7));
//输出-3
2.Math.floor()向下取整
System.out.println(Math.floor(3.4));
//输出3
System.out.println(Math.floor(3.7));
//输出3
System.out.println(Math.floor(-3.4));
//输出-4
System.out.println(Math.floor(-3.7));
//输出-4
3.Math.round() 四舍五入【特别注意：负数的五入是向大的数入】
System.out.println(Math.round(3.4));
//输出3
System.out.println(Math.round(3.7));
//输出4
System.out.println(Math.round(-3.4));
//输出-3
System.out.println(Math.floor(-3.7));
//输出-4
System.out.println(Math.floor(-3.5));
//输出-3
4.补充：int() 去零，不管后面大于5还是小于5，都要去掉
System.out.println(Math.int(3.4));
//输出3
System.out.println(Math.int(3.7));
//输出3
System.out.println(Math.int(-3.4));
//输出-3
System.out.println(Math.int(-3.7));
//输出-3