Find duplicate value in array list and print the duplicated value

Question

I have an Arraylist of names, I want to see duplicated values if exist and print this value. The problem is that I'm getting confused on whether to use contains method or not, the below code is not working.

ArrayList<String> list=new ArrayList();
        list.add("Sagio Mane");
        list.add("Karius");
        list.add("Mo Salah");
        list.add("Firmino");
        list.add("Lovren");
        list.add("Steven Gerrard");
        list.add("Karius");
        list.add("Mo Salah");

    for(int i =0; i < list.size(); i++) {
         if list.contains(list.get(i)) {
             System.out.println(list.get(i)+" is duplicated")
         }
    }

This should print "karius is duplicated"

First of all, describe algorythm you want to apply to get duplicated words list — WeGa
– WeGa, Commented May 27, 2018 at 7:36
you can't use that method unless you remove the searched string element first, then use "contains", then add it back — mangusta
– mangusta, Commented May 27, 2018 at 7:38
karius and Mo Salah are duplicated (but not only karius) — Oleg Cherednik
– Oleg Cherednik, Commented May 27, 2018 at 7:50
You had some syntax error in your error, I have modified your code. — Nitin Bisht
– Nitin Bisht, Commented May 28, 2018 at 15:06

Youcef LAIDANI · Accepted Answer · 2018-05-27 08:06:49Z

9

If you are using Java 8+ you can use :

//Get frequencies of each element
Map<String, Long> frequencies = list.stream()
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

//then filter only the inputs which have frequency great than 1
frequencies.entrySet().stream()
        .filter(entry -> entry.getValue() > 1)
        .forEach(entry -> System.out.println(entry.getKey()));

Outputs

Karius
Mo Salah

answered May 27, 2018 at 8:06

Youcef LAIDANI

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Comments

Nitin Bisht · Accepted Answer · 2018-05-28 15:14:24Z

6

Try this working code:

ArrayList<String> list = new ArrayList<String>();
        list.add("Sagio Mane");
        list.add("Karius");
        list.add("Mo Salah");
        list.add("Firmino");
        list.add("Lovren");
        list.add("Steven Gerrard");
        list.add("Karius");
        list.add("Mo Salah");

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

        for(String name : list) {
            if(s.add(name) == false)
                System.out.println(name + "is duplicated");
        }

Output:

Kariusis duplicated
Mo Salahis duplicated

answered May 28, 2018 at 15:14

Nitin Bisht

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Comments

Joakim Danielson · Accepted Answer · 2018-05-27 07:41:51Z

3

You can use lastIndexOf to check for duplicates

for(int i =0; i < list.size(); i++) {
  if (list.lastIndexOf(list.get(i)) != i)  {
     System.out.println(list.get(i)+" is duplicated");
  }
}

Note that if you have triplicates or more this will print "xxx is duplicated" several times for the same name but you only asked to check for duplicates so this solution should be enough.

answered May 27, 2018 at 7:41

Joakim Danielson

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Comments

Anshul Tyagi · Accepted Answer · 2022-07-19 20:38:54Z

1

If you want to reduce some time complexity at the cost of O(n) space, you can use a Set

Set<String> set = new HashSet<>();
for(int i =0; i < list.size(); i++) {
    if (set.contains(list.get(i))) {
         System.out.println(list.get(i)+" is duplicated");
    } else set.add(list.get(i));
}

edited Jul 19, 2022 at 20:38

Anshul Tyagi

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answered May 27, 2018 at 7:51

stinepike

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1 Comment

shinjw Over a year ago

Set::add returns a boolean to specify if the record was added or not. Set::contains wouldn't be necessary.

Oleg Cherednik · Accepted Answer · 2018-05-27 08:03:22Z

0

You can use two sets. This is bettwer for performance O(n), because you have to loop an array only once, but less effective with the memory O(n):

public static Set<String> getDuplicates(List<String> list) {
    Set<String> res = new HashSet<>();
    Set<String> existed = new HashSet<>();

    list.forEach(str -> {
        if (!existed.add(str))
            res.add(str);
    });

    return res;
}

Another option, is to use streams:

public static Set<String> getDuplicates(List<String> list) {
    return list.stream().filter(i -> Collections.frequency(list, i) > 1).collect(Collectors.toSet());
}

edited May 27, 2018 at 8:03

answered May 27, 2018 at 7:48

Oleg Cherednik

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5 Comments

mangusta Over a year ago

why does he need to use two sets? one hashset is enough - for each element in the list: he takes an element, checks whether it has been added to the set before, if yes, then output it, otherwise put it into hashset

Oleg Cherednik Over a year ago

@mangusta In case print to the console is what you need - yes, one set is enough. But this is not my style, I prefer to prepare data and then do what I want. This method retrieved duplicated and does not responsible for output to console.

Oleg Cherednik Over a year ago

@shinjw No :-). Try to think why.

shinjw Over a year ago

I'd recommend mentioning the time complexity for the stream solution.

shinjw Over a year ago

Set<String> set = new HashSet<>(); return list.filter(v -> !set.add(v)).collect(Collectors.toSet()); Would be more optimal in runtime.

Faiz Ahmed · Accepted Answer · 2018-05-27 08:30:50Z

you can follow this Find the duplicate elements in arraylist and display

 public static void main(String[] args) {
    ArrayList<String> list = new ArrayList();
    list.add("Sagio Mane");
    list.add("Karius");
    list.add("Mo Salah");
    list.add("Firmino");
    list.add("Lovren");
    list.add("Steven Gerrard");
    list.add("Karius");
    list.add("Mo Salah");

    HashMap<String, Integer> map = new LinkedHashMap<>(); // If you want insertion order same

    list.forEach((string) -> {
        if (map.containsKey(string)) {
            map.put(string, map.get(string) + 1);
        } else {
            map.put(string, 1);
        }
    });
    for (Map.Entry<String, Integer> entry : map.entrySet()) {
        String key = entry.getKey();
        Integer value = entry.getValue();
        if (value > 1) {
            System.out.println(key + " is duplicated");
        }
    }
}

anothernode · Accepted Answer · 2018-05-27 09:51:46Z

0

List<String> newList = new ArrayList();

Java 8:

oldList.forEach( {

    if (newList.contains(e)) {
        Sytem.out.println(e+" is duplicated");
    } else {
        newList.add(e);
    }
});

Java 7: you can use customised for loop to get each from oldList.

edited May 27, 2018 at 9:51

anothernode

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answered May 27, 2018 at 8:13

Tarun Sawlani

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Comments

Harish Kotikalapudi · Accepted Answer · 2021-08-04 15:36:38Z

0

import java.util.ArrayList;

import java.util.HashMap;

public class CollectionsEx2 {

public static void main(String[] args) {
    ArrayList<String> list=new ArrayList();
    list.add("Sagio Mane");
    list.add("Karius");
    list.add("Mo Salah");
    list.add("Firmino");
    list.add("Lovren");
    list.add("Steven Gerrard");
    list.add("Karius");
    list.add("Mo Salah");
    
    HashMap<String,Integer> hm = new HashMap<String,Integer>();
    for(String i: list) {
        if (hm.containsKey(i)) hm.put(i, hm.get(i)+1);
        else hm.put(i, 1);
    }
    System.out.println("HashMap:" + hm);

}

}

O/p: HashMap:{Firmino=1, Sagio Mane=1, Lovren=1, Karius=2, Steven Gerrard=1, Mo Salah=2}

answered Aug 4, 2021 at 15:36

Harish Kotikalapudi

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