I think I understand why. You need to be able to set the Comparator in line 320 for PnlAbstractUniqueValue. (By the way, this is the only place you use comp, so there is no real need to keep a reference. You could delete line 291 and change 320 to newRL.setComparator(getComparator()).) And getComparator() is overridden only in PnlAbstractUniqueValue.