@Override
  public Collection<String> getComponents(String service) {
    return componentsForService.containsKey(service) ?
      new HashSet<>(componentsForService.get(service)) :
        Collections.emptySet();
  }
Get the components for the specified service which are associated with the host group.

@param service  service name

@return set of component names
ambari-server/src/main/java/org/apache/ambari/server/topology/HostGroupImpl.java