### Question:
CommandUtil { public static Set<String> fetchMasterAddrByClusterName(final MQAdminExt adminExt, final String clusterName) throws InterruptedException, RemotingConnectException, RemotingTimeoutException, RemotingSendRequestException, MQBrokerException { Set<String> masterSet = new HashSet<String>(); ClusterInfo clusterInfoSerializeWrapper = adminExt.examineBrokerClusterInfo(); Set<String> brokerNameSet = clusterInfoSerializeWrapper.getClusterAddrTable().get(clusterName); if (brokerNameSet != null) { for (String brokerName : brokerNameSet) { BrokerData brokerData = clusterInfoSerializeWrapper.getBrokerAddrTable().get(brokerName); if (brokerData != null) { String addr = brokerData.getBrokerAddrs().get(MixAll.MASTER_ID); if (addr != null) { masterSet.add(addr); } } } } else { System.out .printf("[error] Make sure the specified clusterName exists or the nameserver which connected is correct."); } return masterSet; }  static Map<String/*master addr*/, List<String>/*slave addr*/> fetchMasterAndSlaveDistinguish(
        final MQAdminExt adminExt, final String clusterName); static Set<String> fetchMasterAddrByClusterName(final MQAdminExt adminExt, final String clusterName); static Set<String> fetchMasterAndSlaveAddrByClusterName(final MQAdminExt adminExt, final String clusterName); static Set<String> fetchBrokerNameByClusterName(final MQAdminExt adminExt, final String clusterName); static String fetchBrokerNameByAddr(final MQAdminExt adminExt, final String addr);  }### Answer:
@Test public void testFetchMasterAddrByClusterName() throws InterruptedException, MQBrokerException, RemotingTimeoutException, RemotingSendRequestException, RemotingConnectException { Set<String> result = CommandUtil.fetchMasterAddrByClusterName(defaultMQAdminExtImpl, "default-cluster"); assertThat(result.size()).isEqualTo(0); }