CODEWITHSUNDEEP
javaarraylist
Valek
Valek

Reputation: 115

Find duplicate value in array list and print the duplicated value

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"

Upvotes: 8

Views: 40926

Answers (8)

stinepike
stinepike

Reputation: 54672

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));
}

Upvotes: 1

Harish Kotikalapudi
Harish Kotikalapudi

Reputation: 29

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}

Upvotes: 0

Nitin Bisht
Nitin Bisht

Reputation: 5331

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

Upvotes: 6

Tarun Sawlani
Tarun Sawlani

Reputation: 34

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.

Upvotes: 0

Faiz Ahmed
Faiz Ahmed

Reputation: 415

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");
        }
    }
}

Upvotes: 0

Youcef LAIDANI
Youcef LAIDANI

Reputation: 59950

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

Upvotes: 8

Oleg Cherednik
Oleg Cherednik

Reputation: 18245

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());
}

Upvotes: 0

Joakim Danielson
Joakim Danielson

Reputation: 51892

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.

Upvotes: 3

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