Java BigDecimal.remainder() 的问题 - 不合理且不准确 returns

Question

我正在修改return程序，这个看似无法解决的问题实在令人沮丧。

我正在使用 BigDecimal，因此我可以使用精确的结果，但是我的九个 BigDecimal.remainder() 方法中有两个是 returning 非理性和不准确的值（其他七个工作正常），这使我的最终结果不准确。

例如，如果输入为 70.70 和 100，则输出将是一个额外的 5 美分硬币。

我已经突出显示了我的代码中的问题。非常感谢任何帮助。提前谢谢你。

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(": %.0f \n: %.0f \n: %.0f \n: %.0f \n: %.0f \n: %.0f \n[=13=].50: %.0f \n[=13=].20: %.0f \n[=13=].10: %.0f \n[=13=].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，您在声明代表钞票和硬币的对象时也会引入不精确：

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双精度值，但其中一些值无法以 64 位双精度格式准确捕获。

您应该改用基于 String 的构造函数：

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

这会给你正确的输出：

Cost: 70.70
$ Received: 100
Total change returned: 29.30
0.30
0.10
0.00
: 0 
: 1 
: 0 
: 1 
: 2 
: 0 
[=12=].50: 0 
[=12=].20: 1 
[=12=].10: 1 
[=12=].05: 0

Answer 2

永远不要使用 BigDecimal(double) 构造函数。正如 javadoc 所说：

The results of this constructor can be somewhat unpredictable.

阅读 javadoc 了解原因。

对于您的代码，您有以下选项：

对整数使用 BigDecimal(int) 构造函数，例如5 但不是 0.50
对任意数量使用 BigDecimal(String) 构造函数，例如"5" 和 "0.50"
如果需要输入为 double 值，请使用 BigDecimal.valueOf(double) 静态方法。

Java BigDecimal.remainder() 的问题 - 不合理且不准确 returns

Problem with Java BigDecimal.remainder() - irrational and inaccurate returns

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