public int groupSize(final int group) {
        final Integer start = this.groupToStart.get(group);
        if (start == null) {
            return 0;
        }
        final Map.Entry<Integer, Integer> endEntry = this.groupToStart.higherEntry(group);
        return endEntry == null ? size() - start : endEntry.getValue() - start;
    }
Returns number of elements for given group, or 0 if group isn't present in list
main/src/main/java/cgeo/geocaching/utils/GroupedList.java