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<p>一提到Java里面的商业计算，我们都知道不能用float和double，因为他们无法进行精确计算。但是Java的设计者给编程人员提供了一个很有用的类BigDecimal，他可以完善float和double类无法进行精确计算的缺憾。BigDecimal类位于java.maths类包下。首先我们来看下如何构造一个BigDecimal对象。它的构造函数很多，我挑最常用的两个来演示一下：一个就是BigDecimal(double val)，另一个就是BigDecimal(String str)。这两个看上去没什么太大区别，但是正像API描述中说的那样： </p> <p>view plaincopy to clipboardprint? <br>/*The results of this constructor can be somewhat unpredictable. One might assume that <br>new BigDecimal(.1) is exactly equal to .1, but it is actually equal <br>to .1000000000000000055511151231257827021181583404541015625. This is so because .1 <br>cannot be represented exactly as a double (or, for that matter, as a binary fraction <br>of any finite length). Thus, the long value that is being passed in to the constructor <br>is not exactly equal to .1, appearances nonwithstanding. <br>The (String) constructor, on the other hand, is perfectly predictable: new BigDecimal <br>(".1") is exactly equal to .1, as one would expect. Therefore, it is generally <br>recommended that the (String) constructor be used in preference to this one.*/ <br>/*The results of this constructor can be somewhat unpredictable. One might assume that <br>new BigDecimal(.1) is exactly equal to .1, but it is actually equal <br>to .1000000000000000055511151231257827021181583404541015625. This is so because .1 <br>cannot be represented exactly as a double (or, for that matter, as a binary fraction <br>of any finite length). Thus, the long value that is being passed in to the constructor <br>is not exactly equal to .1, appearances nonwithstanding. <br>The (String) constructor, on the other hand, is perfectly predictable: new BigDecimal <br>(".1") is exactly equal to .1, as one would expect. Therefore, it is generally <br>recommended that the (String) constructor be used in preference to this one.*/ </p> <p>也就是说利用double作为参数的构造函数，无法精确构造一个BigDecimal对象，需要自己指定一个上下文的环境，也就是指定精确位。而利用String对象作为参数传入的构造函数能精确的构造出一个BigDecimal对象。请看下面的代码： </p> <p>view plaincopy to clipboardprint? <br>import java.math.*; <br>public class TestBigDecimal { <br> public static void main(String args[]){ <br> BigDecimal bd = new BigDecimal("10.123"); <br> BigDecimal bd1 = new BigDecimal(10.123); <br> System.out.println(bd +"/n"+ bd1); <br> } <br>} <br>import java.math.*; </p> <p>public class TestBigDecimal { <br> public static void main(String args[]){ <br> BigDecimal bd = new BigDecimal("10.123"); <br> BigDecimal bd1 = new BigDecimal(10.123);</p> <p> System.out.println(bd +"/n"+ bd1); </p> <p> } <br>} </p> <p>输出： </p> <p>view plaincopy to clipboardprint? <br>10.123 <br>10.1229999999999993320898283855058252811431884765625 <br>10.123 <br>10.1229999999999993320898283855058252811431884765625 </p> <p>所以我们在选择构造函数时，要看具体需求而定。 </p> <p>另外，很多人会问到怎么将基本类型，如int，float，double，long，和BigDecimal对象相互转换。很简单： </p> <p>基本类型通过构造函数转换成对应的BigDecimal对象，而BigDecimal类提供了诸如intValue(), floatValue(), doubleValue(), longValue()方法来将BigDecimal对象转换成对应的值。 </p> <p>关于BigDecimal是如何计算的，我以论坛中一个人的提问帖子为例，来简单的写出BigDecimal的运算方法。题目是：李白无事街上走，提壶去买酒。遇店加一倍，见花喝一斗，五遇花和店，喝光壶中酒，试问李白壶中原有多少斗酒？ </p> <p>这道题应该是从后往前推，并且要逆运算，最后得出原有酒的体积。 </p> <p>view plaincopy to clipboardprint? <br>import java.math.*; <br>public class Libai { <br> public static void main(String args[]){ <br> BigDecimal volumn = new BigDecimal("0"); <br> for (int i=0; i<5; i++){ <br> volumn = volumn.add(new BigDecimal("1")); <br> volumn = volumn.divide(new BigDecimal("2")); <br> } <br> System.out.print(volumn); <br> } <br>} <br>import java.math.*; </p> <p>public class Libai { <br> public static void main(String args[]){ <br> BigDecimal volumn = new BigDecimal("0"); <br> for (int i=0; i<5; i++){ <br> volumn = volumn.add(new BigDecimal("1")); <br> volumn = volumn.divide(new BigDecimal("2")); <br> } <br> System.out.print(volumn); <br> } <br>} </p> <p>结果： </p> <p>view plaincopy to clipboardprint? <br>0.96875 </p>转载地址：http://zdaji.baihongyu.com/