TreeMap:

TreeMap is a Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

This implementation provides guaranteed log(n) time cost for the containsKey, get, putand remove operations.

SortedMap<String, String> fileExtensions = new TreeMap<>(new Comparator<String>() {
            @Override
            public int compare(String s1, String s2) {
                return s2.compareTo(s1);
            }
        });