BigDecimal初始化不要用double类型

Posted 2022-09-26 expiator

在进行单价、总价相关的计算时，就会用到BigDecimal。
在初始化时，一个不小心，就可能给自己挖坑。

示例如下：

public class BigDecimalInitTest 
    public static void main(String[] args) 
        BigDecimal amount1=new BigDecimal("0.06");
        BigDecimal amount2=new BigDecimal(0.06);
        System.out.println(amount1);
        System.out.println(amount2);
    

运行之后，结果为：

0.06
0.059999999999999997779553950749686919152736663818359375

源码注释

打开BigDecimal的构造方法，可以发现:

   /**
     * Translates a @code double into a @code BigDecimal which
     * is the exact decimal representation of the @code double's
     * binary floating-point value.  The scale of the returned
     * @code BigDecimal is the smallest value such that
     * <tt>(10<sup>scale</sup> &times; val)</tt> is an integer.
     * <p>
     * <b>Notes:</b>
     * <ol>
     * <li>
     * The results of this constructor can be somewhat unpredictable.
     * One might assume that writing @code new BigDecimal(0.1) in
     * Java creates a @code 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
     * @code double (or, for that matter, as a binary fraction of
     * any finite length).  Thus, the value that is being passed
     * <i>in</i> to the constructor is not exactly equal to 0.1,
     * appearances notwithstanding.
     *
     * <li>
     * The @code String constructor, on the other hand, is
     * perfectly predictable: writing @code new BigDecimal("0.1")
     * creates a @code BigDecimal which is <i>exactly</i> equal to
     * 0.1, as one would expect.  Therefore, it is generally
     * recommended that the @linkplain #BigDecimal(String)
     * <tt>String</tt> constructor be used in preference to this one.
     *
     * <li>
     * When a @code double must be used as a source for a
     * @code BigDecimal, note that this constructor provides an
     * exact conversion; it does not give the same result as
     * converting the @code double to a @code String using the
     * @link Double#toString(double) method and then using the
     * @link #BigDecimal(String) constructor.  To get that result,
     * use the @code static @link #valueOf(double) method.
     * </ol>
     *
     * @param val @code double value to be converted to
     *        @code BigDecimal.
     * @throws NumberFormatException if @code val is infinite or NaN.
     */
    public BigDecimal(double val) 
        this(val,MathContext.UNLIMITED);
    

大体意思就是，BigDecimal(double val)这个构造方法有时是无法精确预料的，
传入0.1，有可能变成0.1000000000000000055511151231257827021181583404541015625。
因为double类型无法精确地存储0.1

IDEA编码提示

IDEA也会在编码时给出提示。

结论

通过double类型初始化的BigDecimal类型，是不精确的。
最好用String类型的数值字符串来初始化。