### Question:
MQClientInstance { public boolean registerConsumer(final String group, final MQConsumerInner consumer) { if (null == group || null == consumer) { return false; } MQConsumerInner prev = this.consumerTable.putIfAbsent(group, consumer); if (prev != null) { log.warn("the consumer group[" + group + "] exist already."); return false; } return true; } MQClientInstance(ClientConfig clientConfig, int instanceIndex, String clientId);  MQClientInstance(ClientConfig clientConfig, int instanceIndex, String clientId, RPCHook rpcHook); static TopicPublishInfo topicRouteData2TopicPublishInfo(final String topic, final TopicRouteData route); static Set<MessageQueue> topicRouteData2TopicSubscribeInfo(final String topic, final TopicRouteData route); void start(); String getClientId(); void updateTopicRouteInfoFromNameServer(); void checkClientInBroker(); void sendHeartbeatToAllBrokerWithLock(); void adjustThreadPool(); boolean updateTopicRouteInfoFromNameServer(final String topic); boolean updateTopicRouteInfoFromNameServer(final String topic, boolean isDefault, DefaultMQProducer defaultMQProducer); void shutdown(); boolean registerConsumer(final String group, final MQConsumerInner consumer); void unregisterConsumer(final String group); boolean registerProducer(final String group, final DefaultMQProducerImpl producer); void unregisterProducer(final String group); boolean registerAdminExt(final String group, final MQAdminExtInner admin); void unregisterAdminExt(final String group); void rebalanceImmediately(); void doRebalance(); MQProducerInner selectProducer(final String group); MQConsumerInner selectConsumer(final String group); FindBrokerResult findBrokerAddressInAdmin(final String brokerName); String findBrokerAddressInPublish(final String brokerName); FindBrokerResult findBrokerAddressInSubscribe(//
        final String brokerName, //
        final long brokerId, //
        final boolean onlyThisBroker//
    ); int findBrokerVersion(String brokerName, String brokerAddr); List<String> findConsumerIdList(final String topic, final String group); String findBrokerAddrByTopic(final String topic); void resetOffset(String topic, String group, Map<MessageQueue, Long> offsetTable); Map<MessageQueue, Long> getConsumerStatus(String topic, String group); TopicRouteData getAnExistTopicRouteData(final String topic); MQClientAPIImpl getMQClientAPIImpl(); MQAdminImpl getMQAdminImpl(); long getBootTimestamp(); ScheduledExecutorService getScheduledExecutorService(); PullMessageService getPullMessageService(); DefaultMQProducer getDefaultMQProducer(); ConcurrentMap<String, TopicRouteData> getTopicRouteTable(); ConsumeMessageDirectlyResult consumeMessageDirectly(final MessageExt msg, //
        final String consumerGroup, //
        final String brokerName); ConsumerRunningInfo consumerRunningInfo(final String consumerGroup); ConsumerStatsManager getConsumerStatsManager(); NettyClientConfig getNettyClientConfig();  }### Answer:
@Test public void testRegisterConsumer() throws RemotingException, InterruptedException, MQBrokerException { boolean flag = mqClientInstance.registerConsumer(group, mock(MQConsumerInner.class)); assertThat(flag).isTrue(); flag = mqClientInstance.registerConsumer(group, mock(MQConsumerInner.class)); assertThat(flag).isFalse(); mqClientInstance.unregisterConsumer(group); flag = mqClientInstance.registerConsumer(group, mock(MQConsumerInner.class)); assertThat(flag).isTrue(); }