Link to the old Java-based exercise

README.md

@@ -8,7 +8,8 @@ it's time to write your own tests in the latter part of this exercise.
 This exercise uses [JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript) (ES6/ECMAScript 2015),
 [Mocha](https://mochajs.org/) testing framework and [Chai](https://www.chaijs.com/)
 assertions, but the concepts are language agnostic. It would be quite easy to translate the example tests to another
-programming language as you go.
+programming language as you go. There also exists an
+older [Java-based predecessor](https://github.com/luontola/tdd-tetris-tutorial) of this exercise.
 
 Refer to the course material at <https://tdd-mooc.luontola.fi> to learn more about TDD.