Java Double 值 = 0.01 更改为 0.009999999999999787

Java Double value = 0.01 changes to 0.009999999999999787(Java Double 值 = 0.01 更改为 0.009999999999999787)

问题描述

可能重复:
为什么不使用 Double 或 Float 来表示货币?

我正在为我的高中课程用 Java 编写一个基本的命令行程序.我们现在只使用变量.它用于计算购买后您找零中任何类型的纸币和硬币的数量.这是我的程序:

I'm writing a basic command-line program in Java for my high school course. We're only working with variables right now. It's used to calculate the amount of bills and coins of whatever type in your change after a purchase. This is my program:

class Assign2c {
    public static void main(String[] args) {
        double cost = 10.990;
        int paid = 20;
        double change = paid - cost;
        int five, toonie, loonies, quarter, dime, nickel, penny;

        five = (int)(change / 5.0);
        change -= five * 5.0;

        toonie = (int)(change / 2.0);
        change -= toonie * 2.0;

        loonies = (int)change;
        change -= loonies;

        quarter = (int)(change / 0.25);
        change -= quarter * 0.25;

        dime = (int)(change / 0.1);
        change -= dime * 0.1;

        nickel = (int)(change / 0.05);
        change -= nickel * 0.05;

        penny = (int)(change * 100);
        change -= penny * 0.01;

        System.out.println("$5   :" + five);
        System.out.println("$2   :" + toonie);
        System.out.println("$1   :" + loonies);
        System.out.println("$0.25:" + quarter);
        System.out.println("$0.10:" + dime);
        System.out.println("$0.05:" + nickel);
        System.out.println("$0.01:" + penny);
    }
}

它应该一切正常，但在最后一步，当剩下 0.01 美元时，便士的数量应该是 1，但实际上是 0.在进入代码并将更改值输出到控制台几分钟后，我已经发现在最后一步当change = 0.01时，它变为0.009999999999999787.为什么会这样?

It should all work but at the last step when there's $0.01 leftover, number of pennies should be 1 but instead, it's 0. After a few minutes of stepping into the code and outputting the change value to the console, I've found out that at the last step when change = 0.01, it changes to 0.009999999999999787. Why is this happening?

推荐答案

使用 double 作为货币是个坏主意，为什么不使用 Double 或 Float 来表示货币?.我建议使用 BigDecimal 或以美分计算.

Using double for currency is a bad idea, Why not use Double or Float to represent currency?. I recommend using BigDecimal or doing every calculation in cents.

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