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Javier Salinas

Software Engineer located in London, interested in simple designs and clean code using TDD and DDD.

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Practicing Test-Driven-Development (TDD) with the Roman Numerals Kata.

Uncle Bob defines the 3 rules of TDD

  1. You are not allowed to write any production code unless it is to make a failing unit test pass.
  2. You are not allowed to write any more of a unit test than is sufficient to fail; and compilation failures are failures.
  3. You are not allowed to write any more production code than is sufficient to pass the one failing unit test.

I wanted to practice TDD and some Java8, so I tried to follow the rules and solve the problem using streams and lambda expressions.

To start with, I wrote the most simple test scenario.

assertThat(RomanNumeral.romanFrom(1), is("I"));

It was straight forward, so once I wrote the code to make it pass, I added more test cases:

assertThat(RomanNumeral.romanFrom(1), is("I"));
assertThat(RomanNumeral.romanFrom(2), is("II"));
assertThat(RomanNumeral.romanFrom(3), is("III"));

Once the code made the tests pass, it was clear that this code needed refactoring.

public static String romanFrom(int number) {
    if(number == 3) return "III";
    if(number == 2) return "II";
    return "I";

So I refactored it.

I didn’t want to add the cases where the number is reduced by subtraction like e.g. 4 -> IV, instead I added a new letter to convert 5 -> V.

After having passed this test, I continued by adding more numbers like 6 -> VI, 8 -> VIII…

Before adding the numbers that require subtraction, I decided to test the algorithm with more letters, so I added 10 -> X.

Once the new code made the test pass…

public static String romanFrom(int number) {
    if (number == 5) return "V";
    return IntStream.iterate(number, i -> i - (i >= 10 ? 10 : i >= 5 ? 5 : 1))
        .filter(i -> i > 0)
        .mapToObj(i -> i >= 10 ? "X" : i >= 5 ? "V" : "I")

and again it needed refactoring.

So I refactored it by adding an Enum containing letters and decimals numbers like so:

enum RomanToDecimal {

After the Enum was added, it seemed easy to add more numbers, excluding those that require subtraction.

I added roman numerals until thousand (M) and the algorithm was working, so I felt comfortable to add the remaining numbers which require subtraction.

enum RomanToDecimal {
  M(1000), CM(900),
  D(500), CD(400),
  C(100), XC(90),
  L(50), XL(40),
  X(10), IX(9),
  V(5), IV(4),

After adding these cases to the Enum, the implementation was working fine.

I am pretty sure, that if I would have tried to solve this problem without following TDD, the result would have been different and probably not so clean.

To follow the steps for solving the kata, have a look at the history commits.

Before seeing my solution, I would encourage you to try to solve it and share your solution. I would also recommend to watch this video from Sandro Mancuso where he solves this problem.

Full source is located on a github repo roman-numerals