Roman Numerals II

import java.util.*;
import java.io.*;

public class RomanNumerals {
    // Holds expanded roman character representations of each digit, for the first four places
    static String[][] lookupTable = new String[4][10];

    static void initLookupTable() {
        // Roman representation of One, Five, Ten for each place
        String[][] romanChars = {
            {"I", "V", "X"},  // ---X ones
            {"X", "L", "C"},  // --X- tens
            {"C", "D", "M"},  // -X-- hundreds
            {"M", "?", "?"}   // X--- thousands
        };

        // Compute lookup table for each of 4 places (ones, tens, hundreds, thousands)
        for (int i = 0; i < 4; i++) {
            String one = romanChars[i][0];
            String five = romanChars[i][1];
            String ten = romanChars[i][2];

            lookupTable[i][0] = "";
            lookupTable[i][1] = one;
            lookupTable[i][2] = one + one;
            lookupTable[i][3] = one + one + one;
            lookupTable[i][4] = one + five;
            lookupTable[i][5] = five;
            lookupTable[i][6] = five + one;
            lookupTable[i][7] = five + one + one;
            lookupTable[i][8] = five + one + one + one;
            lookupTable[i][9] = one + ten;
        }
    }

    // Initialize the lookup table when the class loads
    static { initLookupTable(); }

    //  Main; run tests, then interactively convert numerals
    public static void main(String[] args) throws IOException {
        testRomanNumerals();
        interactiveRomanNumerals();
    }

    // Read numbers from stdin, output roman numeral representation of input to stdout
    private static void interactiveRomanNumerals() throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String input;
        String output;
        while ((input = br.readLine()) != null && input.length() > 0) {
            System.out.println(toRomanNumerals(input));
        }
    }

    /**
     * Converts a number, as an input string, into the roman numeral representation
     * @param input a 0-4 digit number, represented as a string
     * @return the roman numeral representation of the input
     */
    private static String toRomanNumerals(String input) {
        String output;
    // Reverse string order, it's easier to think of the "places" this way. index 0 == ones, 1 == tens, etc.
        StringBuffer data = new StringBuffer(input).reverse();
        output = "";
        for (int i = 0; i < data.length(); i++) {
            // For each char in the input, get the Roman representation, prepend it to the output string (to reverse the order again)
            output = getRomanNumeral(data.charAt(i), i) + output;
        }
        return output;
    }

    /**
     * @param value one digit of a number, as a character (eg: '1')
     * @param placeIdx the digit's "place" (0 == ones, 1 == tens, 2 == hundreds, 3 == thousands)
     * @return the roman numeral representation of the one input digit
     */
    private static String getRomanNumeral(char value, int placeIdx) {
        return lookupTable[placeIdx][value - '0'];
    }

    private static void testRomanNumerals() {
        testEquals("I", toRomanNumerals("1"));
        testEquals("VI", toRomanNumerals("6"));
        testEquals("XXXI", toRomanNumerals("31"));
        testEquals("XXIV", toRomanNumerals("24"));
        testEquals("L", toRomanNumerals("50"));
        testEquals("CCCLXIX", toRomanNumerals("369"));
        testEquals("CD", toRomanNumerals("400"));
        testEquals("CDXLVIII", toRomanNumerals("448"));
        testEquals("MCMXCVIII", toRomanNumerals("1998"));
        testEquals("MMDCCLI", toRomanNumerals("2751"));
        System.out.println("All test cases passed.");
    }

    private static void testEquals(String left, String right) {
        if (!left.equals(right))
            throw new RuntimeException("Test failed. left != right (" + left + " != " + right + ")");
    }
}