How to remove duplicates elements from ArrayList in Java - Example

You can remove duplicates or repeated elements from ArrayList in Java by converting ArrayList into HashSet in Java. but before doing that just keep in mind that the set doesn't preserver insertion order which is guaranteed by List, in fact, that’s the main difference between List and Set in Java. So when you convert ArrayList to HashSet all duplicates elements will be removed but the insertion order will be lost. Let’s see this in action by writing a Java program to remove duplicates from ArrayList in Java.

In this Java collection tutorial, we will see both approaches of deleting duplicates from ArrayList e.g using HashSet and LinkedHashSet and compare the order of elements in the final ArrayList which contains no duplicates.

If you are not very familiar with What is an ArrayList and HashSet in Java collection framework, I suggest reading Java ArrayList Example and 10 HashSet Example in Java. These articles contain a good introduction to most commonly used collection in Java i.e. ArrayList and HashSet.

Java program to delete duplicates from ArrayList

Here is a quick example of removing duplicates from ArrayList in Java. Suppose you have an ArrayList of String which contains 4 Strings out of those ones is duplicate and we want to remove that duplicate:

//ArrayList with duplicates String
List<String> duplicateList = (List<String>) Arrays.asList("Android" , "Android", "iOS", "Windows mobile");
System.out.println("size of Arraylist with duplicates: " + duplicateList.size()); //should print 4 becaues of duplicates Android

System.out.println(duplicateList);
     
//Converting ArrayList to HashSet to remove duplicates
HashSet<String> listToSet = new HashSet<String>(duplicateList);
     
//Creating Arraylist without duplicate values
List<String> listWithoutDuplicates = new ArrayList<String>(listToSet);
System.out.println("size of ArrayList without duplicates: " + listToSet.size()); //should print 3 becaues of duplicates Android removed

System.out.println(listWithoutDuplicates);

Output:
size of Arraylist with duplicates: 4
[Android, Android, iOS, Windows mobile]
size of ArrayList without duplicates: 3
[Android, Windows mobile, iOS]

Now if you have noticed here duplicate entry "Android" has been removed from ArrayList but the order of ArrayList is not the same. 

Since we have converted ArrayList to HashSet we have lost the insertion order of elements. but don't worry there is another way of removing duplicates from ArrayList without losing the order of elements, that instead of HashSet we need to use LinkedHashSet, Which guarantees insertion order. 

Just remember checking whether ArrayList contains duplicates or not is completely different than removing it, which is what we are doing here. Here is another example of removing duplicate entries from ArrayList without losing insertion order or entries:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashSet;
import java.util.List;

public class RemoveDuplicatesFromArrayList {

    public static void main(String args[]) {
   
        //ArrayList with duplicates String
        List<String> duplicateList = (List<String>) Arrays.asList("Android" , "Android", "iOS", "Windows mobile");
       

        //should print 4 becaues of duplicates Android

        System.out.println("size of Arraylist with duplicates: " + duplicateList.size());      
        System.out.println("ArrayList with duplicates: " + duplicateList);
     
        //Converting ArrayList to HashSet to remove duplicates
        LinkedHashSet<String> listToSet = new LinkedHashSet<String>(duplicateList);
     
        //Creating Arraylist without duplicate values
        List<String> listWithoutDuplicates = new ArrayList<String>(listToSet);

        //should print 3 because of duplicates Android removed
        System.out.println("size of ArrayList without duplicates: " + listToSet.size());
        System.out.println("ArrayList after removing duplicates in same order: " + listWithoutDuplicates);
     
     
    }
 
}

Output:
size of Arraylist with duplicates: 4
ArrayList with duplicates: [Android, Android, iOS, Windows mobile]
size of ArrayList without duplicates: 3
ArrayList after removing duplicates in the same order: [Android, iOS, Windows mobile]

So now we know how to remove duplicates from ArrayList in Java and also know how to preserve the order of elements while removing duplicates from  ArrayList. If you don't prefer converting List to Set than you can still go with copying data from one ArrayList to other ArrayList and removing duplicates by checking with ArrayList.contains() method.

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