### Question:
SystemUserNotificationFilter implements NotificationFilter, ToggleableNotificationFilter { @Override public ExpressionNode filterExpression(DocumentReference user, Collection<NotificationFilterPreference> filterPreferences, NotificationPreference preference) { return null; }  @Override FilterPolicy filterEvent(Event event, DocumentReference user,
            Collection<NotificationFilterPreference> filterPreferences, NotificationFormat format); @Override boolean matchesPreference(NotificationPreference preference); @Override ExpressionNode filterExpression(DocumentReference user,
            Collection<NotificationFilterPreference> filterPreferences, NotificationPreference preference); @Override ExpressionNode filterExpression(DocumentReference user,
            Collection<NotificationFilterPreference> filterPreferences,
            NotificationFilterType type, NotificationFormat format); @Override String getName(); static final String FILTER_NAME; }### Answer:
@Test public void filterExpression() throws Exception { NotificationPreference fakePreference = mock(NotificationPreference.class); assertNull(mocker.getComponentUnderTest().filterExpression(randomUser, Collections.emptyList(), fakePreference)); assertEquals("USER <> \"serializedSystemUser\"", mocker.getComponentUnderTest().filterExpression(randomUser, Collections.emptyList(), NotificationFilterType.EXCLUSIVE, null).toString()); }