Check nbo decimal values of double

Question

Hi there!
I need some advice on a topic:
What is the best practice to check the number of decimal places, double values have (e.g. two decimal places are allowed, when there are more an exception has to be thrown)?
I did some research, but could not find a truely satisfying solution.
Hope you can help!
greetz

Answer 1

YoungWinston wrote:
JoachimSauer wrote:
You could try to get the String representation using String.valueOf() and check how many characters it contains after the decimal point. If the double value is really the result of the parsing of that String value, then the result should not have more digits than the original input...Are you sure that every possible value created by a string with up to 2 decimal places will convert back to the same string Joachim? It's possible, but I've never seen any confirmation of it.I'm not 100% sure, but I seem to remember some documentation to that effect. I was just too lazy to look it up ... I'm looking into it now.
I think I found it:
[Double.toString(double)|http://java.sun.com/javase/6/docs/api/java/lang/Double.html#toString(double)] says:
How many digits must be printed for the fractional part of m or a? There must be at least one digit to represent the fractional part, and beyond that as many, but only as many, more digits as are needed to uniquely distinguish the argument value from adjacent values of type double.(emphasis mine)
If my understanding is correct, then this rule ensures that the shortest possible String literal that parses into the same double value must be produced. So if the String literal "0.2" parses to the double value that you have, then Double.toString() must not produce 0.20, because that's more digits than would be necessary.
Of course this approach breaks down if you exceed the possible precision of the double (i.e. if you have 15 digits before the decimal point, then anything after the decimal point will be lost/effectively random).