BigDecimal从Double错误值？ [英] BigDecimal from Double incorrect value?

问题描述

我试图从一个字符串中做一个BigDecimal。不要问我为什么，我只需要它！这是我的代码：

  Double theDouble = new Double（。3）; 
 System.out.println（The Double：+ theDouble.toString（））; 
 BigDecimal theBigDecimal = new BigDecimal（theDouble）; 
 System.out.println（The Big：+ theBigDecimal.toString（））; 
  

这是我得到的输出？

  The Double：0.3 
 The Big：0.299999999999999988897769753748434595763683319091796875 
  

任何想法？

解决方案

当您创建一个double时，值0.3无法正确表示。你可以从一个没有中间值的字符串创建一个BigDecimal，如

  new BigDecimal（0.3）
  

浮点数表示为二进制分数和指数。因此，有一些数字无法正确表示。在基数10中有类似的问题，数字为1/3，为0.333333333 ..... 1/3的任何十进制数表示不准确。这发生在二进制中的一组不同的分数，0.3是二进制中不精确的集合之一。

I am trying to make a BigDecimal from a string. Don't ask me why, I just need it! This is my code:

Double theDouble = new Double(".3");
System.out.println("The Double: " + theDouble.toString());
BigDecimal theBigDecimal = new BigDecimal(theDouble);
System.out.println("The Big: " + theBigDecimal.toString());

This is the output I get?

The Double: 0.3
The Big: 0.299999999999999988897769753748434595763683319091796875

Any ideas?

解决方案

When you create a double, the value 0.3 cannot be represented exactly. You can create a BigDecimal from a string without the intermediate double, as in

new BigDecimal("0.3")

A floating point number is represented as a binary fraction and an exponent. Therefore there are some number that cannot be represented exactly. There is an analogous problem in base 10 with numbers like 1/3, which is 0.333333333..... Any decimal representation of 1/3 is inexact. This happens to a DIFFERENT set of fractions in binary, and 0.3 is one of the set that is inexact in binary.

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