def apiClassExtractor2 = Mock(ApiClassExtractor)

        def resourceHasher1 = AbiExtractingClasspathResourceHasher.withFallback(apiClassExtractor1)
        def resourceHasher2 = AbiExtractingClasspathResourceHasher.withFallback(apiClassExtractor2)

        when:
        def configurationHash1 = configurationHashOf(resourceHasher1)
        def configurationHash2 = configurationHashOf(resourceHasher2)

    private static final Logger LOGGER = LoggerFactory.getLogger(AbiExtractingClasspathResourceHasher.class);
    public static final AbiExtractingClasspathResourceHasher DEFAULT = withFallback(new ApiClassExtractor(Collections.emptySet()));

    private final ApiClassExtractor extractor;
    private final FallbackStrategy fallbackStrategy;

@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public final class Futures extends GwtFuturesCatchingSpecialization {

  // A note on memory visibility.
  // Many of the utilities in this class (transform, withFallback, withTimeout, asList, combine)
  // have two requirements that significantly complicate their design.
  // 1. Cancellation should propagate from the returned future to the input future(s).

@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public final class Futures extends GwtFuturesCatchingSpecialization {

  // A note on memory visibility.
  // Many of the utilities in this class (transform, withFallback, withTimeout, asList, combine)
  // have two requirements that significantly complicate their design.
  // 1. Cancellation should propagate from the returned future to the input future(s).

      @Override
      public ListenableFuture<V> apply(X t) {
        throw new AssertionError("Unexpected fallback", t);
      }
    };
  }

  // catchingAsync tests cloned from the old withFallback tests:

  public void testCatchingAsync_inputDoesNotRaiseException() throws Exception {
    AsyncFunction<Throwable, Integer> fallback = unexpectedAsyncFunction();

      @Override
      public ListenableFuture<V> apply(X t) {
        throw new AssertionError("Unexpected fallback", t);
      }
    };
  }

  // catchingAsync tests cloned from the old withFallback tests:

  public void testCatchingAsync_inputDoesNotRaiseException() throws Exception {
    AsyncFunction<Throwable, Integer> fallback = unexpectedAsyncFunction();