How to remove duplicate elements in ArrayList

Learn to remove duplicate elements in ArrayList in Java using different techniques such as LinkedHashSet in Collections framework and using java 8 stream apis.

1. Remove duplicate elements in arraylist using LinkedHashSet

The LinkedHashSet is the best approach for removing duplicate elements in an arraylist. LinkedHashSet does two things internally :

• Remove duplicate elements
• Maintain the order of elements added to it

Java example to remove duplicates in arraylist using LinkedHashSet. In given example, numbersList is an arraylist containing integers and some of them are duplicate numbers e.g. 1, 3 and 5. We add the list to LinkedHashSet, and then get back the content back into the list. The result arraylist does not have duplicate integers.

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

public class ArrayListExample 
{
    public static void main(String[] args) 
    {
        // ArrayList with duplicate elements
        ArrayList<Integer> numbersList = new ArrayList<>(Arrays.asList(1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8));
        
        System.out.println(numbersList);

        LinkedHashSet<Integer> hashSet = new LinkedHashSet<>(numbersList);
        
        ArrayList<Integer> listWithoutDuplicates = new ArrayList<>(hashSet);
        
        System.out.println(listWithoutDuplicates);
    }
}

Program Output.

[1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8]

2. Remove duplicates in arraylist – Java 8

To remove the duplicates from the arraylist, we can use the java 8 stream api as well. Use steam’s distinct() method which returns a stream consisting of the distinct elements comparing by object’s equals() method.

Collect all district elements as List using Collectors.toList().

Java program to remove duplicates from arraylist in java without using Set.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class ArrayListExample 
{
    public static void main(String[] args) 
    {
        // ArrayList with duplicate elements
        ArrayList<Integer> numbersList = new ArrayList<>(Arrays.asList(1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8));
        
        System.out.println(numbersList);

        List<Integer> listWithoutDuplicates = numbersList.stream().distinct().collect(Collectors.toList());
        
        System.out.println(listWithoutDuplicates);
    }
}

Program Output.

[1, 1, 2, 3, 3, 3, 4, 5, 6, 6, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8]

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