### Question:
CustomTagService { public void contributorMergedEventHandler(@Observes ContributorMergedEvent event) { if (event != null && event.getContributorCodeFrom() != null && event.getContributorCodeTo() != null) { customTagPersistenceService.changeOwnershipOfTags(event.getContributorCodeFrom(), event.getContributorCodeTo()); } else { log.warning("Invalid event " + event); } }  void contentDeletedEventHandler(@Observes ContentDeletedEvent event); void contentBeforeIndexedHandler(@Observes ContentBeforeIndexedEvent event); void contributorMergedEventHandler(@Observes ContributorMergedEvent event); void contributorCodeChangedEventHandler(@Observes ContributorCodeChangedEvent event); @SuppressWarnings("unchecked") void updateSysTagsField(Map<String, Object> source);  }### Answer:
@Test public void contributorMergedEventHandler() { CustomTagService tested = getTested(); tested.contributorMergedEventHandler(null); verifyZeroInteractions(tested.customTagPersistenceService); reset(tested.customTagPersistenceService); tested.contributorMergedEventHandler(new ContributorMergedEvent(null, null)); verifyZeroInteractions(tested.customTagPersistenceService); reset(tested.customTagPersistenceService); tested.contributorMergedEventHandler(new ContributorMergedEvent("idFrom", null)); verifyZeroInteractions(tested.customTagPersistenceService); reset(tested.customTagPersistenceService); tested.contributorMergedEventHandler(new ContributorMergedEvent(null, "idTo")); verifyZeroInteractions(tested.customTagPersistenceService); reset(tested.customTagPersistenceService); tested.contributorMergedEventHandler(new ContributorMergedEvent("idFrom", "idTo")); verify(tested.customTagPersistenceService).changeOwnershipOfTags("idFrom", "idTo"); }