Get k highest values from HashMap

Question

In the code I have posted below, I need to remove the duplicates from the HashMap (the highest alphabetical value gets to stay in the map) and print the keys of the k highest values after the duplicates are removed. How do I do this? I tried with a HashSet but I am pretty clueless.

public ArrayList<String> mostOften(int k)
    {
        ArrayList<String> lista = new ArrayList<String>();
        HashMap<String,Integer> temp = new HashMap<String, Integer>();

        for(String it : wordList)
        {
            if(temp.containsKey(it))
                temp.put(it, temp.get(it)+1);
            else
                temp.put(it, 1);
        }

        temp = sortByValues(temp);

        Set<Integer> set = new HashSet<Integer>(temp.values());
        System.out.println(set);

        return lista;
    }

    private static HashMap sortByValues(HashMap map) 
    { 
        List list = new LinkedList(map.entrySet());
        Collections.sort(list, new Comparator() 
                         {
                             public int compare(Object o1, Object o2) 
                             {
                                 return ((Comparable)((Map.Entry) (o1)).getValue()).compareTo(((Map.Entry) (o2)).getValue());
                             }
                         });

        HashMap sortedHashMap = new LinkedHashMap();
        for (Iterator it = list.iterator(); it.hasNext();) 
        {
            Map.Entry entry = (Map.Entry) it.next();
            sortedHashMap.put(entry.getKey(), entry.getValue());
        } 
        return sortedHashMap;
    }

`HashMap` is unordered by designed. Why not use a collection which is designed to be sorted like `TreeMap`? — Peter Lawrey, Dec 11 '15 at 23:14
It says in the definition, the key is a string and the value is an integer. — Hydroxis, Dec 11 '15 at 23:14
see my answer there: http://stackoverflow.com/questions/34233528/java-get-top-15-from-hashmap-by-rule/34233828#34233828 It inverts value and key and sort by value — guillaume girod-vitouchkina, Dec 11 '15 at 23:15
@PeterLawrey because I need it to be ordered by the values, not by the keys. — Hydroxis, Dec 11 '15 at 23:15
or sort a MultiMap (inverting key and value), by this: http://stackoverflow.com/questions/5501468/having-a-multimap-sorted-on-keys-only-in-java — guillaume girod-vitouchkina, Dec 11 '15 at 23:23

Peter Lawrey · Answer 1 · 2015-12-11T23:30:59.493

6

If you are trying to do a frequency count of words you are heading down the wrong road. Java 8 does this much easier and cleaner.

You need these imports

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

The code

public static void main(String[] args) {
    printTopWords(Arrays.asList("Hello World Hello , Bye World".split(" ")), 2);
}

public static void printTopWords(List<String> words, int limit) {
    // using the Stream API
    words.stream()
            // create a map of words with the count of those words
            .collect(Collectors.groupingBy(w -> w, Collectors.counting()))
            // take that map as a stream of entries
            .entrySet().stream()
            // sort them by count in reverse order
            .sorted(Comparator.comparing(Map.Entry<String, Long>::getValue).reversed())
            // limit the number to get top Strings
            .limit(limit)
            // keep just the key ie. drop the count.
            .map(Map.Entry::getKey)
            // print them
            .forEach(System.out::println);
}

prints

Hello
World

edited Dec 11 '15 at 23:30

answered Dec 11 '15 at 23:20

Peter Lawrey

525,659
79
751
1,130

Do I have to import anything? I am getting – Hydroxis Dec 11 '15 at 23:25
illegal start of expression .collect(Collectors.groupingBy(w -> w, Collectors.counting())) – Hydroxis Dec 11 '15 at 23:25
@Hydroxis Your IDE should help you put in the right imports. I don't look at them very often as it does it for me. I have added them to my answer. – Peter Lawrey Dec 11 '15 at 23:31
It is possible that we do not have Java 8 on the platform I have to submit my solution to :/ – Hydroxis Dec 11 '15 at 23:45
@Hydroxis what you can do is break up the problem the same way. It would take more code but it would still work. – Peter Lawrey Dec 11 '15 at 23:49
I already did - I have a sorted HashMap. Now I need to print the last K keys from behind, which was my question. – Hydroxis Dec 12 '15 at 00:10

Madushan Perera · Answer 2 · 2015-12-12T00:46:45.240

If you are not familiar with java 8 streams and lambdas then below answer would be helpful to you :

public class Java7Way {

public static void main(String[] args) {
    Map<String, Integer> myMap = new HashMap<>();
    myMap.put("A", 20);
    myMap.put("A", 38);
    myMap.put("B", 40);
    myMap.put("K", 23);
    System.out.println(sortByValue(myMap,2).toString());
}

public static <K, V extends Comparable<? super V>> Map<K, V>
        sortByValue(Map<K, V> map,int limit) {
    List<Map.Entry<K, V>> list
            = new LinkedList<>(map.entrySet());
    Collections.sort(list, new Comparator<Map.Entry<K, V>>() {
        @Override
        public int compare(Map.Entry<K, V> o1, Map.Entry<K, V> o2) {
            return (o1.getValue()).compareTo(o2.getValue());
        }
    }
            .reversed()//to arrange it in decending order
    );
    Map<K, V> result = new LinkedHashMap<>();//maintains the order which the entries were put into the map
    for (Map.Entry<K, V> entry : list) {
        if (limit == 0) {
            break;
        }
        result.put(entry.getKey(), entry.getValue());
        limit--;
    }
    return result;
}

}

Out-put :

{B=40, A=38}

Sorry Updated my answer :) – Madushan Perera Dec 12 '15 at 00:34 — Madushan Perera, Dec 12 '15 at 00:34

Slava Vedenin · Answer 3 · 2015-12-11T23:37:47.687

I recommend using TreeBidiMap from Apache Commons Collection. In this structure all keys and all values sorted according to the natural order for the key's and value's classes.

For your code:

    BidiMap<String,Integer> temp = new TreeBidiMap<String, Integer>();
    for(String it : wordList)
    {
        if(temp.containsKey(it))
            temp.put(it, temp.get(it)+1);
        else
            temp.put(it, 1);
    }

    // print values unsing natural sorting in reverse order          
    BidiMap inverse = temp.inverseBidiMap();
    for (MapIterator it = inverse.mapIterator(); it.hasPrevious();) {
       String k = it.next();
       Integer s = it.getValue();  
       System.out.printLn(s + " = " + k);
    }

Get k highest values from HashMap

3 Answers3