Java Program to Remove duplicate elements from ArrayList

Example 1: Remove duplicate elements from ArrayList using Set

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

class Main {
  public static void main(String[] args) {

    // create an arraylist from the array
    // using asList() method of the Arrays class
    ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 1, 3));
    System.out.println("ArrayList with duplicate elements: " + numbers);

    // convert the arraylist into a set
    Set<Integer> set = new LinkedHashSet<>();
    set.addAll(numbers);

    // delete al elements of arraylist
    numbers.clear();

    // add element from set to arraylist
    numbers.addAll(set);
    System.out.println("ArrayList without duplicate elements: " + numbers);
  }
}

Output

ArrayList with duplicate elements: [1, 2, 3, 4, 1, 3]
ArrayList without duplicate elements: [1, 2, 3, 4]

In the above example, we have created an arraylist named numbers. The arraylist contains duplicate elements.

To remove duplicate elements from the arraylist, we have

• add all elements from arraylist to set
• empty the arraylist using clear() method
• add all elements from set to arraylist

Here, we have used the LinkedHashSet to create a set. It is because it removes the duplicate elements and maintains insertion order. To learn more, visit Java LinkedHashSet.

Example 2: Remove duplicate elements from ArrayList using Stream

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

class Main {
  public static void main(String[] args) {

    // create an arraylist from the array
    // using asList() method of the Arrays class
    ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 1, 3));
    System.out.println("ArrayList with duplicate elements: " + numbers);

    // create a stream from arraylist
    Stream<Integer> stream = numbers.stream();

    // call the distinct() of Stream
    // to remove duplicate elements
    stream = stream.distinct();

    // convert the stream to arraylist
    numbers = (ArrayList<Integer>)stream.collect(Collectors.toList());
    System.out.println("ArrayList without duplicate elements: " + numbers);

  }
}

Output

ArrayList with duplicate elements: [1, 2, 3, 4, 1, 3]
ArrayList without duplicate elements: [1, 2, 3, 4]

In the above example, we have created an arraylist named numbers. The arraylist contains duplicate elements.

Here, we have used the Stream class to remove duplicate elements from the arraylist.

• numbers.stream() - create a stream from the arraylist
• stream.distinct() - removes duplicate elements
• stream.collect(Collectors.toList()) - returns a list from the stream

Here, we have used typecasting to convert the returned list into an arraylist.