Get key of minimum value in a Hashtable

Question

I have a Hashtable in java like below and I'm trying to get the key that has the minimum value. Obviously I can iterate through all the elements to find it but is there an easier way to do it?

Hashtable<Object, Integer> hash= new Hashtable<Object, Integer>();

Answer 1

Using a Hashtable, no. But you could instead use a TreeMap.

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.

It has a method firstKey() which provides the exact functionality you want.

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Grr, values, not keys. No, then you will need to iterate.

I'd say in that case you should use a separate Map (Multimap?) to store the reverse association.

Map<Object, Integer> hash= new Hashtable<Object, Integer>();
SortedSetMultimap<Integer, Object> reverse = TreeMultimap.create();

whenever you put key, value something into hash, also put value, key into reverse. then retrieve the lowest value using reverse.keySet().first()

(this solution requires Guava)

Answer 2

Instead of iterating yourself you can use library function Collections.min(Collection,Comparator) over entrySet().

SAMPLE

public static void main(String[] args) {


    HashMap<String,Integer> map = new HashMap<String,Integer>();

    map.put("A", 1);
    map.put("B", 2);
    map.put("C", 3);

    System.out.println( 

    Collections.min(map.entrySet(), new Comparator<Map.Entry<String,Integer>>() {

        @Override
        public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2) {

            return o1.getValue().intValue() - o2.getValue().intValue();

        }})

        .getKey()


    );
}