LinkedHashSet Class in Java

Last Updated: 30 September, 2023

The LinkedHashSet class of the Java collections framework was introduced in the JDK 1.4 version and is available in the java.util package.

LinkedHashSet is the Hashtable and linked list implementation of the Set interface with preserved iteration order. The linked list defines the iteration order, which is the order in which elements are inserted into the set. Insertion order is not affected if an element is re-inserted into the set.

LinkedHashSet is a child class of HashSet, so it has all the functionalities of the HashSet class. The HashSet does not maintain the insertion order of elements, whereas the LinkedHashSet maintains the element insertion order. This one is the main difference between both.

👉 Java LinkedHashSet important features:

• The underlying data structure is a hash table + linked list. It uses the implementation of a doubly-linked list on the hash table.
• LinkedHashSet only stores unique elements; no duplicates are allowed.
• LinkedHashSet maintains insertion order using a doubly linked list.
• LinkedHashSet also supported inserting a single null value.
• No new methods are added in the LinkedHashSet class; all are extended from HashSet.
• LinkedHashSet is not synchronized.
• Uses a hashing technique to store elements at the specified index based on the hashcode.
• LinkedHashSet is the best choice to develop cache-based applications where duplicates are not allowed and insertion orders must be preserved.

👉 LinkedHashSet Class Hierarchy

As we can see from the complete hierarchy above, LinkedHashSet extends the HashSet class and implements the Set interface as well as Serializable and Cloneable interfaces.

👉 LinkedHashSet Class Desclaration

public class LinkedHashSet<E> extends HashSet<E> implements Set<E>, Cloneable, Serializable

Here, E defines the type of elements that the Set will contain.

👉 Java LinkedHashSet Class Example

import java.util.LinkedHashSet; public class LinkedHasSetExample { public static void main(String[] args) { // Creating a LinkedHashSet LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<Integer>(); // Adding elements into the LinkedHashSet linkedHashSet.add(800); linkedHashSet.add(500); linkedHashSet.add(null); // Adding null as an element linkedHashSet.add(200); linkedHashSet.add(500); // Adding duplicate element will be ignored // Printing LinkedHashSet elements System.out.println("LinkedHashSet elements: " + linkedHashSet); } }

Output

LinkedHashSet elements: [800, 500, null, 200]

👉 Java LinkedHashSet Class Constructors

The Java LinkedHashSet class has the following constructors:

LinkedHashSet Class Methods Examples

The Java LinkedHashSet class provided methods are implemented in the below given examples. Let's go through the examples one by one and understand the uses for each method.

Example 1 : Methods for Adding elements in LinkedHashSet

import java.util.Arrays; import java.util.LinkedHashSet; public class AddElementsInLinkedHashSet { public static void main(String[] args) { /*********************************************** LinkedHashSet - add(), addAll() Methods Example ************************************************/ // Creating a LinkedHashSet LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<Integer>(); // Adding elements into the LinkedHashSet linkedHashSet.add(800); linkedHashSet.add(500); linkedHashSet.add(null); // Adding null as an element linkedHashSet.add(200); linkedHashSet.add(500); // Adding duplicate element // Printing LinkedHashSet elements System.out.println("LinkedHashSet elements: " + linkedHashSet); // Creating another LinkedHashSet LinkedHashSet<Integer> childSet = new LinkedHashSet<Integer>(Arrays.asList(10, 20, 30, 40)); // Adding LinkedHashSet into another LinkedHashSet linkedHashSet.addAll(childSet); // Printing LinkedHashSet elements System.out.println("\nLinkedHashSet elements after adding another LinkedHashSet"); System.out.println(linkedHashSet); } }

Output

LinkedHashSet elements: [800, 500, null, 200]

LinkedHashSet elements after adding another LinkedHashSet
[800, 500, null, 200, 10, 20, 30, 40]

Example 2 : Methods for Accessing elements from LinkedHashSet

import java.util.Arrays; import java.util.Iterator; import java.util.LinkedHashSet; public class AccessElementsFromLinkedHashSet { public static void main(String[] args) { // Creating and adding elements into LinkedHashSet LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<Integer>(Arrays.asList(10, 20, 30, 40, 50)); System.out.println("LinkedHashSet elements: " + linkedHashSet); // Access LinkedHashSet using Iterator System.out.println("\nAccess LinkedHashSet using Iterator"); Iterator<Integer> num = linkedHashSet.iterator(); while (num.hasNext()) System.out.println(num.next()); // Access LinkedHashSet using for loop System.out.println("\nAccess LinkedHashSet using for loop"); for (Integer i : linkedHashSet) System.out.println(i); } }

