Java LinkedList Remove Duplicate Elements Example

This example shows how to remove duplicate elements from the LinkedList in Java. This example also shows how to remove duplicates using the HashSet and LinkedHashSet classes.

How to remove duplicate elements from the LinkedList in Java?

The LinkedList class is a Java linked list implementation of the List interface. The LinkedList class does not prevent us from adding duplicate elements to it.

However, if you want to remove duplicate elements from the LinkedList object, you can do that in a couple of ways.

1. Using the HashSet or LinkedHashSet class

The Java HashSet class is a Set implementation and does not allow duplicate elements. We can use this property of the Set to remove duplicate elements from the LinkedList as given below.

import java.util.HashSet;

import java.util.LinkedList;

public class LinkedListRemoveDuplicatesExample {

public static void main(String[] args) {

LinkedList<Integer> linkedListNumbers = new LinkedList<Integer>();

linkedListNumbers.add(1);

linkedListNumbers.add(3);

linkedListNumbers.add(5);

linkedListNumbers.add(3);

linkedListNumbers.add(2);

* Convert LinkedList to HashSet using the

* HashSet constructor to remove duplicates

HashSet<Integer> hSetNumbers = new HashSet<Integer>( linkedListNumbers );

//delete all elements from the original linked list

linkedListNumbers.clear();

//add all elements from the set to the linked list

linkedListNumbers.addAll(hSetNumbers);

System.out.println( linkedListNumbers );

}

Output

1	[1, 2, 3, 5]

As you can see from the output, all the duplicate elements from the LinkedList were not added to the HashSet object.

However, if you look closely, the order of the elements is different from the original. That is because the HashSet class does not guarantee to maintain the insertion order of its elements.

If you want to retain the order of the elements of the LinkedList, use the LinkedHashSet class instead of the HashSet class as given below.

LinkedList<Integer> linkedListNumbers = new LinkedList<Integer>();

linkedListNumbers.add(1);

linkedListNumbers.add(3);

linkedListNumbers.add(5);

linkedListNumbers.add(3);

linkedListNumbers.add(2);

* Convert LinkedList to LinkedHashSet using the

* LinkedHashSet constructor.

LinkedHashSet<Integer> lhSetNumbers = new LinkedHashSet<Integer>( linkedListNumbers );

//delete all elements from the original linked list

linkedListNumbers.clear();

//add all elements from the set to the linked list

linkedListNumbers.addAll(lhSetNumbers);

System.out.println( linkedListNumbers );

Output

1	[1, 3, 5, 2]

As you can see from the output, now the order of the elements in the LinkedList object is the same as before.

2. Using the Stream (Java 8 and above)

If you are using Java 8 or later version, you use the stream to remove duplicates from the linked list as given below.

LinkedList<Integer> linkedListNumbers = new LinkedList<Integer>();

linkedListNumbers.add(1);

linkedListNumbers.add(3);

linkedListNumbers.add(5);

linkedListNumbers.add(3);

linkedListNumbers.add(2);

List<Integer> list = linkedListNumbers.stream().distinct().collect( Collectors.toList() );

System.out.println( list );

Output

1	[1, 3, 5, 2]

How to remove duplicate objects of a custom class from the LinkedList?

The above examples use the Integer class as the linked list elements. The Integer class has implemented the equals method. That is the reason the above examples worked.

If the equals method is not implemented in the custom class, then the equals method from the Object class will be used which compares only object references not the actual objects. If the LinkedList object contains the custom class objects, then the custom class must implement the equals and hashCode methods to work.

This example is a part of the LinkedList in Java tutorial.

Please let me know your views in the comments section below.

References:
Java 8 LinkedList