I'd like types for all the currencies.

USD rent = 2000

EUR salary = 6000.53

USD disposableIncome = salary - rent

Then:

disposableIncome.toDecimal()

disposableIncome.toInteger()

disposableIncome.toString()

USD interest = disposableIncome.compoundInterest(startDate, endDate, apy)

USD tip = billTotal.tip(percentage)

USD and EUR would inherit from Money

Not sure how to implement it, might be a JVM magic thing.

What you want is Money, currency is a _unit_ type of a Money quantity, just as meter is a unit type of Length.

Money cost = 25M USD;

cost += 12M EUR;

display(cost.to(EUR));

I don't think this makes sense? You need to specify an exchange rate somewhere.

> just as meter is a unit type of Length

The difference is that length units are fixed, but exchange rates are constantly changing.

> You need to specify an exchange rate somewhere.

Right. As I mentioned earlier exchange rate services would need to be integrated into the API.

Also, exchange rate services would not be needed until a conversion is necessary. For instance, when adding money of different currencies internally the Money type maintains currencies separately, subtotals for USD, EUR, etc. are managed. If at a later time the sum of Money should be displayed, deposited, etc. as a single currency, only then is an exchange service involved.

> Money ask = 2.1M USD;

Feels like it should be

  Money<USD> ask = 2.1M USD;

Then methods like .plus() wouldn't even be defined for incompatible currencies. Of course that doesn't work for .equals(), so you'd still have to fall back to exceptions in that case.

Generally, the idea is that there is no incompatibility between units. Money is a bit exotic given its abstract nature wrt units / exchange rates.

For instance, physical dimensions store values as SI units, regardless of supplied unit, but retain the supplied unit type as a display hint.

Here, the Length dimension illustrates working with amounts of any unit.

Length orbit = 250 mi;

Length moon = 238000 km;

Length landing = moon + orbit;

display(landing); // prints 148,136.344 mi

display(landing.to(km)); // prints 238,402.336 km

// where

display(landing.to(mi) == landing.to(km)); // prints true

// perhaps a better example

Rate rate = 1.41 km/s;

Time duration = 76 hours;

Length distance = rate * duration;

// make your own unit constants

RateUnit kps = km/s;

rate = 1.41 kps;

You can play with the manifold-science[1] project, it implements the complete SI physical dimensions and many derived dimensions along with comprehensive operator overloading[2] support etc. to make working with and reading code using quantities more productive in Java.

1. https://github.com/manifold-systems/manifold/tree/master/man...

2. https://github.com/manifold-systems/manifold/tree/master/man...

The JSR 354 API can provide access to exchange rate services via the ExchangeRateProvider interface. The reference implementation (Moneta) includes a few implementations of ExchangeRateProviders that connect to online services to look up exchange rates.