Remove duplicate values from HashMap in Java

Question

I have a map with duplicate values:

("A", "1");
("B", "2");
("C", "2");
("D", "3");
("E", "3");

I would like to the map to have

("A", "1");
("B", "2");
("D", "3");

Do you know how to get rid of the duplicate values?

At present, I get 'java.util.ConcurrentModificationException' error.

Thank you.

public static void main(String[] args) {

    HashMap<String, String> map = new HashMap<String, String>();
    map.put("A", "1");
    map.put("B", "2");
    map.put("C", "2");
    map.put("D", "3");
    map.put("E", "3");

    Set<String> keys = map.keySet(); // The set of keys in the map.

    Iterator<String> keyIter = keys.iterator();

    while (keyIter.hasNext()) {
        String key = keyIter.next();
        String value = map.get(key);

        System.out.println(key + "\t" + value);

        String nextValue = map.get(key);

        if (value.equals(nextValue)) {
            map.remove(key);
        }
    }
    System.out.println(map);
}

Why do you keep `B` and leave out `C`? Remember, `HashMap` doesn't maintain the order of elements inserted. — Rohit Jain, Jul 23 '13 at 14:03
Can you tell the exact requirement? Looks like you want unique values. HashMap can give you unique keys. Will inverting the key and value be enough for you? — midhunhk, Jul 23 '13 at 14:05

score 9 · Answer 1 · edited May 11 '19 at 16:04

9

make a reverse HashMap!

HashMap<String, String> map = new HashMap<String, String>();
Set<String> keys = map.keySet(); // The set of keys in the map.

Iterator<String> keyIter = keys.iterator();

while (keyIter.hasNext()) {
    String key = keyIter.next();
    String value = map.get(key);
    map.put(value, key);
}

now that you have the hashMap you need reverse it or print it.

in anyway do not delete while iterating hashMap. save the values in a list and delete them in an outer loop

edited May 11 '19 at 16:04

Community

1
1

answered Jul 23 '13 at 14:06

No Idea For Name

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3

It will still be random which element will be kept (because the order of a `HashMap` is undefined), but if that's no issue, this works fine. – Vincent van der Weele Jul 23 '13 at 14:17
2

@Heuster i agree, but he didn't said it's an issue – No Idea For Name Jul 23 '13 at 15:04
@NoIdeaForName why there is map.add() and not map.put() – bot13 May 05 '19 at 15:36
@bot13 can't say i remember if there was a reason for this, it was 6 years back. works with `put` as well. – No Idea For Name May 07 '19 at 08:49
It is only possible if both key and value are of same type – Saroj Kumar Sahoo Feb 26 '20 at 08:04

Nicolas Filotto · Answer 2 · 2017-06-22T15:33:05.527

8

Assuming that you use Java 8, it could be done using the Stream API with a Set<String> that will store the existing values:

Map<String, String> map = new HashMap<>();
map.put("A", "1");
...
System.out.printf("Before: %s%n", map);

// Set in which we keep the existing values
Set<String> existing = new HashSet<>();
map = map.entrySet()
    .stream()
    .filter(entry -> existing.add(entry.getValue()))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
System.out.printf("After: %s%n", map);

Output:

Before: {A=1, B=2, C=2, D=3, E=3}
After: {A=1, B=2, D=3}

NB: Strictly speaking a predicate of a filter is not supposed to be stateful, it should be stateless as mentioned into the javadoc in order to ensure that the result remain deterministic and correct even if we use a parallel stream. However here, I assume that you don't intend to use a parallel stream such that this approach remains valid.

edited Jun 22 '17 at 15:33

answered Oct 19 '16 at 14:53

Nicolas Filotto

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Strictly speaking streams shouldn't really be used with a side-effecting filter. – Graeme Moss Jun 22 '17 at 14:22
@GraemeMoss that's right, I added a comment to avoid misuse/misunderstanding – Nicolas Filotto Jun 22 '17 at 15:34
You can replace .filter() by .distinct() – Diggi55 Nov 26 '20 at 09:59

score 5 · Answer 3 · answered Oct 01 '14 at 06:41

5

    Map<String,Object> mapValues = new HashMap<String,Object>(5);
    mapValues.put("1", "TJ");
    mapValues.put("2", "Arun");
    mapValues.put("3", "TJ");
    mapValues.put("4", "Venkat");
    mapValues.put("5", "Arun");

    Collection<Object> list = mapValues.values();
    for(Iterator<Object> itr = list.iterator(); itr.hasNext();)
    {
        if(Collections.frequency(list, itr.next())>1)
        {
            itr.remove();
        }
    }

answered Oct 01 '14 at 06:41

Thananjayan N

77
1
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Some explanation or links for further details would be helpful. Especially if asked why some Exception thrown there is need. – hc_dev Feb 14 '19 at 21:53

score 2 · Answer 4 · edited May 23 '17 at 11:33

2

ConcurrentModificationException happening,because you are removing from map

  if (value.equals(nextValue)) {
            map.remove(key);
        }

You have to remove from iterator

if (value.equals(nextValue)) {
            keyIter.remove(key);
        }

Coming to the duplicate entry issue,Its pretty simple :Find duplicate values in Java Map?

