Collection<Document> () { Collection<Document> readOnlyDocs = new ArrayList<>(); for (UndoableAction action : myUndoableGroup.getActions()) { if (action instanceof MentionOnlyUndoableAction) continue; DocumentReference[] refs = action.getAffectedDocuments(); if (refs == null) continue; for (DocumentReference ref : refs) { if (ref instanceof DocumentReferenceByDocument docRef) { Document doc = docRef.getDocument(); if (!doc.isWritable()) readOnlyDocs.add(doc); } } } return readOnlyDocs; }