Remove repeated elements from ArrayList in Java

An ArrayList can contain duplicate or repeated elements. In Java, there are several ways to remove these repeated elements while keeping only one occurrence of each value.

• A Set does not allow duplicate elements, so it can be used to remove repeated values.
• LinkedHashSet removes duplicates while preserving the original insertion order.

Examples:

Input: [1, 2, 2, 3, 4, 4, 4]
Output: [1, 2, 3, 4]

Input: [12, 23, 23, 34, 45, 45, 45, 57, 67]
Output: [12, 23, 34, 45, 57, 67]

Methods to Remove Repeated Elements from ArrayList

1. Using LinkedHashSet

A LinkedHashSet is a collection that does not allow duplicate elements and also maintains the insertion order of the elements.

Approach

1. Create an ArrayList containing duplicate elements.
2. Create a LinkedHashSet from the ArrayList.
3. Duplicate elements are automatically removed.
4. Convert the LinkedHashSet back into an ArrayList.
5. Print the resulting list.

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;

public class RemoveDuplicates {

    public static void main(String[] args)
    {

        ArrayList<String> list = new ArrayList<>();

        list.add("Geeks");
        list.add("for");
        list.add("Geeks");
        list.add("Java");
        list.add("for");

        System.out.println("Original ArrayList: " + list);

        LinkedHashSet<String> set
            = new LinkedHashSet<>(list);

        List<String> result = new ArrayList<>(set);

        System.out.println(
            "ArrayList after removing duplicates: "
            + result);
    }
}

Original ArrayList: [Geeks, for, Geeks, Java, for]
ArrayList after removing duplicates: [Geeks, for, Java]

Explanation: LinkedHashSet does not allow duplicate elements. When the ArrayList is converted into a LinkedHashSet, repeated values are removed automatically. It also maintains the insertion order, so the first occurrence of each element is retained.

2. Using HashSet

A HashSet can also be used to remove repeated elements because it does not allow duplicate values.

Approach

1. Create an ArrayList containing duplicate elements.
2. Convert the ArrayList into a HashSet.
3. The duplicate elements are removed automatically.
4. Convert the HashSet back into an ArrayList.
5. Print the resulting list.

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

public class RemoveDuplicates {

    public static void main(String[] args)
    {

        ArrayList<Integer> list = new ArrayList<>();

        list.add(10);
        list.add(20);
        list.add(10);
        list.add(30);
        list.add(20);
        list.add(40);

        System.out.println("Original ArrayList: " + list);

        HashSet<Integer> set = new HashSet<>(list);

        List<Integer> result = new ArrayList<>(set);

        System.out.println(
            "ArrayList after removing duplicates: "
            + result);
    }
}

Original ArrayList: [10, 20, 10, 30, 20, 40]
ArrayList after removing duplicates: [20, 40, 10, 30]

Explanation: The HashSet stores only unique elements. Therefore, when the ArrayList is converted into a HashSet, all repeated values are removed.

Note: If you need to preserve the original order, use LinkedHashSet instead of HashSet.

3. Using Stream distinct() Method

Java 8 introduced the Stream API, which provides the distinct() method to remove duplicate elements from a stream.

Approach

1. Create an ArrayList containing duplicate elements.
2. Convert the ArrayList into a stream using stream().
3. Use the distinct() method to remove duplicate elements.
4. Collect the resulting elements into a new List.
5. Print the resulting list.

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

public class RemoveDuplicates{

    public static void main(String[] args)
    {

        ArrayList<String> list = new ArrayList<>();

        list.add("Geeks");
        list.add("for");
        list.add("Geeks");
        list.add("Java");
        list.add("for");

        System.out.println("Original ArrayList: " + list);

        List<String> result= list.stream().distinct().collect(Collectors.toList());

        System.out.println("ArrayList after removing duplicates: "+ result);
    }
}

Original ArrayList: [Geeks, for, Geeks, Java, for]
ArrayList after removing duplicates: [Geeks, for, Java]

Explanation: The stream() method converts the ArrayList into a stream. The distinct() method removes repeated elements from the stream.