How to Remove Duplicates from ArrayList in Java

How to Remove Duplicates from ArrayList in Java

ArrayList is most used class from Java's Collection framework, but it allows duplicates.There are many ways to remove duplicates from the ArrayList.

1). Using Set

Set which is primarily designed to store unique elements,To remove duplicates objects from ArrayList ,copy them to a Set like HashSet and then copy it back to ArrayList

public class RemoveDuplicates1 {
	public static void main(String[] args) {
		List <String> fruitsList = new ArrayList <String>();

		fruitsList.add("Mango");
		fruitsList.add("Kiwi");
		fruitsList.add("Mango");
		fruitsList.add("Banana");
		fruitsList.add("Watermellon");
		fruitsList.add("Apple");

		System.out.println(fruitsList.toString());
		Set <String> fruitsSet = new LinkedHashSet <String> (fruitsSet);
        
        fruitsList.clear(); 
  
        fruitsList.addAll(fruitsSet); 
        
		System.out.println("The fruits list without duplicates : "+fruitsList);
	}
}

2). Using Iterator

In iterator we have traverse each element in the arraylist and put the element to new arraylist.Before put element to new arrayList check if element present in new list.For this arralist has contains() method.

1). Create new ArrayList.

2). Traverse through the first arraylist and store the first appearance of each element into the second arraylist using contains() method.

3). The second ArrayList contains the elements with duplicates removed.

import java.util.* ;

public class RemoveDuplicates2 {

	// dFunction to remove duplicates from an ArrayList 
	public static ArrayList <String> removeDuplicates(ArrayList <String> fruitsList) {

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

		for (String element: list) {

			// If this element is not present in newList 
			// then add it 
			if (!newFruitsList.contains(element)) {

				newFruitsList.add(element);
			}
		}

		return newFruitsList;
	}

	public static void main(String args[]) {

		List <String> fruitsList = new ArrayList <String> ();

		fruitsList.add("Mango");
		fruitsList.add("Kiwi");
		fruitsList.add("Mango");
		fruitsList.add("Apple");
		fruitsList.add("Banana");
		fruitsList.add("Watermellon");
		fruitsList.add("Apple");
		fruitsList.add("Kiwi");

		System.out.println("Original ArrayList with duplicates: " + fruitsList);

		// Remove duplicates 
		ArrayList <String> newFruitsList = removeDuplicates(fruitsList);

		// Print the ArrayList without duplicates 
		System.out.println("New ArrayList without duplicates : " + fruitsList);
	}
}

3). Using Java 8 Stream.distinct()

Java 8 Stream contains distinct method which removes duplocated from the arrayList. The distinct() method return a new Stream without duplicates elements based on the result returned by equals() method.

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

class RemoveDuplicates3 {
	public static void main(String[] args) {

		List <String> fruitsList = new ArrayList<String>();

		fruitsList.add("Mango");
		fruitsList.add("Kiwi");
		fruitsList.add("Mango");
		fruitsList.add("Apple");
		fruitsList.add("Banana");
		fruitsList.add("Watermellon");
		fruitsList.add("Apple");
		fruitsList.add("Kiwi");

		System.out.println("Original ArrayList with duplicates : " + fruitsList);

		List <String> newfruitsList = fruitsList.stream().distinct().collect(Collectors.toList());

		System.out.println("ArrayList without duplicates : " + newfruitsList);
	}
}

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