persistable sclass SynchronizedSortedMap<K,V>
    extends SynchronizedMap<K,V>
    implements SortedMap<K,V>
{
    // not final because persistence
    private /*final*/ SortedMap<K,V> sm;

    SynchronizedSortedMap(SortedMap<K,V> m) {
        super(m);
        sm = m;
    }
    SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
        super(m, mutex);
        sm = m;
    }

    public Comparator<? super K> comparator() {
        synchronized (mutex) {return sm.comparator();}
    }

    public SortedMap<K,V> subMap(K fromKey, K toKey) {
        synchronized (mutex) {
            return new SynchronizedSortedMap<>(
                sm.subMap(fromKey, toKey), mutex);
        }
    }
    public SortedMap<K,V> headMap(K toKey) {
        synchronized (mutex) {
            return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
        }
    }
    public SortedMap<K,V> tailMap(K fromKey) {
        synchronized (mutex) {
           return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
        }
    }

    public K firstKey() {
        synchronized (mutex) {return sm.firstKey();}
    }
    public K lastKey() {
        synchronized (mutex) {return sm.lastKey();}
    }
}

Began life as a copy of #1031164