1.绝对值

就变个符号

public static double abs(double a) {
    return (a <= 0.0D) ? 0.0D - a : a;
}
 public static float abs(float a) {
        return (a <= 0.0F) ? 0.0F - a : a;
    }
   public static int abs(int a) {
        return (a < 0) ? -a : a;
    }
  @IntrinsicCandidate
    public static long abs(long a) {
        return (a < 0) ? -a : a;
    }

public static void main(String[] args) {
        int i1 = 5;
        int i2 = -5;
        double d1 = 5.88;
        double d2 =-5.88;
        System.out.println("Math.abs(i2) = " + Math.abs(i2));
    }

2.取整

1.向上取整 ceil

按自然顺序进行取整的
注意这里的返回值还是double

double d1 = 5.88;
double d2 =-5.88;
这里的结果是  >=d  也就是大于等于他的最小整数
System.out.println("正数向上取整：Math.ceil(d1) = " + Math.ceil(d1));
System.out.println("负数向上取整：Math.ceil(d2) = " + Math.ceil(d2));
        System.out.println("Math.ceil(5.0) = " + Math.ceil(5.0));
        System.out.println("Math.ceil(5.1) = " + Math.ceil(5.1));

2.向下取整

小于等于他的最大整数

System.out.println("Math.floor(d1) = " + Math.floor(d1));
System.out.println("Math.floor(d2) = " + Math.floor(d2));

3.四舍五入

double d1 = 5.88;
double d2 =-5.88;
double d3 = 5.11;
double d4 = -5.11;
System.out.println("Math.round(d1) = " + Math.round(d1));
System.out.println("Math.round(d2) = " + Math.round(d2));
System.out.println("Math.round(d3) = " + Math.round(d3));
System.out.println("Math.round(d4) = " + Math.round(d4));

4.比大小

static int max(int a, int b) ->求两个数之间的较大值 
static int min(int a, int b) ->求两个数之间的较小值