14 March 2020

Java Remove Duplicates From List

development

This post provides examples, showing how to remove duplicate items from an ArrayList in Java.

Remove Duplicate Strings From ArrayList

Since a Set cannot hold duplicate elements, we can instantiate a Set object passing in the ArrayList with duplicates as a parameter.

For example:

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

public class RemoveDuplicatesFromArrayList {

    public static void main(String[] args) {
        ArrayList<String> pets = new ArrayList<>();

        pets.add("cat");
        pets.add("dog");
        pets.add("cat");
        pets.add("hamster");

        System.out.println(pets);

        Set<String> hashSet = new LinkedHashSet(pets);
        ArrayList<String> removedDuplicates = new ArrayList(hashSet);

        System.out.println(removedDuplicates);
    }
}

Output:

[cat, dog, cat, hamster]
[cat, dog, hamster]

Remove Duplicate Integers From List

Likewise, we can use the same methodology to remove duplicate integers.

import java.util.*;

public class RemoveDuplicatesFromArrayList {

    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1,2,2,2,3,5);

        System.out.println(numbers);

        Set<Integer> hashSet = new LinkedHashSet(numbers);
        ArrayList<Integer> removedDuplicates = new ArrayList(hashSet);

        System.out.println(removedDuplicates);
    }
}

Output:

[1, 2, 2, 2, 3, 5]
[1, 2, 3, 5]

Remove Duplicates From List Using Java 8 Lambdas

import java.util.*;
import java.util.stream.Collectors;

public class RemoveDuplicatesFromArrayList {

    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1,2,2,2,3,5);

        System.out.println(numbers);

        List<Integer> removedDuplicates = numbers.stream()
                .distinct()
                .collect(Collectors.toList());

        System.out.println(removedDuplicates);
    }
}

Output:

[1, 2, 2, 2, 3, 5]
[1, 2, 3, 5]

14 March 2020