}

  public void testKeySet() {
    Map<String, Integer> map = create();
    Set<String> keySet = map.keySet();
    assertTrue(keySet instanceof SynchronizedSet);
    assertSame(mutex, ((SynchronizedSet<?>) keySet).mutex);
  }

  public void testValues() {
    Map<String, Integer> map = create();
    Collection<Integer> values = map.values();

/**
 * Fetches HTML from a requested URL, follows the links, and repeats.
 */
public final class Crawler {
  private final OkHttpClient client;
  private final Set<HttpUrl> fetchedUrls = Collections.synchronizedSet(new LinkedHashSet<>());
  private final BlockingQueue<HttpUrl> queue;
  private final ConcurrentHashMap<String, AtomicInteger> hostnames = new ConcurrentHashMap<>();
  private final int hostLimit;