RomanNumeral.java

package tudelft.sqt;

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

public class RomanNumeral {

	private static Map<Character, Integer> map;

	static {
		map = new HashMap<Character, Integer>() {{
			put('I', 1);
			put('V', 5);
			put('X', 10);
			put('L', 50);
			put('C', 100);
			put('D', 500);
			put('M', 1000);
		}};
	}

	public int romanToInt(String s) {
		int convertedNumber = 0;
		for(int i = 0; i < s.length(); i++) {

			if(!map.containsKey(s.charAt(i)))
				throw new RuntimeException("Invalid number");

			int currentNumber = map.get(s.charAt(i));
			int next = i+1 < s.length() ?
					map.get(s.charAt(i+1)) : 0;

			if(currentNumber > next)
				convertedNumber += currentNumber;
			else
				convertedNumber -= currentNumber;
		}

		return convertedNumber;

	}
}

RomanNumeralTest.java

package tudelft.sqt;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import static org.junit.jupiter.api.Assertions.assertThrows;

public class RomanNumeralTest {

	@Test
	void singleDigit() {
		Assertions.assertEquals(1, new RomanNumeral().romanToInt("I"));
		Assertions.assertEquals(5, new RomanNumeral().romanToInt("V"));
		Assertions.assertEquals(10, new RomanNumeral().romanToInt("X"));
		Assertions.assertEquals(50, new RomanNumeral().romanToInt("L"));
		Assertions.assertEquals(100, new RomanNumeral().romanToInt("C"));
		Assertions.assertEquals(500, new RomanNumeral().romanToInt("D"));
		Assertions.assertEquals(1000, new RomanNumeral().romanToInt("M"));
	}

	@Test
	void repetition() {
		Assertions.assertEquals(2, new RomanNumeral().romanToInt("II"));
		Assertions.assertEquals(20, new RomanNumeral().romanToInt("XX"));
	}

	@Test
	void manyLettersInOrder() {
		Assertions.assertEquals(6, new RomanNumeral().romanToInt("VI"));
		Assertions.assertEquals(15, new RomanNumeral().romanToInt("XV"));
	}

	@Test
	void subtractiveNotation() {
		Assertions.assertEquals(4, new RomanNumeral().romanToInt("IV"));
		Assertions.assertEquals(9, new RomanNumeral().romanToInt("IX"));
	}

	@Test
	void withAndWithoutSubtractiveNotation() {
		Assertions.assertEquals(14, new RomanNumeral().romanToInt("XIV"));
		Assertions.assertEquals(159, new RomanNumeral().romanToInt("CLIX"));
	}

	@Test
	void minAndMax() {
		Assertions.assertEquals(1, new RomanNumeral().romanToInt("I"));
		Assertions.assertEquals(3999, new RomanNumeral().romanToInt("MMMCMXCIX"));
	}

	@Test
	void invalidLetter() {
		Exception e = assertThrows(RuntimeException.class, () -> new RomanNumeral().romanToInt("Y"));

		Assertions.assertEquals("Invalid number", e.getMessage());
	}

	// TODO: We still miss testing for invalid numbers, such as 'IIIVII'.


}