if (up.isPresent()) {
                had++;
                for (Map.Entry<String, String> entry : up.get().entrySet()) {
                    items.putIfAbsent(entry.getKey(), entry.getValue());
                }
            }
        }
        return had == 0 ? Optional.empty() : Optional.of(Map.copyOf(items));
    }

    assertThat(cache.getIfPresent(10)).isEqualTo(50);
    assertThat(asMap.get(10)).isEqualTo(50);
    assertThat(cache.getIfPresent(2)).isNull();
    assertThat(asMap.get(2)).isNull();

    asMap.putIfAbsent(2, 20);
    asMap.putIfAbsent(10, 20);

    assertThat(cache.getIfPresent(2)).isEqualTo(20);
    assertThat(asMap.get(2)).isEqualTo(20);
    assertThat(cache.getIfPresent(10)).isEqualTo(50);

      }
      checkArgument(occurrences > 0, "Invalid occurrences: %s", occurrences);

      while (true) {
        int current = count(element);
        if (current == 0) {
          if (countMap.putIfAbsent(element, occurrences) == null) {
            return 0;
          }
        } else {
          checkArgument(
              occurrences <= Integer.MAX_VALUE - current,

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final @Nullable V putIfAbsent(K key, V value) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always

    }

    private final ConcurrentHashMap<TransferResourceIdentifier, Boolean> ongoing = new ConcurrentHashMap<>();

    @Override
    public void transferInitiated(TransferEvent event) {
        ongoing.putIfAbsent(new TransferResourceIdentifier(event.getResource()), Boolean.TRUE);
        put(event, false);
    }

    @Override
    public void transferStarted(TransferEvent event) throws TransferCancelledException {

      }
    }
  }

  public void fetch(HttpUrl url) throws IOException {
    // Skip hosts that we've visited many times.
    AtomicInteger hostnameCount = new AtomicInteger();
    AtomicInteger previous = hostnames.putIfAbsent(url.host(), hostnameCount);
    if (previous != null) hostnameCount = previous;
    if (hostnameCount.getAndIncrement() >= hostLimit) return;

    Request request = new Request.Builder()
        .url(url)

            E result = (E) canonical;
            return result;
          }
        }

        // didn't see it, trying to put it instead...
        Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
        if (sneaky == null) {
          return sample;
        } else {
          /* Someone beat us to it! Trying again...
           *

    }

    private final ConcurrentHashMap<TransferResourceIdentifier, Boolean> ongoing = new ConcurrentHashMap<>();

    @Override
    public void transferInitiated(TransferEvent event) {
        ongoing.putIfAbsent(new TransferResourceIdentifier(event.getResource()), Boolean.TRUE);
        put(event, false);
    }

    @Override
    public void transferStarted(TransferEvent event) throws TransferCancelledException {

    assertThat(cache.getIfPresent(10)).isEqualTo(50);
    assertThat(asMap.get(10)).isEqualTo(50);
    assertThat(cache.getIfPresent(2)).isNull();
    assertThat(asMap.get(2)).isNull();

    asMap.putIfAbsent(2, 20);
    asMap.putIfAbsent(10, 20);

    assertThat(cache.getIfPresent(2)).isEqualTo(20);
    assertThat(asMap.get(2)).isEqualTo(20);
    assertThat(cache.getIfPresent(10)).isEqualTo(50);

            E result = (E) canonical;
            return result;
          }
        }

        // didn't see it, trying to put it instead...
        Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
        if (sneaky == null) {
          return sample;
        } else {
          /* Someone beat us to it! Trying again...
           *