@Override public int compare(PeerReviewDuplicateGroup x, PeerReviewDuplicateGroup y) { PeerReviewSummary summaryX = (PeerReviewSummary) x.getActivities().iterator().next(); PeerReviewSummary summaryY = (PeerReviewSummary) y.getActivities().iterator().next(); if (summaryX.getCompletionDate() == null && summaryY.getCompletionDate() == null) { return 0; } if (summaryY.getCompletionDate() == null) { return -1; } if (summaryX.getCompletionDate() == null) { return 1; } return summaryY.getCompletionDate().compareTo(summaryX.getCompletionDate()); }