Five Ways to Remove Duplicates from a Java ArrayList

The following introduces five different methods to remove duplicate data from a Java ArrayList.

1. Remove duplicates from an ArrayList using LinkedHashSet

LinkedHashSet is an optimal way to delete duplicate data while preserving insertion order. The example creates a list of integers with repeated values, adds it to a LinkedHashSet, and converts the set back to a list.

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. Remove duplicates using Java 8 Stream API

The Stream API’s distinct() method returns a stream of unique elements based on equals(). The distinct stream is collected back into 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. Remove duplicates using HashSet while preserving order manually

Because HashSet does not guarantee order, we use it only as a duplicate‑check while building a new list that retains the original order.

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

4. Remove duplicates by iterating with contains and rebuilding the list

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

5. Remove duplicates using a double‑for loop

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));
        }
    }
}

These five approaches provide developers with options ranging from concise modern APIs to explicit manual algorithms for de‑duplicating an ArrayList in Java.