用Java精确地将两个数字相乘 [英] Multiply two numbers precisely in Java

问题描述

我正在寻找一种精确的方法来在Java中将两个浮点数相乘，并且我读到我应该使用BigDecimal，但是它不能按预期工作.我在做什么错了?

I was searching for a precise way to multiply two floating point numbers in Java, and I read that I should use BigDecimal, however it doesn't work as expected. What am I doing wrong?

我的代码:

BigDecimal a = new BigDecimal(3.53);
BigDecimal b = new BigDecimal(3.59);
BigDecimal c = a.multiply(b);

System.out.println(c);

结果:

12.672699999999998796873512674210388041622697702955394242845497954075284496866515837609767913818359375

预期结果:

12.6727

推荐答案

使用

When you use a BigDecimal(double) constructor it cannot be more precise than a double, use the String form instead. Like,

BigDecimal a = new BigDecimal("3.53");
BigDecimal b = new BigDecimal("3.59");
BigDecimal c = a.multiply(b);
System.out.println(c);

哪个输出

12.6727

链接的Javadoc部分说明-

The linked Javadoc says in part -

注意:

1. 此构造函数的结果可能有些不可预测.可能会假定使用Java编写新的BigDecimal(0.1)会创建一个BigDecimal，该BigDecimal完全等于0.1(未缩放的值1，小数位为1)，但实际上等于0.1000000000000000055511151231257827021181583404541015625.这是因为0.1不能精确地表示为double(或者就此而言，可以表示为任何有限长度的二进制分数).因此，尽管出现，传递给构造函数的值并不完全等于0.1.

1. The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.

String构造函数是完全可预测的:如人们所期望的那样，编写new BigDecimal("0.1")会创建一个精确等于0.1的BigDecimal.因此，通常建议字符串构造函数优先于此函数使用.

The String constructor, on the other hand, is perfectly predictable: writing new BigDecimal("0.1") creates a BigDecimal which is exactly equal to 0.1, as one would expect. Therefore, it is generally recommended that the String constructor be used in preference to this one.

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