### Question:
ClusteredObservationBus implements ObservationBus { public void setClusterName( String clusterName ) { CheckArg.isNotNull(clusterName, "clusterName"); if (channel != null) { String name = this.clusterName; throw new IllegalStateException(ClusteringI18n.clusteringChannelIsRunningAndCannotBeChangedUnlessShutdown.text(name)); } this.clusterName = clusterName; }  String getConfiguration(); void setConfiguration( String configuration ); String getClusterName(); void setClusterName( String clusterName ); @Override synchronized void start(); @Override void notify( Changes changes ); @Override boolean register( Observer observer ); @Override boolean unregister( Observer observer ); @Override boolean hasObservers(); boolean isStarted(); @Override synchronized void shutdown();  }### Answer:
@Test( expected = IllegalArgumentException.class ) public void shouldNotAllowSettingClusterNameToNull() { bus.setClusterName(null); }