Output

LinkedHashSet elements: [10, 20, 30, 40, 50]

Access LinkedHashSet using Iterator
10
20
30
40
50

Access LinkedHashSet using for loop
10
20
30
40
50

Example 3 : Methods for Removing elements from LinkedHashSet

import java.util.Arrays; import java.util.LinkedHashSet; public class RemoveElementsFromLinkedHashSet { public static void main(String[] args) { /****************************************************************************** LinkedHashSet - remove(), removeAll(), removeIf(), retainAll() Methods Example *******************************************************************************/ // Creating and adding elements into LinkedHashSet LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<Integer>(Arrays.asList(10, 20, 30, 40, 50, 60, 70)); System.out.println("LinkedHashSet elements: " + linkedHashSet); // Removing the specified element from LinkedHashSet linkedHashSet.remove(40); System.out.println("After calling remove() method: " + linkedHashSet); // Creating another LinkedHashSet and adding elements LinkedHashSet<Integer> hashSetSecond = new LinkedHashSet<Integer>(Arrays.asList(20, 60, 70)); // Removing all those elements that matches with second LinkedHashSet elements linkedHashSet.removeAll(hashSetSecond); System.out.println("After calling removeAll() method: " + linkedHashSet); // Removing elements on the basis of specified condition linkedHashSet.removeIf(i -> i == 10); System.out.println("After calling removeIf() method: "+ linkedHashSet); // Removes from the LinkedHashSet all of its elements that are not contained in the specified collection linkedHashSet.retainAll(hashSetSecond); System.out.println("After calling retainingAll() method: " + linkedHashSet); // Removing all the elements from the LinkedHashSet linkedHashSet.addAll(hashSetSecond); System.out.println("HashSet before calling clear() method: " + linkedHashSet); linkedHashSet.clear(); System.out.println("After calling clear() method: " + linkedHashSet); } }

Output

LinkedHashSet elements: [10, 20, 30, 40, 50, 60, 70]
After calling remove() method: [10, 20, 30, 50, 60, 70]
After calling removeAll() method: [10, 30, 50]
After calling removeIf() method: [30, 50]
After calling retainingAll() method: []
HashSet before calling clear() method: [20, 60, 70]
After calling clear() method: []

Example 4 : Utility methods of LinkedHashSet

import java.util.ArrayList; import java.util.Arrays; import java.util.LinkedHashSet; public class LinkedHashSetUtilityMethods { public static void main(String[] args) { // Creating and adding elements into LinkedHashSet LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<Integer>(Arrays.asList(50, 30, 70, 10, 40, 20, 80)); System.out.println("LinkedHashSet elements: " + linkedHashSet); // Printing LinkedHashSet Size System.out.println("LinkedHashSet Size: " + linkedHashSet.size()); // Returns true if the LinkedHashSet contains no elements. System.out.println("Is LinkedHashSet Empty: " + linkedHashSet.isEmpty()); // Returns true if the LinkedHashSet contains the specified element System.out.println("LinkedHashSet contains() method: " + linkedHashSet.contains(40)); // Creating another LinkedHashSet and adding elements LinkedHashSet<Integer> linkedHashSetSecond = new LinkedHashSet<Integer>(Arrays.asList(20, 60, 70)); // Returns true if the LinkedHashSet contains all of the elements of the specified collection System.out.println("LinkedHashSet containsAll() method: " + linkedHashSet.containsAll(linkedHashSetSecond)); // Converting LinkedHashSet into ArrayList ArrayList<Integer> arrayList = new ArrayList<Integer>(linkedHashSet); System.out.println("ArrayList elements: " + arrayList); } }

Output

LinkedHashSet elements: [50, 30, 70, 10, 40, 20, 80]
LinkedHashSet Size: 7
Is LinkedHashSet Empty: false
LinkedHashSet contains() method: true
LinkedHashSet containsAll() method: false
ArrayList elements: [50, 30, 70, 10, 40, 20, 80]

Reference: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/LinkedHashSet.html