java - Java BigDecimal.remainder() 的问题 - 不合理且不准确的返回

Question

我正在制定找零返回计划，这个看似无法解决的问题确实令人沮丧。

我正在使用 BigDecimal，因此我可以处理精确的结果，但我的九个 BigDecimal.remainder() 方法中有两个返回无理且不准确的值(其他七个工作正常)，这使我的最终结果不准确。

例如，如果输入是 70.70 和 100，则输出将是额外的 1 个 5 美分硬币。

我已经在代码中突出显示了该问题。非常感谢任何帮助。预先感谢您。

import java.math.BigDecimal;
import java.math.*;
import java.util.Scanner;

public class SP010Main {

    public static void main(String[] args) {

        // Declaring the values of notes and coins
        BigDecimal fifty = new BigDecimal(50.0);
        BigDecimal twenty = new BigDecimal(20.0);
        BigDecimal ten = new BigDecimal(10.0);
        BigDecimal five = new BigDecimal(5.0);
        BigDecimal two = new BigDecimal(2.0);
        BigDecimal one = new BigDecimal(1.0);
        BigDecimal fiftyC = new BigDecimal(0.5);
        BigDecimal twentyC = new BigDecimal(0.2);
        BigDecimal tenC = new BigDecimal(0.1);
        BigDecimal fiveC = new BigDecimal(0.05);


        // Getting input of cost and $ received
        Scanner scanner = new Scanner(System.in);
        System.out.print("Cost: ");
        BigDecimal cost = scanner.nextBigDecimal();
        System.out.print("$ Received: ");
        BigDecimal received = scanner.nextBigDecimal();
        BigDecimal totalC = received.subtract(cost);
        System.out.println("Total change returned: " + totalC);


        // Checking how many of each value is needed
        BigDecimal fiftyI = (totalC.divide(fifty, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(fifty);
        BigDecimal twentyI = (totalC.divide(twenty, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(twenty);
        BigDecimal tenI = (totalC.divide(ten, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(ten);
        BigDecimal fiveI = (totalC.divide(five, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(five);
        BigDecimal twoI = (totalC.divide(two, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(two);
        BigDecimal oneI = (totalC.divide(one, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(one);
        BigDecimal fiftyCI = (totalC.divide(fiftyC, 0, RoundingMode.FLOOR));
        totalC = totalC.remainder(fiftyC);

        // What should be happening with the problem----------------------------
        // E.g. if input is 70.70 and 100,
        // Following line will return 0.30
        System.out.println(totalC);
        // ---------------------------------------------------------------------


        BigDecimal twentyCI = (totalC.divide(twentyC, 0, RoundingMode.FLOOR));


        // The problem ---------------------------------------------------------
        // E.g. if input is 70.70 and 100,
        // Following outputs will both be 0.0999999999999999888977697537484345
        // 95763683319091796875
        totalC = totalC.remainder(twentyC);
        System.out.println(totalC);
        BigDecimal tenCI = (totalC.divide(tenC, RoundingMode.FLOOR));
        totalC = totalC.remainder(tenC);
        System.out.println(totalC);
        // End of the problem --------------------------------------------------    


        BigDecimal fiveCI = (totalC.divide(fiveC, 0, RoundingMode.FLOOR));

        // Display output
        System.out.printf("$50: %.0f \n$20: %.0f \n$10: %.0f \n$5: %.0f \n$2: %.0f \n$1: %.0f \n$0.50: %.0f \n$0.20: %.0f \n$0.10: %.0f \n$0.05: %.0f \n",fiftyI, twentyI, tenI, fiveI, twoI, oneI, fiftyCI, twentyCI, tenCI, fiveCI);

    }

}

Answer 1

即使您使用 BigDecimal 对象来获得精确的结果，在声明代表钞票和硬币的对象时也会引入不精确性:

BigDecimal fifty = new BigDecimal(50.0);
BigDecimal twenty = new BigDecimal(20.0);
// ...

传递给构造函数的值被解释为 double ，但其中一些值无法以 64 位 double 格式准确捕获。

您应该使用基于String的构造函数:

BigDecimal fifty = new BigDecimal("50.0");
BigDecimal twenty = new BigDecimal("20.0");
// ...

这将为您提供正确的输出:

Cost: 70.70
$ Received: 100
Total change returned: 29.30
0.30
0.10
0.00
$50: 0 
$20: 1 
$10: 0 
$5: 1 
$2: 2 
$1: 0 
$0.50: 0 
$0.20: 1 
$0.10: 1 
$0.05: 0