RomanConverter.java

package tdd;

import java.util.HashMap;
import java.util.Map;

/**
 * Created by mauricioaniche on 26/05/2017.
 */
public class RomanConverter {


    private static Map<String, Integer> table;

    static {
        table = new HashMap<>();
        table.put("I", 1);
        table.put("V", 5);
        table.put("X", 10);
        table.put("L", 50);
        table.put("C", 100);
        table.put("D", 500);
        table.put("M", 1000);
    }


    public int convert(String romanNumeral) {

        int finalValue = 0;
        for(int i = 0; i < romanNumeral.length(); i++) {

            int currentValue = roman(romanNumeral.charAt(i));

            boolean notLastDigit = i < romanNumeral.length() - 1;
            if(notLastDigit && roman(romanNumeral.charAt(i+1)) > currentValue) {
                currentValue *= -1;
            }

            finalValue += currentValue;
        }

        return finalValue;
    }

    private Integer roman(char romanNumeral) {
        return table.get(String.valueOf(romanNumeral));
    }
}

RomanConverterTest.java

package tdd;

import org.assertj.core.api.Assertions;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvSource;

import static org.assertj.core.api.Assertions.assertThat;

/**
 * Created by mauricioaniche on 26/05/2017.
 */
public class RomanConverterTest {

    @ParameterizedTest(name = "roman numbers with a single digit")
    @CsvSource({"I, 1", "V, 5", "X, 10"})
    void simpleRomanNumber(String romanNumber, int value) {
        int number = new RomanConverter().convert(romanNumber);

        assertThat(number).isEqualTo(value);
    }


    @ParameterizedTest(name="roman number with many digits: {0}")
    @CsvSource({"II, 2", "III, 3", "VII, 7", "CLXIII, 163"})
    void manyDigits(String romanNumber, int value) {
        int number = new RomanConverter().convert(romanNumber);

        assertThat(number).isEqualTo(value);
    }


    @ParameterizedTest(name="inverse order: {0}")
    @CsvSource({"IV, 4", "XIV, 14", "CDXXXII, 432"})
    void inverseOrder(String romanNumber, int value) {
        int number = new RomanConverter().convert(romanNumber);

        assertThat(number).isEqualTo(value);
    }
}