Bhuwan Upadhyay

Talks all about software engineering

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Implement a URL shortener with alphanumeric string



Implement a URL shortener with six-character alphanumeric string.


Implement a URL shortener with the following methods:

  • shorten(url), which shortens the url into a six-character alphanumeric string, such as zLg6wl.
  • restore(short), which expands the shortened string into the original url.
  • If no such shortened string exists, return null.

What if we enter the same URL twice?


class Solution {
    public static final int MAX = 6;
    private final Map<URL, URL> urlCache = new HashMap<>();
    private final AlphanumericRandomizer randomizer = new AlphanumericRandomizer();
    URL shorten(String url) {
        try {
            URL longUrl = URI.create(url).toURL();
            String shortValue = randomizer.next(MAX);
            URL shortUrl = new URL(longUrl.getProtocol(), longUrl.getHost(), "/" + shortValue);
            this.urlCache.put(shortUrl, longUrl);
            return shortUrl;
        } catch (MalformedURLException e) {
            throw new RuntimeException(e);

    URL restore(URL shortValue) {
        return this.urlCache.get(shortValue);
     * Alphanumeric characters are A to Z, a to z and 0 to 9
    static class AlphanumericRandomizer {
        private static final String CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        private final Random r;
        private final String alphaNumeric;

        public AlphanumericRandomizer() {
            r = new Random();
            this.alphaNumeric = CHARS + CHARS.toLowerCase() + "0123456789";

        public String next(int size) {
            final int length = this.alphaNumeric.length();
            return IntStream.range(0, size)
                    .mapToObj(value -> r.nextInt(length))
                    .map(index -> String.valueOf(alphaNumeric.charAt(index)))

Test Cases

class SolutionTest {
    private Solution solution;

    static Collection<Object[]> data() {
        return Arrays.asList(new Object[][]{
                {"http://hello.com/test", "http", "hello.com", "/[A-Za-z0-9]{6}"},
                {"http://hello.com/test", "http", "hello.com", "/[A-Za-z0-9]{6}"},
                {"https://hello.com/test", "https", "hello.com", "/[A-Za-z0-9]{6}"},

    void setUp() {
        this.solution = new Solution();

    void testSolution(String url, String expectedProtocol, String expectedHost, String expectedPathRegEx) {
        URL actual = this.solution.shorten(url);
        assertEquals(expectedProtocol, actual.getProtocol());
        assertEquals(expectedHost, actual.getHost());
        assertTrue(actual.getPath().matches(expectedPathRegEx), "The shorten url " + actual + " is not six characters alphanumeric.");

    void testSolution(String url) throws MalformedURLException {
        URL shorten = this.solution.shorten(url);
        URL actual = this.solution.restore(shorten);
        URL expected = URI.create(url).toURL();
        assertEquals(expected, actual);