edited May 23 '17 at 11:33

Community

1
1

answered Jul 23 '13 at 14:03

Suresh Atta

120,458
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3

Well, if you see his code clearly, this won't really solve his problem. – Rohit Jain Jul 23 '13 at 14:09
This code is wrong , it won't compile and neither does it solves the problem . – AllTooSir Jul 23 '13 at 14:11
iterator ,`Iterator keyIter = keys.iterator();` – Suresh Atta Jul 23 '13 at 14:15
And what is [keyIter.remove(key)](http://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html#remove()) ? – AllTooSir Jul 23 '13 at 14:15
@TheNewIdiot :That avoids the exception `The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.` – Suresh Atta Jul 23 '13 at 14:18
Please go through the syntax first and then the documentation . – AllTooSir Jul 23 '13 at 14:18
Please feel free to edit, If you found some mistake,Or please convey the exact reason why I it won't compile,It's working fine In my IDE. – Suresh Atta Jul 23 '13 at 14:26

score 2 · Answer 5 · answered Mar 04 '18 at 07:38

This can be done using Java 8. The concept of stream is required. The pseudocode, is stream().filter().collect(). If the Initial Map : {A=1, B=2, C=2, D=3, E=3}. Then the required answer after removing the duplicates is {A=1, B=2, D=3} .

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class RemoveDuplicates1 {
   public static void main(String[] args) {

        //Initial Map : {A=1, B=2, C=2, D=3, E=3}
        //After =>  {A=1, B=2, D=3} 

      Map<String , String > map = new HashMap<>();
        map.put("A", "1");
        map.put("B", "2");
        map.put("C", "2");
        map.put("D", "3");
        map.put("E", "3");

        System.out.printf("before :   " +map );
        System.out.println("\n");

        Set<String> set = new  HashSet<>();

        map = map.entrySet().stream()
                .filter(entry -> set.add(entry.getValue()))
                .collect(Collectors.toMap(Map.Entry :: getKey ,  Map.Entry :: getValue));
        System.out.printf("after => " + map);

   }
}

score 1 · Answer 6 · answered Jul 23 '13 at 14:47

1

If this is yours frequent requirement then DualHashBidiMap calss of apache's commons.collections will help you more instead of using HashMap.

answered Jul 23 '13 at 14:47

Prabhaker A

8,317
1
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score 1 · Answer 7 · answered Aug 06 '14 at 13:26

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    map.put("A", "1");
    map.put("B", "2");
    map.put("C", "2");
    map.put("D", "3");
    map.put("E", "3");
    System.out.println("Initial Map : " + map);
    for (String s : new ConcurrentHashMap<>(map).keySet()) {
        String value = map.get(s);
        for (Map.Entry<String, String> ss : new ConcurrentHashMap<>(map)
                .entrySet()) {
            if (s != ss.getKey() && value == ss.getValue()) {
                map.remove(ss.getKey());
            }
        }
    }
    System.out.println("Final Map : " + map);
}

score 0 · Answer 8 · answered Feb 06 '14 at 05:28

This can be easily done by putting your hashmap into arraylist. This arraylist is of hashmap type.

ArrayList<HashMap<String, String>> mArrayList=new ArrayList<>();
HashMap<String, String> map=new HashMap<>();
map.put("1", "1");
        mArrayList.add(map);
        map=new HashMap<>();
        map.put("1", "1"); 
        mArrayList.add(map);
        map=new HashMap<>();
        map.put("1", "2");
        mArrayList.add(map);
        map=new HashMap<>();
        map.put("1", "3");
        mArrayList.add(map);
        map=new HashMap<>();
        map.put("1", "2");
        mArrayList.add(map);

for(int i=0;i<mArrayList.size();i++)
        {
            temp=mArrayList.get(i).get("1");
            for(int k=i+1;k<mArrayList.size();k++)
            {
                if(temp.equals(mArrayList.get(k).get("1")))
                {
                    mArrayList.remove(k); 
                } 
            }

        }

Now print your arraylist...all the duplicate values from the hashmap easily removed...This is the easiest way to remove duplicacy

Santhanam · Answer 9 · 2014-04-24T10:34:36.257

This will be helpful to remove duplicate values from map.

    Map<String, String> myMap = new TreeMap<String, String>();
    myMap.put("1", "One");
    myMap.put("2", "Two");
    myMap.put("3", "One");
    myMap.put("4", "Three");
    myMap.put("5", "Two");
    myMap.put("6", "Three");

    Set<String> mySet = new HashSet<String>();

    for (Iterator itr = myMap.entrySet().iterator(); itr.hasNext();)
    {
        Map.Entry<String, String> entrySet = (Map.Entry) itr.next();

        String value = entrySet.getValue();

        if (!mySet.add(value))
        {
            itr.remove();               
        }
    }

System.out.println("mymap :" + mymap);

Output:

mymap :{1=One, 2=Two, 4=Three}

Einstein_AB · Answer 10 · 2019-02-17T19:03:27.120

0

If you are looking just to remove the concurrentModification exception, then just replace your HashMap with ConcurrentHashMap.

To know more about ConcurrentHashMap look here

edited Feb 17 '19 at 19:03

answered Jan 23 '19 at 13:41

Einstein_AB

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score 0 · Answer 11 · answered Jul 16 '22 at 09:26

It can be done without mutating the original map:

public static <K,V> Map<K,V> getDistinctValues(Map<K,V> map) {
    Set<V> values = new HashSet<>();

    return map.entrySet().stream()
            .filter(entry -> values.add(entry.getValue()))
            .collect(Collectors.toMap(
                    Map.Entry::getKey,
                    Map.Entry::getValue
            ));
}

Remove duplicate values from HashMap in Java

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