5 Simple Ways to Remove Duplicates from a Java ArrayList

Five Different Methods to Remove Duplicates from a Java ArrayList

1. Use LinkedHashSet

LinkedHashSet is an optimal way to delete duplicate data while preserving insertion order.

Removes duplicate data

Maintains the order of elements

Java example using LinkedHashSet:

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

public class ArrayListExample {
    public static void main(String[] args) {
        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);
    }
}

Output:

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

2. Use Java 8 Stream distinct()

Java 8 streams provide the distinct() method, which returns a stream of unique elements based on equals().

Collect the result back to a list:

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<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);
    }
}

Output:

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

3. Use HashSet (order not guaranteed)

Insert elements into a HashSet to filter duplicates, then rebuild the list.

private static void removeDuplicate(List<String> list) {
    HashSet<String> set = new HashSet<>(list.size());
    List<String> result = new ArrayList<>(list.size());
    for (String str : list) {
        if (set.add(str)) {
            result.add(str);
        }
    }
    list.clear();
    list.addAll(result);
}

4. Use List.contains() in a loop

private static void removeDuplicate(List<String> list) {
    List<String> result = new ArrayList<>(list.size());
    for (String str : list) {
        if (!result.contains(str)) {
            result.add(str);
        }
    }
    list.clear();
    list.addAll(result);
}

5. Double for‑loop removal

for (int i = 0; i < list.size(); i++) {
    for (int j = 0; j < list.size(); j++) {
        if (i != j && list.get(i) == list.get(j)) {
            list.remove(list.get(j));
        }
    }
}