How to remove duplicate elements from an ArrayList in Java

How to remove duplicate elements from an ArrayList in Java.

Ø  ArrayList allows adding duplicate elements, sometime there may be a requirement to remove duplicates from a list.

Ø  In this post will talk about the way to do it.

1^St  Approach:

Ø  First thing  that will come to mind is create a new list and loop through the created old list.

Ø  Use  contains() method to check if element is already existed to the new list.

Ø  If element is  not existed then add it to the new list otherwise discard it.

Program:

                  package com.coding.core;

                  import java.util.ArrayList;

                  import java.util.List;

              public class RemoveDuplicateElements {

              public static void main(String[] args) {

List<String> gameList = new ArrayList<String>();

gameList.add("Portal");

gameList.add("Chess");

gameList.add("Roulette");

gameList.add("poker");

gameList.add("poker");

gameList.add("poker");

gameList.add("Carrom");

// Create a new ArrayList

System.out.println("********************************************************");

List<String> newGameList = new ArrayList<String>();

for (String gameName : gameList) {

if (!newGameList.contains(gameName)) {

newGameList.add(gameName);

}

System.out.println("Duplicate gameList : " + gameName);

}

System.out.println("********************************************************");

for (String uniqueGameName : newGameList) {

System.out.println("Game Names : " + uniqueGameName);

}

System.out.println("********************************************************");

}

   OUTPUT :

  ********************************************************************************

                         Game Name : Portal

                         Game Name : Chess

                         Game Name : Roulette

                         Game Name : poker

                         Game Name : Carrom

    

  ********************************************************************************

  

2^nd Approach :  

    But that much code is really not required.

    Collection framework itself provides several options to remove duplicates from an arraylist.

    We can simply use a HashSet to do the trick of removing the duplicate elements.

    HashSet only allows unique elements and we'll use that feature of HashSet to remove duplicates.

    Only problem is it won't retain the order of the list.

   Program:

    package com.coding.core;

    import java.util.ArrayList;

    import java.util.HashSet;

    import java.util.List;

    import java.util.Set;

    public class RemovingDuplicatesByUsingHashSet {

public static void main(String[] args) {

List<String> gameList = new ArrayList<String>();

gameList.add("Portal");

gameList.add("Chess");

gameList.add("Roulette");

gameList.add("poker");

gameList.add("poker");

gameList.add("poker");

gameList.add("Carrom");

// Create HashSet

Set<String> gameSet = new HashSet<String>(gameList);

// remove all the elements from the list

gameList.clear();

gameList.addAll(gameSet);

System.out.println("********************************************************");

for (String name : gameList) {

System.out.println("Game Name - " + name);

}

System.out.println("********************************************************");

}

     ========================================================================

    If  you want retain the order then you will go with LinkedHashSet:

    Please find below program:

    package com.coding.core;

    import java.util.ArrayList;

    import java.util.HashSet;

    import java.util.LinkedHashSet;

    import java.util.List;

    import java.util.Set;

    public class RemovingDuplicatesByUsingHashSet {

public static void main(String[] args) {

// TODO Auto-generated method stub

List<String> gameList = new ArrayList<String>();

gameList.add("Portal");

gameList.add("Chess");

gameList.add("Roulette");

gameList.add("poker");

gameList.add("poker");

gameList.add("poker");

gameList.add("Carrom");

// Create HashSet

//Set<String> gameSet = new HashSet<String>(gameList);

Set<String> gameSet = new LinkedHashSet<String>(gameList);

// remove all the elements from the list

gameList.clear();

gameList.addAll(gameSet);

System.out.println("********************************************************");

for (String name : gameList) {

System.out.println("Game Name - " + name);

}

System.out.println("********************************************************");

} }

    Out Put :

   ********************************************************

     Game Name - Portal

    Game Name - Chess

    Game Name - Roulette

    Game Name - poker

    Game Name - Carrom

  ********************************************************

Java 8 streams provide a very simple way to remove duplicate elements from a list. 

Using the distinct method.

package com.coding.core.sort;

import java.util.ArrayList;

import java.util.List;

import java.util.stream.Collectors;

public class Java8RemoveDuplicates {

public static void main(String[] args) {

// TODO Auto-generated method stub

List<String> gameList = new ArrayList<String>();

gameList.add("Portal");

gameList.add("Chess");

gameList.add("Roulette");

gameList.add("poker");

gameList.add("poker");

gameList.add("poker");

gameList.add("Carrom");

gameList = gameList.stream().distinct().collect(Collectors.toList());

System.out.println("*****************************************************");

for (String name : gameList) {

System.out.println("Game Name - " + name);

}

System.out.println("******************************************************");

}

}}

output:

     ******

   ********************************************************

        Game Name - Portal

        Game Name - Chess

        Game Name - Roulette

        Game Name - poker

        Game Name - Carrom

   ********************************************************

you can see that the duplicate element is removed and original order is retained too and all that is done in a single line.