In this article, we will discuss LinkedHashSet class – one of the Set implemented classes in detail

1. Key points about LinkedHashSet:

• LinkedHashSet is exactly same as that of HashSet
• but it preserves insertion-order

2. LinkedHashSet:

• LinkedHashSet is implementation class of Set interface (i.e.; LinkedHashSet implements Set)
• This is introduced in Java 1.4 version
• LinkedHashSet uses combination of LinkedList & hashtable to store element/objects
• Duplicate element/objects are NOT allowed
• If duplicate value is added again and again, there won’t be any compile-time or runtime errors
• Simply add(object); method returns false for already containing object inside LinkedHashSet
• At any time, LinkedHashSet contains only unique element/objects
• Insertion-order is maintained
• While iterating through LinkedHashSet, we will get items as per insertion-order
• Allows NULL insertion but maximum of only one NULL value
• Without generics, LinkedHashSet allows to insert any type of objects;
• with generics, it is type-bounded (except, if we take Object as type within angle brackets)
• LinkedHashSet is non-synchronized
• Present in java.util package and extends java.util.HashSet implements java.util.Set interface
• Also, implements java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to LinkedHashSet (provided by JVM at run time) like,
• java.lang.Cloneable: to create a duplicate object or to clone an object
• java.io.Serializable: to transfer objects across network

Source: Team BenchResources.Net

3. LinkedHashSet constructors:

3.1 LinkedHashSet hs = new LinkedHashSet();

• creates an empty LinkedHashSet object of size 16
• with default fill ratio 0.75

3.2 LinkedHashSet hs = new LinkedHashSet(int initialCapacity);

• creates an empty LinkedHashSet object of specified size (or initial capacity)
• with default fill ratio 0.75

3.3 LinkedHashSet hs = new LinkedHashSet(int initialCapacity, float loadFactor);

• creates an empty LinkedHashSet object of specified size (or initial capacity)
• and specified fill ratio (for example 0.85)

3.4 LinkedHashSet hs = new LinkedHashSet(Collection c);

• created an equivalent LinkedHashSet object for the specified collection
• it is basically used for inter-conversion between collection objects

4. Fill ratio (or Load factor)

• Fill ratio is also known as Load factor
• This factor determines when to increase the size of LinkedHashSet automatically
• For example, for the 1^st two constructors the default load factor is 75 –> which means after filling 75 % of LinkedHashSet, new LinkedHashSet of bigger size will be created
• For 3^rd constructor, programmer can define load factor while creating LinkedHashSet object. If programmer define it to be 0.95, then after filling 95% of LinkedHashSet, size of LinkedHashSet will be increased automatically
• The value of Load factor should be in between 0 to 1.0

5. LinkedHashSet examples:

LinkedHashSetAddAndRemove.java

package in.bench.resources.java.collection;

import java.util.LinkedHashSet;
import java.util.Iterator;

public class LinkedHashSetAddAndRemove {

	public static void main(String[] args) {

		// creating LinkedHashSet object of type String
		LinkedHashSet<String> lhs = new LinkedHashSet<String>();

		// adding elements to LinkedHashSet object
		lhs.add("Sundar Pichai");
		lhs.add("Satya Nadella");
		lhs.add("Shiv Nadar");
		lhs.add("Shantanu Narayen");
		lhs.add("Sundar Pichai"); // adding duplicate element
		lhs.add("Francisco D’Souza");

		// adding null element to LinkedHashSet lhs
		lhs.add(null);
		lhs.add(null); // 2nd null is added, lhs will have one NULL

		// creating Iterator reference
		Iterator<String> ceo = lhs.iterator();

		System.out.println("Iterating using Iterator\n");

		// iterating using while loop
		while (ceo.hasNext()){
			System.out.println(ceo.next());
		}

		System.out.println("\n\nprinting inside square brackets []");
		System.out.println(lhs);

	}
}

Output:

Iterating using Iterator

Sundar Pichai
Satya Nadella
Shiv Nadar
Shantanu Narayen
Francisco D’Souza
null

printing inside square brackets []
[Sundar Pichai, Satya Nadella, Shiv Nadar, Shantanu Narayen,
Francisco D’Souza, null]

Q) Difference between HashSet and LinkedHashSet ?

• The main difference between HashSet and LinkedHashSet is insertion-order
• HashSet doesn’t maintain insertion-order and print values in random-order while iterating
• whereas LinkedHashSet maintains insertion-order as seen in above example
• Note: All methods of HashSet/LinkedHashSet is non-synchronized

Q) How to make LinkedHashSet synchronized ?

• LinkedHashSet can be easily converted into synchronized LinkedHashSet
• using utility method synchronizedSet(lhs); of java.util.Collections class

Set set = Collections.synchronizedSet(lhs);

Related Articles:

• How to get size or length of HashSet
• How to delete a element and delete all elements of HashSet
• retainAll() method explanation with HashSet
• How to check whether particular element is present in HashSet
• Comparing two HashSet objects using containsAll() method
• Adding one HashSet to another HashSet using addAll() method
• Various ways to iterate through HashSet
• How to reverse LinkedHashSet contents
• Sorting HashSet contents in ascending and descending order
• How to Sort HashSet in Java – 2 ways

References:

• https://docs.oracle.com/javase/tutorial/collections/intro/
• https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/list.html
• https://docs.oracle.com/javase/tutorial/collections/interfaces/set.html
• https://docs.oracle.com/javase/7/docs/api/java/util/Set.html
• https://docs.oracle.com/javase/7/docs/api/java/util/class-use/HashSet.html
• https://docs.oracle.com/javase/7/docs/api/java/util/HashSet.html
• https://docs.oracle.com/javase/7/docs/api/java/util/LinkedHashSet.html
• http://docs.oracle.com/javase/7/docs/api/index.html?java/util/class-use/LinkedHashSet.html

Happy Coding !!
Happy Learning !!