Whenever you need to implement a histogram this will save time. Just invoke update(K key) to add one more ocurrence to the underlying map. When you want the histogram in ascending order, invoke histogram() and you will get all the entries and number of ocurrences in a Entry<K,Integer> array.

HistoHashMap.java

import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class HistoHashMap<K> {
    private final Map<K,Integer> map = new HashMap<K, Integer>();

    public int update(K key) {
        int r = 1;
        if (map.containsKey(key)) {
            r = 1 + map.get(key);
        }
        map.put(key, r);
        return r;
    }
    
    public Integer get(K key) {
        return map.get(key);
    }
    
    public Entry<K,Integer>[] histogram() {
        Set<Entry<K, Integer>> entrySet = map.entrySet();
        @SuppressWarnings("unchecked")
        Entry<K, Integer>[] array = entrySet.toArray(new Entry[0]);
        Arrays.sort(array, new Comparator<Entry<K,Integer>>() {
            @Override
            public int compare(Entry<K, Integer> o1, Entry<K, Integer> o2) {
                return o1.getValue().compareTo(o2.getValue());
            }
        });
        return array;
    }
}