Do you know Bill Karwin's joke about handling money with FLOAT? No? Then you should come to my talk. Most of the systems we develop deal with monetary amounts, and yet we know so little about money handling intricacies. This can lead to serious bugs costing millions of dollars. I will share my experience with the Java Money standard and talk about developing complex e-commerce and payment systems.

1. How to count money
and not lose it
Piotr Horzycki
www.peterdev.pl | twitter.com/peterdevpl

2. System.out.println(0.10 + 0.20);
0.30000000000000004
Bugs...

3. if (parseInt(amount) > 0) {
/* proceed with payment */
} else {
/* disable payment button */
}
Why does 0.5 not work?
Bugs...

4. How does IEEE-754 work?

6. “A large proportion of the computers in this world
manipulate money, so it's always puzzled me that
money isn't actually a first class data type in any
mainstream programming language. The lack of a
type causes problems, the most obvious surrounding
currencies. (...) The more subtle problem is with
rounding. Monetary calculations are often rounded to
the smallest currency unit. When you do this it's easy to
lose pennies (or your local equivalent) because of
rounding errors.
The good thing about object-oriented programming is
that you can fix these problems by creating a Money
class that handles them. Of course, it's still surprising
that none of the mainstream base class libraries
actually do this.”
https://martinfowler.com/eaaCatalog/money.html

7. JavaMoney “Moneta”: implementation of the Money pattern
(JSR-354)
Money amount1 = Money.of(123.45, "EUR");
Money amount2 = Monetary.getAmountFactory(Money.class)
.setCurrency("EUR")
.setNumber(123.45)
.create();

8. “Monetary values are a key feature of many applications, yet the JDK
provides little or no support. The existing java.util.Currency class is
strictly a structure used for representing current ISO-4217 currencies,
but not associated values or custom currencies. The JDK also provides
no support for monetary arithmetic or currency conversion, nor
for a standard value type to represent a monetary amount.”
https://download.oracle.com/otndocs/jcp/money_currency-1_0-fr-eval-spec/

9. public final class Money implements MonetaryAmount,
Comparable<MonetaryAmount>, Serializable {
/**
* The currency of this amount.
*/
private final CurrencyUnit currency;
/**
* the {@link MonetaryContext} used by this instance, e.g. on division.
*/
private final MonetaryContext monetaryContext;
/**
* The numeric part of this amount.
*/
private final BigDecimal number;
/* ... */
}

10. Money amount1 = Money.of(123.45, "EUR");
Money amount2 = Money.of(43.21, "EUR");
Money amount3 = amount1.add(amount2);
Money amount4 = amount1.subtract(amount2);
Money amount5 = amount1.multiply(1.23);
Money amount6 = amount1.divide(2);
Money amount7 = amount1.remainder(3);
Basic money arithmetics

11. final Money amount1 = Money.of(123.45, "EUR");
final Money amount2 = Money.of(123.45, "USD");
amount1.isEqualTo(amount2); // false
amount1.isGreaterThan(amount2); // MonetaryException
Comparing money objects

12. final Money[] amounts = new Money[] {
Money.of(10, "EUR"),
Money.of(20, "EUR")
};
final MonetarySummaryStatistics statistics =
DefaultMonetarySummaryStatistics
.of(Monetary.getCurrency("EUR"));
Arrays.stream(amounts).forEach(statistics::accept);
System.out.println(statistics.getSum().getNumber());
Statistics: sum, min, max, average...

13. Immutability
final Money jimPrice = Money.of(25.00, "EUR");
final Money hannahPrice = jimPrice;
final Money coupon = Money.of(5, "EUR");
// wrong
jimPrice.subtract(coupon);
jimPrice.isEqualTo(hannahPrice); // true!
// correct
final Money jimDiscountedPrice = jimPrice.subtract(coupon);
jimDiscountedPrice.isLessThan(jimPrice); // true
$jimPrice and $hannahPrice are immutable value objects

14. Database
INT, BIGINT…
VARCHAR...
DECIMAL (MySQL)
NUMERIC (Oracle)
FLOAT, DOUBLE...

15. Database
INT, BIGINT…
VARCHAR...
DECIMAL (MySQL)
NUMERIC (Oracle)
FLOAT, DOUBLE...
DECIMAL(10, 2)
total decimal
digits places
max 99,999,999.99

16. Exchange rates...
https://www.bankofengland.co.uk/boeapps/database/Rates.asp

17. https://en.wikipedia.org/wiki/Redenomination

18. https://www.theguardian.com/world/2018/aug/20/venezuela-bolivars-hyperinflation-banknotes

19. https://www.amusingplanet.com/2018/08/hungarys-hyperinflation-worst-case-of.html
Hungary’s hyperinflation in 1946

20. Converting currencies
final ExchangeRateProvider rateProvider = MonetaryConversions
.getExchangeRateProvider();
final CurrencyConversion conversion = rateProvider
.getCurrencyConversion("CHF");
final Money amountUsd = Money.of(10, "USD");
final Money amountChf = amountUsd.with(conversion);

21. Save the conversion rate!

22. $100
€100
£100
¥100
...
Price formatting

23. $100
€100
£100
¥100
...
100 zł
Price formatting

24. final Money amount = Money.of(123.45, "EUR");
final MonetaryAmountFormat formatter = MonetaryFormats
.getAmountFormat(Locale.ENGLISH);
System.out.println(formatter.format(amount));
EUR123.45
Price formatting

25. final Money amount = Money.of(123.45, "PLN");
final MonetaryAmountFormat formatter = MonetaryFormats
.getAmountFormat(new Locale("pl", "PL"));
System.out.println(formatter.format(amount));
123,45 PLN
Price formatting

26. final Money amount = Money.of(123.45, "PLN");
final MonetaryAmountFormat formatter = MonetaryFormats
.getAmountFormat(
AmountFormatQueryBuilder.of(new Locale("pl", "PL"))
.set(CurrencyStyle.SYMBOL)
.build()
);
System.out.println(formatter.format(amount));
123,45 zł
Price formatting

27. https://en.wikipedia.org/wiki/Dollar_sign

28. Rounding
●
https://en.wikipedia.org/wiki/Cash_rounding
(aka Swedish rounding)
●
Polish income tax: round to the nearest whole zloty

29. Architecture
public final class Invoice {
private final InvoiceId number;
private final CurrencyUnit currency;
private Sender sender;
private Recipient recipient;
private PaymentTerms paymentTerms;
private LocalDate issueDate;
private final List<InvoiceItem> items;
public Invoice addItem(InvoiceItem item) { /* ... */ }
public MonetaryAmount getTotalNetAmount() { /* ... */}
public MonetaryAmount getTotalVatAmount() { /* ... */}
public MonetaryAmount getTotalGrossAmount() { /* ... */ }
/* ... */
}

30. Architecture
interface InvoiceBuilder {
InvoiceBuilder fromSeller(Contractor seller);
InvoiceBuilder toBuyer(Contractor buyer);
InvoiceBuilder addItem(MonetaryAmount unitPrice, int quantity,
VatRate tax);
/* ... */
Invoice build();
}

31. Further reading
●
https://www.h-schmidt.net/FloatConverter/IEEE754.html
●
https://martinfowler.com/eaaCatalog/money.html
●
https://download.oracle.com/otndocs/jcp/money_currency-1_0-fr-eval-spec/
●
https://github.com/JavaMoney/
●
https://www.baeldung.com/java-money-and-currency
●
https://github.com/peterdevpl/invoices

32. Thank you!
Piotr Horzycki
www.peterdev.pl | twitter.com/peterdevpl