@NonNull
    private String getComparable(final int position) {
        if (position < 0 || position >= list.size()) {
            return " ";
        }
        try {
            return sortContext.getSort().getComparator().getSortableSection(list.get(position));
        } catch (NullPointerException e) {
            return " ";
        }
    }
detect whether all caches in this list belong to a series with similar names
main/src/main/java/cgeo/geocaching/ui/CacheListAdapter.java