插曲：BigDecimal

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参考文章链接：http://www.opentaps.org/docs/index.php/How_to_Use_Java_BigDecimal:_A_Tutorial

*************************************************************************以下是对该技术文章的总结***********************************************************************************

简略介绍下BigDecimal 作用：能够在程序运算中控制：运算数、运算结果的小数精度；按何种规则舍入运算数。

BigDecimal example：

一、 BigDecimal 构造示例

1、double参数的BigDecimal例子：

bd = new BigDecimal(1.5); bd.toString(); // => 0.1499999999999999944488848768742172978818416595458984375

2、String入参例子：

bd = new BigDecimal("1.5");

3、以下例子中，设置它的舍入位数：

 <pre name="code" class="java">bd = new BigDecimal(1.5); // is actually 1.4999.... bd.setScale(1); // throws ArithmeticException

但是很不幸，抛异常。原因：他不知道该按照怎样的舍入规则。利用方法：.setScale(scale, roundingMode),就阔以了。

bd = new BigDecimal(1.5); // is actually 1.4999.... bd.setScale(1, RoundingMode.HALF_UP);

二、介绍8中舍入规则

ROUND_CEILING: Ceiling function

                 0.333  ->   0.34                 -0.333  ->  -0.33

ROUND_DOWN: Round towards zero

                 0.333  ->   0.33                 -0.333  ->  -0.33

ROUND_FLOOR: Floor function

                 0.333  ->   0.33                -0.333  ->  -0.34

ROUND_HALF_UP: Round up if decimal >= .5

                 0.5  ->  1.0                 0.4  ->  0.0

ROUND_HALF_DOWN: Round up if decimal > .5

                 0.5  ->  0.0                 0.6  ->  1.0

ROUND_HALF_EVEN: 前提是舍入数字为5，它左边是偶数就向下舍入；如果为奇数，向上舍入。可自行做测试！

 a = new BigDecimal("2.5"); // digit left of 5 is even, so round down b = new BigDecimal("1.5"); // digit left of 5 is odd, so round up a.setScale(0, BigDecimal.ROUND_HALF_EVEN).toString() // => 2 b.setScale(0, BigDecimal.ROUND_HALF_EVEN).toString() // => 2

ROUND_UNNECESSARY:

Use ROUND_UNNECESSARY when you need to use one of the methods that requires input of a rounding mode but you know the result won't need to be rounded.

文章提了下 bigdecimal 中的除法运算，详情：

When dividing BigDecimals, be careful to specify the rounding in the .divide(...) method. Otherwise, you could run into an ArithmeticException if there is no precisely rounded resulting value, such as 1/3. Thus, you should always do:

  a = b.divide(c, decimals, rounding);

就是说，如果两数相除，如果最终的结果是个无线循环的小数，不要忘记告诉程序要舍入位数以及该使用何种舍入规则。

三、使用bigdecimal 注意点（对于bigdecimal做基本运算时，都会产生一个新的对象，这个新的对象里才包含运算结果！）：

So how do we do math then? The methods .add(), .multiply(), and so on all return a new BD value containing the result. For example, when you want to keep a running total of the order amount,

 amount = amount.add( thisAmount );

Make sure you don't do this,

 amount.add( thisAmount );

THIS IS THE MOST COMMON MISTAKE MADE WITH BIGDECIMALS!

四、两个bigdecimal比较

 BigDecimal a = new BigDecimal("2.00"); BigDecimal b = new BigDecimal("2.0"); print(a.equals(b)); // false

Instead, we should use the .compareTo() and .signum() methods.

 a.compareTo(b);  // returns (-1 if a < b), (0 if a == b), (1 if a > b) a.signum(); // returns (-1 if a < 0), (0 if a == 0), (1 if a > 0)

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