Java默认bigdecimal初始值_BigDecimal初始化不要用double类型

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

* (10^scale × val) is an integer.

*

* Notes:

*

*

* 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

* in to the constructor is not exactly equal to 0.1,

* appearances notwithstanding.

*

*

* The {@code String} constructor, on the other hand, is

* perfectly predictable: writing {@code new BigDecimal("0.1")}

* creates a {@code BigDecimal} which is exactly equal to

* 0.1, as one would expect. Therefore, it is generally

* recommended that the {@linkplain #BigDecimal(String)

* String constructor} be used in preference to this one.

*

*

* 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.

*

*

* @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。