Using BigDecimal to work with currencies

Question

I was trying to make my own class for currencies using longs  but apparently I should use BigDecimal instead  Could someone help me get started  What would be the best way to use BigDecimals for dollar currencies  like making it at least but no more than 2 decimal places for the cents  etc  The API for BigDecimal is huge  and I don t know which methods to use  Also  BigDecimal has better precision  but isn t that all lost if it passes through a double  if I do new BigDecimal 24 99   how will it be different than using a double  Or should I use the constructor that uses a String instead

User · Answer

Or  wait for JSR-354  Java Money and Currency API coming soon

User · Answer

There is an extensive example of how to do this on javapractices com  See in particular the Money class  which is meant to make monetary calculations simpler than using BigDecimal directly    The design of this Money class is intended to make expressions more natural  For example   if   amount lt hundred       cost   amount times price        The WEB4J tool has a similar class  called Decimal  which is a bit more polished than the Money class

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Primitive numeric types are useful for storing single values in memory   But when dealing with calculation using double and float types  there is a problems with the rounding It happens because memory representation doesn t map exactly to the value   For example  a double value is supposed to take 64 bits but Java doesn t use all 64 bits It only stores what it thinks the important parts of the number  So you can arrive to the wrong values when you adding values together of the float or double type    Please see a short clip https   youtu be EXxUSz9x7BM

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1  If you are limited to the double precision  one reason to use BigDecimals is to realize operations with the BigDecimals created from the doubles   2  The BigDecimal consists of an arbitrary precision integer unscaled value and a non-negative 32-bit integer scale  while the double wraps a value of the primitive type double in an object  An object of type Double contains a single field whose type is double  3  It should make no difference  You should have no difficulties with the   and precision  One way to do it is using System out printf

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NumberFormat getNumberInstance java util Locale US  format num

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I would recommend a little research on Money Pattern  Martin Fowler in his book Analysis pattern has covered this in more detail   public class Money        private static final Currency USD   Currency getInstance  USD        private static final RoundingMode DEFAULT ROUNDING   RoundingMode HALF EVEN       private final BigDecimal amount      private final Currency currency          public static Money dollars BigDecimal amount            return new Money amount  USD              Money BigDecimal amount  Currency currency            this amount  currency  DEFAULT ROUNDING              Money BigDecimal amount  Currency currency  RoundingMode rounding            this currency   currency                this amount   amount setScale currency getDefaultFractionDigits    rounding              public BigDecimal getAmount             return amount             public Currency getCurrency             return currency              Override     public String toString             return getCurrency   getSymbol           getAmount               public String toString Locale locale            return getCurrency   getSymbol locale          getAmount                 Coming to the usage   You would represent all monies using Money object as opposed to BigDecimal  Representing money as big decimal will mean that you will have the to format the money every where you display it  Just imagine if the display standard changes  You will have to make the edits all over the place  Instead using the Money pattern you centralize the formatting of money to a single location   Money price   Money dollars 38 28   System out println price

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Here are a few hints    Use BigDecimal for computations if you need the precision that it offers  Money values often need this   Use the NumberFormat class for display  This class will take care of localization issues for amounts in different currencies  However  it will take in only primitives  therefore  if you can accept the small change in accuracy due to transformation to a double  you could use this class  When using the NumberFormat class  use the scale   method on the BigDecimal instance to set the precision and the rounding method    PS  In case you were wondering  BigDecimal is always better than double  when you have to represent money values in Java   PPS   Creating BigDecimal instances  This is fairly simple since BigDecimal provides constructors to take in primitive values  and String objects  You could use those  preferably the one taking the String object  For example   BigDecimal modelVal   new BigDecimal  24 455    BigDecimal displayVal   modelVal setScale 2  RoundingMode HALF EVEN     Displaying BigDecimal instances  You could use the setMinimumFractionDigits and setMaximumFractionDigits method calls to restrict the amount of data being displayed   NumberFormat usdCostFormat   NumberFormat getCurrencyInstance Locale US   usdCostFormat setMinimumFractionDigits  1    usdCostFormat setMaximumFractionDigits  2    System out println  usdCostFormat format displayVal doubleValue

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I would be radical  No BigDecimal   Here is a great article  https   lemnik wordpress com 2011 03 25 bigdecimal-and-your-money   Ideas from here   import java math BigDecimal   public class Main        public static void main String   args            testConstructors            testEqualsAndCompare            testArithmetic               private static void testEqualsAndCompare             final BigDecimal zero   new BigDecimal  0 0            final BigDecimal zerozero   new BigDecimal  0 00             boolean zerosAreEqual   zero equals zerozero           boolean zerosAreEqual2   zerozero equals zero            System out println  zerosAreEqual      zerosAreEqual         zerosAreEqual2            int zerosCompare   zero compareTo zerozero           int zerosCompare2   zerozero compareTo zero           System out println  zerosCompare      zerosCompare         zerosCompare2              private static void testArithmetic             try               BigDecimal value   new BigDecimal 1               value   value divide new BigDecimal 3                System out println value             catch  ArithmeticException e                System out println  Failed to devide      e getMessage                          private static void testConstructors             double doubleValue   35 7          BigDecimal fromDouble   new BigDecimal doubleValue           BigDecimal fromString   new BigDecimal  35 7             boolean decimalsEqual   fromDouble equals fromString           boolean decimalsEqual2   fromString equals fromDouble            System out println  From double      fromDouble           System out println  decimalsEqual      decimalsEqual         decimalsEqual2             It prints  From double  35 7000000000000028421709430404007434844970703125 decimalsEqual  false false zerosAreEqual  false false zerosCompare  0 0 Failed to devide  Non-terminating decimal expansion  no exact representable decimal result    How about storing BigDecimal into a database  Hell  it also stores as a double value    At least  if I use mongoDb without any advanced configuration it will store BigDecimal TEN as 1E1   Possible solutions   I came with one - use String to store BigDecimal in Java as a String into the database  You have validation  for example  NotNull   Min 10   etc    Then you can use a trigger on update or save to check if current string is a number you need  There are no triggers for mongo though  Is there a built-in way for Mongodb trigger function calls   There is one drawback I am having fun around - BigDecimal as String in Swagger defenition  I need to generate swagger  so our front-end team understands that I pass them a number presented as a String  DateTime for example presented as a String   There is another cool solution I read in the article above    Use long to store precise numbers      A standard long value can store the current value of the Unites States national debt  as cents  not dollars  6477 times without any overflow  Whats more  it   s an integer type  not a floating point  This makes it easier and accurate to work with  and a guaranteed behavior       Update  https   stackoverflow com a 27978223 4587961  Maybe in the future MongoDb will add support for BigDecimal  https   jira mongodb org browse SERVER-1393 3 3 8 seems to have this done   It is an example of the second approach  Use scaling  http   www technology-ebay de the-teams mobile-de blog mapping-bigdecimals-with-morphia-for-mongodb html

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Use BigDecimal setScale 2  BigDecimal ROUND HALF UP  when you want to round up to the 2 decimal points for cents  Be aware of rounding off error when you do calculations though  You need to be consistent when you will be doing the rounding of money value  Either do the rounding right at the end just once after all calculations are done  or apply rounding to each value before doing any calculations  Which one to use would depend on your business requirement  but generally  I think doing rounding right at the end seems to make a better sense to me   Use a String when you construct BigDecimal for money value  If you use double  it will have a trailing floating point values at the end  This is due to computer architecture regarding how double float values are represented in binary format

[java] Using BigDecimal to work with currencies

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