不要使用double来构造一个BigDcimal对象。BigDecimal的构造函数有这么一段说明:
The results of this constructor can be somewhat unpredictable. One might assume that writing in Java creates a 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 (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 constructor, on the other hand, is perfectly predictable: writing creates a which is exactly equal to 0.1, as one would expect. Therefore, it is generally recommended that the String constructor be used in preference to this one.由于double本身是不精确的，如果使用double作为构造函数参数，也会导致BigDecimal对象不精确，比如使用浮点数0.1来构造一个BigDecimal对象，它的实际值为 0.1000000000000000055511151231257827021181583404541015625，所以，需要精确计算的场景，推荐使用String类型的构造函数。
总之，在需要精确浮点数计算的场景，不要在任何地方使用double,float类型的变量，应该使用String类型来创建BigDecimal.
