JAVA中控制double和float的精度

本篇介紹了在JAVA中如何對double或者float的浮點數進行精度計算，在JAVA中提供了多種參數來實現精度的不同控制方式。具體例子如下： 

轉自http://stephen830.iteye.com/blog/260776

/*
 * Created on 2005-6-5
 * Author stephen
 * Email zhoujianqiang AT gmail DOT com
 * CopyRight(C)2005-2008 , All rights reserved.
 */
package com.soft4j.utility;

import java.math.BigDecimal;

/**
 * 與小數位精度(四捨五入等)相關的一些常用工具方法.
 *
 * float/double的精度取值方式分爲以下幾種: <br>
 * java.math.BigDecimal.ROUND_UP <br>
 * java.math.BigDecimal.ROUND_DOWN <br>
 * java.math.BigDecimal.ROUND_CEILING <br>
 * java.math.BigDecimal.ROUND_FLOOR <br>
 * java.math.BigDecimal.ROUND_HALF_UP<br>
 * java.math.BigDecimal.ROUND_HALF_DOWN <br>
 * java.math.BigDecimal.ROUND_HALF_EVEN <br>
 *
 * @author stephen
 * @version 1.0.0
 */
public final class RoundTool {

    /**
     * 對double數據進行取精度.
     * <p>
     * For example: <br>
     * double value = 100.345678; <br>
     * double ret = round(value,4,BigDecimal.ROUND_HALF_UP); <br>
     * ret爲100.3457 <br>
     *
     * @param value
     *            double數據.
     * @param scale
     *            精度位數(保留的小數位數).
     * @param roundingMode
     *            精度取值方式.
     * @return 精度計算後的數據.
     */
    public static double round(double value, int scale, int roundingMode) {
        BigDecimal bd = new BigDecimal(value);
        bd = bd.setScale(scale, roundingMode);
        double d = bd.doubleValue();
        bd = null;
        return d;
    }

    /**
     * 測試用的main方法.
     *
     * @param argc
     *            運行參數.
     */
    public static void main(String[] argc) {
        //下面都以保留2位小數爲例

        //ROUND_UP
        //只要第2位後面存在大於0的小數，則第2位就+1
        System.out.println(round(12.3401,2,BigDecimal.ROUND_UP));//12.35
        System.out.println(round(-12.3401,2,BigDecimal.ROUND_UP));//-12.35
        //ROUND_DOWN
        //與ROUND_UP相反
        //直接捨棄第2位後面的所有小數
        System.out.println(round(12.349,2,BigDecimal.ROUND_DOWN));//12.34
        System.out.println(round(-12.349,2,BigDecimal.ROUND_DOWN));//-12.34
        //ROUND_CEILING
        //如果數字>0 則和ROUND_UP作用一樣
        //如果數字<0 則和ROUND_DOWN作用一樣
        System.out.println(round(12.3401,2,BigDecimal.ROUND_CEILING));//12.35
        System.out.println(round(-12.349,2,BigDecimal.ROUND_CEILING));//-12.34
        //ROUND_FLOOR
        //如果數字>0 則和ROUND_DOWN作用一樣
        //如果數字<0 則和ROUND_UP作用一樣
        System.out.println(round(12.349,2,BigDecimal.ROUND_FLOOR));//12.34
        System.out.println(round(-12.3401,2,BigDecimal.ROUND_FLOOR));//-12.35
        //ROUND_HALF_UP [這種方法最常用]
        //如果第3位數字>=5,則第2位數字+1
        //備註:只看第3位數字的值,不會考慮第3位之後的小數的
        System.out.println(round(12.345,2,BigDecimal.ROUND_HALF_UP));//12.35
        System.out.println(round(12.3449,2,BigDecimal.ROUND_HALF_UP));//12.34
        System.out.println(round(-12.345,2,BigDecimal.ROUND_HALF_UP));//-12.35
        System.out.println(round(-12.3449,2,BigDecimal.ROUND_HALF_UP));//-12.34
        //ROUND_HALF_DOWN
        //如果第3位數字>=5,則做ROUND_UP
        //如果第3位數字<5,則做ROUND_DOWN
        System.out.println(round(12.345,2,BigDecimal.ROUND_HALF_DOWN));//12.35
        System.out.println(round(12.3449,2,BigDecimal.ROUND_HALF_DOWN));//12.34
        System.out.println(round(-12.345,2,BigDecimal.ROUND_HALF_DOWN));//-12.35
        System.out.println(round(-12.3449,2,BigDecimal.ROUND_HALF_DOWN));//-12.34
        //ROUND_HALF_EVEN
        //如果第3位是偶數,則做ROUND_HALF_DOWN
        //如果第3位是奇數,則做ROUND_HALF_UP
        System.out.println(round(12.346,2,BigDecimal.ROUND_HALF_EVEN));//12.35
        System.out.println(round(12.345,2,BigDecimal.ROUND_HALF_EVEN));//12.35
    }
}