### Question:
ValidatorApiHandler implements ValidatorApiChannel { @Override public void subscribeToBeaconCommitteeForAggregation( final int committeeIndex, final UInt64 aggregationSlot) { attestationTopicSubscriber.subscribeToCommitteeForAggregation(committeeIndex, aggregationSlot); } ValidatorApiHandler(
      final CombinedChainDataClient combinedChainDataClient,
      final SyncStateTracker syncStateTracker,
      final StateTransition stateTransition,
      final BlockFactory blockFactory,
      final AggregatingAttestationPool attestationPool,
      final AttestationManager attestationManager,
      final AttestationTopicSubscriber attestationTopicSubscriber,
      final EventBus eventBus,
      final DutyMetrics dutyMetrics,
      final PerformanceTracker performanceTracker); @Override SafeFuture<Optional<ForkInfo>> getForkInfo(); @Override SafeFuture<Optional<UInt64>> getGenesisTime(); @Override SafeFuture<Optional<List<ValidatorDuties>>> getDuties(
      final UInt64 epoch, final Collection<BLSPublicKey> publicKeys); @Override SafeFuture<Optional<List<AttesterDuties>>> getAttestationDuties(
      final UInt64 epoch, final Collection<Integer> validatorIndexes); @Override SafeFuture<Optional<List<ProposerDuties>>> getProposerDuties(final UInt64 epoch); @Override SafeFuture<Optional<BeaconBlock>> createUnsignedBlock(
      final UInt64 slot, final BLSSignature randaoReveal, final Optional<Bytes32> graffiti); @Override SafeFuture<Optional<Attestation>> createUnsignedAttestation(
      final UInt64 slot, final int committeeIndex); @Override SafeFuture<Optional<Attestation>> createAggregate(final Bytes32 attestationHashTreeRoot); @Override void subscribeToBeaconCommitteeForAggregation(
      final int committeeIndex, final UInt64 aggregationSlot); @Override void subscribeToPersistentSubnets(Set<SubnetSubscription> subnetSubscriptions); @Override void sendSignedAttestation(
      final Attestation attestation, final Optional<Integer> expectedValidatorIndex); @Override void sendSignedAttestation(final Attestation attestation); @Override void sendAggregateAndProof(final SignedAggregateAndProof aggregateAndProof); @Override void sendSignedBlock(final SignedBeaconBlock block);  }### Answer:
@Test public void subscribeToBeaconCommittee_shouldSubscribeViaAttestationTopicSubscriptions() { final int committeeIndex = 10; final UInt64 aggregationSlot = UInt64.valueOf(13); validatorApiHandler.subscribeToBeaconCommitteeForAggregation(committeeIndex, aggregationSlot); verify(attestationTopicSubscriptions) .subscribeToCommitteeForAggregation(committeeIndex, aggregationSlot); }