Sum

Sum is a reduction method. Given a stream of MonetaryAmount, it returns the sum of its elements. Here's an example.

public class ReduceSumMonetaryAmount {

    public static void main(String[] args) {
        CurrencyUnit dollar = Monetary.getCurrency("USD");

        MonetaryAmount money = Money.of(10, dollar);
        MonetaryAmount money2 = Money.of(10, dollar);
        MonetaryAmount money3 = Money.of(10, dollar);
        MonetaryAmount money4 = Money.of(9, dollar);
        MonetaryAmount money5 = Money.of(8, dollar);

        Optional<MonetaryAmount> result = Stream.of(money, money2, money3, money4, money5).reduce(MonetaryFunctions.sum());
        result.ifPresent(System.out::println);//USD 47

    }
}

Notice that the operation will throw an exception if applied on a stream of money with different currencies as shown below.

public class ReduceSumMonetaryAmountError {

    public static void main(String[] args) {
        CurrencyUnit dollar = Monetary.getCurrency("USD");
        CurrencyUnit real = Monetary.getCurrency("BRL");

        MonetaryAmount money = Money.of(10, dollar);
        MonetaryAmount money2 = Money.of(10, dollar);
        MonetaryAmount money3 = Money.of(10, real);
        MonetaryAmount money4 = Money.of(9, dollar);
        MonetaryAmount money5 = Money.of(8, dollar);

        Optional<MonetaryAmount> result = Stream.of(money, money2, money3, money4, money5).reduce(
                MonetaryFunctions.sum());//javax.money.MonetaryException: Currency mismatch: BRL/USD

    }
}

In Moneta, you can provide an implementation of ExchangeRateProvider in order to perform currency conversions for monetary values with different currencies as shown below.

public class ReduceSumMonetaryAmountExchange {

    public static void main(String[] args) {
        CurrencyUnit dollar = Monetary.getCurrency("USD");
        CurrencyUnit real = Monetary.getCurrency("BRL");
        ExchangeRateProvider provider = MonetaryConversions.getExchangeRateProvider(ExchangeRateType.IMF);

        MonetaryAmount money = Money.of(10, dollar);
        MonetaryAmount money2 = Money.of(10, dollar);
        MonetaryAmount money3 = Money.of(10, real);
        MonetaryAmount money4 = Money.of(9, dollar);
        MonetaryAmount money5 = Money.of(8, dollar);

        Optional<MonetaryAmount> result = Stream.of(money, money2, money3, money4, money5).reduce(
                MonetaryFunctions.sum(provider, dollar));//javax.money.MonetaryException: Currency mismatch: BRL/USD
        result.ifPresent(System.out::println);//money converted in dollar

    }
}