BigDecimal使用注意

最新推荐文章于 2021-05-19 11:30:49 发布

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本文详细探讨了使用Java的BigDecimal类解决浮点数运算中的不精确问题，通过实例展示了如何正确构造BigDecimal对象以获取精确结果，并解释了构造方法中的注意事项。

今天摸着API试验了BigDecimal类，都不是特别理想，实验过程：

浮点数的不精确是广泛存在的。在java中的Math包中有一个BigDecmal类专门用来处理大精度数据。

The BigDecimal class provides operations for arithmetic, scale manipulation, rounding, comparison, hashing, and format conversion. The toString() method provides a canonical representation of a BigDecimal.

double a = 5.899989871234567890987654321;
System.out.println(a); //输出结果：5.899989871234568

BigDecimal bd = new BigDecimal(5.899989871234567890987654321);
System.out.println(bd);//输出结果：5.89998987123456775094609838561154901981353759765625

MathContext mc = new MathContext(28);
BigDecimal bd = new BigDecimal(5.899989871234567890987654321, mc);
System.out.println(bd);//输出结果：5.899989871234567750946098392

百度了下看了下其他人的blog才明白原来要用String作为形参构造，才能得到最精确的结果。如下：

MathContext mc = new MathContext(28);
BigDecimal bd = new BigDecimal("5.899989871234567890987654321", mc);
System.out.println(bd);//输出结果：5.899989871234567890987654321

Note: the results of this constructor can be somewhat unpredictable. One might assume that new BigDecimal(.1) is exactly equal to .1, but it is actually equal to .1000000000000000055511151231257827021181583404541015625. This is so because .1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the long value that is being passed in to the constructor is not exactly equal to .1, appearances nonwithstanding.
The (String) constructor, on the other hand, is perfectly predictable: new BigDecimal(".1") is exactly equal to .1, as one would expect. Therefore, it is generally recommended that the (String) constructor be used in preference to this one.

嗯，note一记！