WroModel { public final WroModel setGroups(final Collection<Group> groups) { notNull(groups, "groups cannot be null!"); LOG.debug("setGroups: {}", groups); identifyDuplicateGroupNames(groups); this.groups = new TreeSet<Group>(groups); return this; }  final Collection<Group> getGroups(); final WroModel setGroups(final Collection<Group> groups); void merge(final WroModel importedModel); WroModel addGroup(final Group group); @Override boolean equals(final Object obj); @Override int hashCode(); @Override String toString();  }
@Test(expected = NullPointerException.class) public void cannotSetNullGroups() { victim.setGroups(null); }