Java Math.round()函数小结

Math类中提供了三个与取整有关的方法：ceil,floor,round,这些方法的作用于它们的英文名称的含义相对应，例如：ceil的英文意义是天花板，该方法就表示向上取整，Math.ceil（11.3）的结果为12，Math.ceil(-11.6)的结果为-11；floor的英文是地板，该方法就表示向下取整，Math.floor(11.6)的结果是11，Math.floor(-11.4)的结果-12；最难掌握的是round方法，他表示“四舍五入”，算法为Math.floor(x+0.5),即将原来的数字加上0.5后再向下取整，所以，Math.round(11.5)的结果是12，Math.round(-11.5)的结果为-11.Math.round( )符合这样的规律：小数点后大于5全部加，等于5正数加，小于5全不加。

让我们看看JDK的说明：  
(1)public static long round(double a)  
returns the closest long to the argument. the result is rounded to an integer by adding 1/2, taking the floor of the result, and casting the result to type long. in other words, the result is equal to the value of the expression:    
    
  (long)math.floor(a  +  0.5d)

(2)public static double floor(double a)  
  returns the largest(closest to positive infinity) double value that is not greater than the argument and is equal to a mathematical integer.special cases:
  if the argument value is already equal to a mathematical integer, then the result is the same as the argument.    
  if the argument is nan or an infinity or positive zero or negative zero, then the result is the same as the argument.  
    
  parameters:  
  a - a value.    
  returns:  
  the smallest (closest to negative infinity) floating-point value that is not less than the argument and is equal to a mathematical integer.

//import java.math.*;

public class RoundTest {

public static void main(String[] args) {

// TODO Auto-generated method stub

// Math.round():Java中的四舍五入函数

System.out.println("Case1：小数点后第一位 = 5");

System.out.println("正数：Math.round(11.5) = " + Math.round(11.5));

System.out.println("负数：Math.round(-11.5) = " + Math.round(-11.5));

System.out.println("Case2：小数点后第一位 < 5");

System.out.println("正数：Math.round(11.49) = " + Math.round(11.49));

System.out.println("负数：Math.round(-11.49) = " + Math.round(-11.49));

System.out.println("Case3：小数点后第一位 > 5");

System.out.println("正数：Math.round(11.69) = " + Math.round(11.69));

System.out.println("负数：Math.round(-11.69) = " + Math.round(-11.69));

System.out.println("结论：正数小数点后大于5则进位；负数小数点后小于以及等于5都舍去，大于5的则进位");

System.out.println("也就是说：小数点后大于5全部加，等于5正数加，小于5全不加");

}

}

Parameters

d	the value to be rounded.

Returns

• the closest integer to the argument.