Kata for Java Generics

Level: Intermediate
Timebox: 1hr

This kata helps you be more comfortable with Java generics beyond simple List<String>. You will practice writing generics classes and methods, and not just using them. You will also practice lambda expressions and method references.

Goal

Write a ExchangeDesk class to change currencies, like the foreign currency desk of an airport. Avoid hard-coding any particular currency class in the converter. Preserve currency types in the conversion.

Part 1

The currency conversion call should look like:

final INR rupees = new INR(1_000);
final USD dollar = exchangeDesk.convert(rupees, USD.class);

So your goal is to change the type declarations in convert from specific currencies to general Currency references using generics.

Hard-code the exchange rate for this part.

Part 2

Remember: Your "exchange desk" will need to know exchange rates (try INR to USD for example rates), but not look them up in real-time—assume static rates for the kata:

exchangeDesk.addRate(USD.class, INR.class, 64.5d);

Adding a rate for USD to INR should also add the corresponding rate for INR to USD.

Rules for success

Always start with a failing test (hint: use the above code in your test)
Keep test coverage at 100%
Keep clean code style
No compiler warnings
No type casting (exception for casting to a generic parameter, if required)
No use of reflection: use lambdas and/or method references

Tips

Fork the kata (https://github.com/binkley/tw-generics-kata) to your github account; work from there, pushing your local changes back to your fork
Ensure your editor has annotation processing enabled for javac, and a Lombok plugin or support turned on
Use the "ABC" pattern to your advantage: git add . && ./gradlew test && git commit (or amend)
Commit every time the tests pass; save ./gradlew clean build for pushing your commit

Extra credit

Each of these are suitable for follow-on katas:

Refer to currencies by string ("INR", "USD") rather than class token (INR.class, USD.class). See also the locale kata
Use ServiceLoader from the JDK to find and use supported currencies. You may find META-INF/services generator helpful.
Use Locale and NumberFormat from the JDK to format currencies. This is not related to generics, but introduces you to more of the JDK. You may find Using Predefined Formats (The Java™ Tutorials > Internationalization > Formatting) helpful.

Updates

2018/10/24 - Refreshed Gradle, dependencies and tooling

ahornace/tw-generics-kata