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rootulp's solution

to Hexadecimal in the Java Track

Published at Jul 13 2018 · 4 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Convert a hexadecimal number, represented as a string (e.g. "10af8c"), to its decimal equivalent using first principles (i.e. no, you may not use built-in or external libraries to accomplish the conversion).

On the web we use hexadecimal to represent colors, e.g. green: 008000, teal: 008080, navy: 000080).

The program should handle invalid hexadecimal strings.

Running the tests

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$ gradle test

in your terminal.


All of Computer Science http://www.wolframalpha.com/examples/NumberBases.html

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import org.junit.Test;
import org.junit.Ignore;
import static org.junit.Assert.assertEquals;

public class HexadecimalTest {

    public void testOne(){
        int expected = 1;
        assertEquals(expected, Hexadecimal.toDecimal("1"));

    public void testC(){
        int expected = 12;
        assertEquals(expected, Hexadecimal.toDecimal("c"));

    public void test10(){
        int expected = 16;
        assertEquals(expected, Hexadecimal.toDecimal("10"));

    public void testAf(){
        int expected = 175;
        assertEquals(expected, Hexadecimal.toDecimal("af"));

    public void test100(){
        int expected = 256;
        assertEquals(expected, Hexadecimal.toDecimal("100"));

    public void test19ace(){
        int expected = 105166;
        assertEquals(expected, Hexadecimal.toDecimal("19ace"));

    public void testInvalid(){
        int expected = 0;
        assertEquals(expected, Hexadecimal.toDecimal("carrot"));

    public void testBlack(){
        int expected = 0;
        assertEquals(expected, Hexadecimal.toDecimal("000000"));

    public void testWhite(){
        int expected = 16777215;
        assertEquals(expected, Hexadecimal.toDecimal("ffffff"));

    public void testYellow(){
        int expected = 16776960;
        assertEquals(expected, Hexadecimal.toDecimal("ffff00"));
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class Hexadecimal {

  private static final Map<Character, Integer> HEX_MAP = hexMap();
  private static final Set<Character> VALID_CHARACTERS = HEX_MAP.keySet();

  private static Map<Character, Integer> hexMap() {
    Map<Character, Integer> hexMap = new HashMap<>();
    hexMap.put('0', 0);
    hexMap.put('1', 1);
    hexMap.put('2', 2);
    hexMap.put('3', 3);
    hexMap.put('4', 4);
    hexMap.put('5', 5);
    hexMap.put('6', 6);
    hexMap.put('7', 7);
    hexMap.put('8', 8);
    hexMap.put('9', 9);
    hexMap.put('a', 10);
    hexMap.put('b', 11);
    hexMap.put('c', 12);
    hexMap.put('d', 13);
    hexMap.put('e', 14);
    hexMap.put('f', 15);
    return hexMap;

  public static int toDecimal(String str) {
    if (!valid(str)) {
      return 0;
    return calculateDecimal(str);

  private static int calculateDecimal(String str) {
    int decimal = 0;
    for (int i = 0; i < str.length(); i++) {
      int multiplier = multiplier(i, str.length());
      int digit =  digitToDecimal(str.charAt(i));
      decimal += digit * multiplier;
    return decimal;

  private static int multiplier(int index, int strLength) {
    return (int) Math.pow(16, (strLength - 1 - index));

  private static int digitToDecimal(Character digit) {
    return HEX_MAP.get(digit);

  private static boolean valid(String str) {
    for (Character c : str.toCharArray()) {
      if (!VALID_CHARACTERS.contains(c)) {
        return false;
    return true;

Community comments

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Avatar of jaebradley

Can you think of an implementation that wouldn't use a Map?

Avatar of jaebradley

valid iterates through the input String once. Then you iterate through the input String again in calculateDecimal. Can you think of a way to incorporate your validation logic such that you only have to iterate through the input String only once?

Avatar of jaebradley

why have multiplier and digitToDecimal (especially the latter)?

Avatar of jaebradley

There's not really a need for VALID_CHARACTERS. I think it's perfectly readable to do something like HEX_MAP.keySet().contains() instead.

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