CODEWITHSUNDEEP
javajava-stream
Bhavya
Bhavya

Reputation: 16192

Find duplicate value in map using Java Stream API

I am trying to write a program that finds the duplicate value in map, and this map is created using the list and utiity methods.

I am able to get the expected output using for loop, but the code is too long using that.

I tried using the java stream API as shown below, but got empty results.

public class PrintListElements {
    public static void main(String[] args) {
        List<String> roles = new ArrayList<>();
        roles.add("1");
        roles.add("2");
        roles.add("3");
        roles.add("4");
        roles.add("5");
        roles.add("6");
        roles.add("7");
        roles.add("1");
        HashMap<String, List<String>> idToMap = new HashMap<>();
        roles.stream().map(role -> {
            if (idToMap.containsKey(role)) {
              return   idToMap.get(role).add(getName(role));
            } else {
                return idToMap.put(role, new ArrayList<>(Arrays.asList(getName(role))));
            }
        })

        idToMap.entrySet().forEach(e-> {
            if(e.getValue().size()>1) {
                System.out.println("found a key which has duplicate value : "+ e.getKey());
            }
        });

    }

    public static List<String> getNameLL(String id) {
        ArrayList<String> ll = new ArrayList<String>();
        ll.add(getName(id));
        return ll;
    }

    public static String getName(String id) {
        switch (id) {
            case "1":
                return "one";
            case "2":
                return "two";
            case "3":
                return "third";
            case "4":
                return "four";
            case "5":
                return "fifth";
            case "6":
                return "six";
            case "7":
                return "seven";
            case "8":
                return "one";
            default:
                return "one";
        }
    }
}

Expected Output:

[one, one]
[two]
[three]
[four]
[five]
[six]
[seven]

Can anyone please help me, to get the above-expected output result using Java Stream API

Upvotes: 1

Views: 1658

Answers (1)

Eklavya
Eklavya

Reputation: 18480

You can use Collectors.groupingBy to group by key and use Collectors.mapping to map values and collect as a list for each key.

Map<String, List<String>> idToMap = 
    roles.stream()
         .collect(Collectors.groupingBy(e -> e, 
                      Collectors.mapping(e -> getName(e), Collectors.toList())));

Or map operation is lazy, so code inside .map is not executed. You can use terminal operation like forEach by refactoring your current code,

roles.forEach(role -> {
    if (idToMap.containsKey(role)) {
       idToMap.get(role).add(getName(role));
    } else {
       idToMap.put(role, new ArrayList<>(Arrays.asList(getName(role))));
    }
});

which can be simplified using Map's merge method

roles.forEach(
    role -> idToMap.merge(role, new ArrayList<>(Arrays.asList(getName(role))), (a, b) -> {
      a.addAll(b);
      return a;
    }));

Update: If you want just print for duplicate value, you can use Collectors.counting() to get the frequency of key as a result collect as Map<String, Integer>

roles.stream()
     .collect(Collectors.groupingBy(e -> e, Collectors.counting()))
     .entrySet()
     .forEach(e -> {
          if (e.getValue() > 1) {
            System.out.println("found a key which has duplicate value : " + e.getKey());
          }
      });

Upvotes: 4

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