Are there any caveats of creating a FastMoney/Money from a double?

Question

I have seen there are issues when creating BigDecimal from double.

Although FastMoney doesn't use BigDecimal (as opposed to Money), I am not sure how either of them will behave when I crate them from a double value.

Is creating a FastMoney/Money instance from a double not recommended? Should I always try to create them from String?

score 1 · Accepted Answer · answered Dec 19 '18 at 23:47

1

double and especially float types are dangerous. This numbers are base-2 numeral system then it's not possible to represent 0.24F directly as the same it's not possible to represent 1/3 in decimal system without recurring decimal period i.e. 1/3=0.3333... or 0.(3).

So the float number 0.24F when printed back to decimal representation is shown as 0.23 with a change due to rounding:

println(0.24F) => 0.23999999463558197021484375

while 0.25F can be shown directly:

println(0.25F) => 0.25

So answering to your question: it depends. For 0.25, 0.5, 0.75 it's ok to use double.

But the FastMoney class uses not a floating-point arithmetic but a fixed-point arithmetic

answered Dec 19 '18 at 23:47

Sergey Ponomarev

2,947
1
33
43

Yes, it uses fixed point arithmetic... But there's a constructor from `double`, so I am not sure how it will behave if I construct him from a `double` with `FastMoney.of(doubleCalculationResult, currency)` – Michel Feinstein Dec 20 '18 at 00:26
As I remember it will autobox to Double and then cast it as a Number – Sergey Ponomarev Dec 20 '18 at 00:38
And are there any side effects? Or recommendations? – Michel Feinstein Dec 20 '18 at 00:39
No any side effects since Double is already implements the Number. The only problem is the value inside of the double in floating point. – Sergey Ponomarev Dec 20 '18 at 00:41
Watch out for a range especially for FastMoney and roundings – Sergey Ponomarev Dec 20 '18 at 00:44
Could you elaborate more on range and roundings? – Michel Feinstein Dec 20 '18 at 01:32

Are there any caveats of creating a FastMoney/Money from a double?

1 Answers1