private List<ServerInstance> getServerGroupList0(String hostName) {
            return map.computeIfAbsent(hostName, k -> new ArrayList<>());
        }
@author emeroad
@author netspider
@author minwoo.jung
@author HyunGil Jeong
web/src/main/java/com/navercorp/pinpoint/web/applicationmap/nodes/ServerGroupList.java