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

1. Key points about HashSet:

• HashSet contains only unique elements
• It doesn’t maintains insertion-order or
• say stores in random-order

2. HashSet:

• HashSet is the implementation class of Set interface (i.e.; HashSet implements Set)
• HashSet is backed by a hashtable (actually a HashMap instance) to store element/objects
• Duplicate element/objects are NOT allowed
• If duplicate value is added again, then there won’t be any compile-time or runtime errors
• Simply add(object) method returns false for already containing object inside HashSet
• At any time, HashSet contains only unique element/objects
• Insertion-order is NOT maintained
• While iterating through HashSet, we will get items in random-order (as against insertion-order)
• Allows NULL insertion but maximum of only one NULL value
• Without generics, HashSet allows to insert any type of objects;
• with generics, it is type-bounded (except, if we take Object as type within angle brackets)
• HashSet is non-synchronized
• Search operation is faster i.e.; searching any element from HashSet is faster, as it uses hashing to store elements
• Present in java.util package and extends java.util.AbstractSet implements java.util.Set interface
• Also, implements java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to HashSet (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. HashSet constructors:

3.1 HashSet hs = new HashSet();

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

3.2 HashSet hs = new HashSet(int initialCapacity);

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

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

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

3.4 HashSet hs = new HashSet(Collection c);

• creates an equivalent HashSet 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 HashSet automatically
• For example, for the 1^st two constructors the default load factor is 0.75 –> which means after filling 75% of HashSet, new HashSet of bigger size will be created
• For 3^rd constructor, programmer can define load factor while creating HashSet object. If programmer defined it to be 0.95, then after filling 95% of HashSet, size of HashSet will be increased automatically
• The value of Load factor should be in between 0.0 to 1.0

5. HashSet examples:

HashSetAddAndRemove.java

package in.bench.resources.java.collection;

import java.util.HashSet;
import java.util.Iterator;

public class HashSetAddAndRemove {

	public static void main(String[] args) {

		// creating HashSet object of type String
		HashSet<String> hs = new HashSet<String>();

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

		// adding null element to HashSet
		hs.add(null);
		hs.add(null); // 2nd null is added to hs

		// creating Iterator reference
		Iterator<String> ceo = hs.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(hs);
	}
}

Output:

Iterating using Iterator

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

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

Note: All methods of HashSet is non-synchronized

Q) How to make HashSet synchronized ?

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

Set set = Collections.synchronizedSet(hs);

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://en.wikipedia.org/wiki/Hash_table

Happy Coding !!
Happy Learning !!