This gist comprises of the code that i use to answer this SO question: http://stackoverflow.com/questions/17584008/find-most-common-value-from-hashmap-of-set-in-java

CommonMapSetValues.java

import java.util.*;

public class CommonMapSetValues {

  public static void main(String[] args) {
		CommonMapSetValues obj = new CommonMapSetValues();
		obj.addKeyValue("abc", new String[] {"ax1","au2","au3"});
		obj.addKeyValue("def", new String[] {"ax1","au6"});
		obj.addKeyValue("ijk", new String[] {"ax1","au2"});
		System.out.println(obj.toString());
		System.out.println(obj.getFrequencyMap());
		System.out.println(obj.getFrequencyMap().getCommon(2));
		System.out.println(obj.getFrequencyMap().getCommon(3));
		System.out.println(obj.getFrequencyMap().getCommon(4));
	}

	private Map<String, Set<String>> map;

	public CommonMapSetValues() {
		map = new HashMap<String, Set<String>>();
	}

	public void addKeyValue(String key, String[] valueArray) {
		Set<String> valueSet;
		if(map.containsKey(key)) {
			valueSet = map.get(key);
		} else {
			valueSet = new HashSet<String>();
			map.put(key, valueSet);
		}
		Collections.addAll(valueSet, valueArray);
	}

	FrequencyMap getFrequencyMap() {
		Map<String, Integer> frequencies = new HashMap<String, Integer>();
		for(String key : this.map.keySet()) {
			for(String element : this.map.get(key)) {
				int count;
				if(frequencies.containsKey(element)) {
					count = frequencies.get(element);
				} else {
					count = 1;
				}
				frequencies.put(element, count + 1);
			}
		}
		return new FrequencyMap(frequencies);
	}

	@Override public String toString() {
		return map.toString();
	}
}

class FrequencyMap implements Comparator<String> {

	Map<String, Integer> map;
    public FrequencyMap(Map<String, Integer> map) {
        this.map = map;
    }

	public int compare(String a, String b) {
        if (map.get(a) >= map.get(b)) {
            return -1;
        } else {
            return 1;
        } // returning 0 would merge keys
    }

    public ArrayList<String> getCommon(int threshold) {
    	ArrayList<String> common = new ArrayList<String>();
    	for(String key : this.map.keySet()) {
    		if(this.map.get(key) >= threshold) {
    			common.add(key);
    		}
    	}
    	return common;
    }

    @Override public String toString() {
		return this.map.toString();
	}
}

CommonMapSetValues.out

{abc=[ax1, au3, au2], def=[ax1, au6], ijk=[ax1, au2]}
{ax1=4, au6=2, au3=2, au2=3}
[ax1, au6, au3, au2]
[ax1, au2]
[ax1]
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