Learn what is HashSet in Java and how to get started with it!

HashSet in Java is one of the most important aspects of Java Collection hierarchy. It is typically used to store unique values in an unordered manner. Through the medium of this article on HashSet in Java, I will be giving you a complete walkthrough of what exactly is a HashSet and how you can use it in your application.

Below are the topics covered in this article:

• HashSet in Java
• Java HashSet vs HashMap
• HashSet Hierarchy
• Constructors of java.util.HashSet Class
• Methods of java.util.HashSet Class
• Implementing HashSet in Java Program

Let’s get started by first understanding what are HashSet in Java.

HashSet in Java

java.util.HashSet class is a member of the Java collections framework which inherits the AbstractSet class and implements the Set interface. It implicitly implements a hashtable for creating and storing a collection of unique elements. Hashtable is nothing but an instance of the HashMap class that uses a hashing mechanism for storing the information within a HashSet.

Hashing is the process of converting the informational content into a unique value that is more popularly known as hash code. This hashcode is then used for indexing the data associated with the key. The entire process of transforming the informational key into the hashcode is performed internally.

Now for a better understanding of HashSet in Java, let me list down a few of its features:

1. A HashSet in Java does not allow duplicate values.
2. It can contain null values.
3. HashSet doesn’t follow the insertion order for storing the data rather it makes use of the hashcode for indexing the values within.
4. It is non-synchronized which automatically makes it thread-unsafe.
5. HashSet class also implements Cloneable and Serializable interfaces.

Now that you are aware of what exactly is HashSet in Java, let’s move further with this article and demystify the differences between HashMap and HashSet in Java.

Java HashSet vs HashMap

Now that you have a clear distinction between HashMap and HashSet, let’s now focus on HashSet again and dive deeper into it. In the next section of this article, I will be introducing you to the complete hierarchy of HashSet in Java.

HashSet Hierarchy in Java

As you can see from the below-given diagram, The HashSet class implements the Set interface. The Set interface further inherits the Collection interface which eventually extends the Iterable interface in a hierarchical order.

Now, moving ahead with this HashSet in Java article, let’s us check out the various constructors supported by this class.

Constructors of java.util.HashSet Class

These were the four constructors of the HashSet class in Java. Let’s now find out what are the various methods defined in Java HashSet.

Methods of java.util.HashSet Class

Along with the above-listed methods, the HashSet class in Java also contains the methods inherited from its superclasses.

Let’s now try implementing these methods and get our feet wet with coding.

Implementing HashSet in Java Program

In the below example, we will try and implement a number of methods provided by the HashSet class.

import java.util.HashSet;
import java.util.*;

public class SampleHashSet {
    public static void main(String[] args) {
        // Creating a HashSet
        Set<String> eduCourses = new HashSet<>();

        // Adding new elements to the HashSet
        eduCourses.add("Big Data");
        eduCourses.add("Node.js");
        eduCourses.add("Java");
        eduCourses.add("Python");
        eduCourses.add("Blockchain");
        eduCourses.add("JavaScript");
        eduCourses.add("Selenium");
        eduCourses.add("AWS");
        eduCourses.add("Machine Learning");
        eduCourses.add("RPA");
        

        // Adding duplicate elements will be ignored
        eduCourses.add("Java");
        eduCourses.add("RPA");

        System.out.println(eduCourses);
        
        // Check if the HashSet contains an specific element
        String myCourse = "Node.js";
        if(eduCourses.contains(myCourse)) {
            System.out.println(myCourse + " is in the courses list.");
        } else {
            System.out.println(myCourse + " is not in the courses list.");
        }
        
        // Sorting eduCourses using List 
        List<String> list = new ArrayList<String>(eduCourses); 
        Collections.sort(list); 
  
        // Printing the sorted elements of the HashSet 
        System.out.println("Printing the Courses in sorted order using List: " + list);
        
       // Removing items from HashSet using remove() 
        eduCourses.remove("Python"); 
          
        // Iterating over HashSet items 
        System.out.println("Iterating over course list after removing Python:"); 
        Iterator<String> i = eduCourses.iterator(); 
        while (i.hasNext()) 
            System.out.println(i.next()); 
        
        
        // Creating another object of HashSet 
        HashSet<String> eduNewCourses = new HashSet<String>(); 
        eduNewCourses.add("Node.js"); 
        eduNewCourses.add("Python");
        eduNewCourses.add("Machine Learning");
                    
        //Removing all the new elements from HashSet  
        eduCourses.removeAll(eduNewCourses);  
        System.out.println("After invoking removeAll() method courses left: "+ eduCourses); 
        
        //Removing elements on the basis of specified condition  
        eduCourses.removeIf(str->str.contains("Java"));    
        System.out.println("After invoking removeIf() method: "+ eduCourses);  
        
        // Removing elements from eduCourses which are specified in eduNewCourses 
        eduCourses.retainAll(eduNewCourses);
        System.out.println("HashSet after " + "retainAll() operation : " + eduNewCourses); 
        
        //Removing all the elements available in the set   
        eduCourses.clear();  
        System.out.println("After invoking clear() method: "+ eduCourses); 
       
    }
}

When you execute the above code, it will give you the below-shown output.

With this, we come to the end of this article. Hope I was able to keep the concepts crisp and clear. You can learn more about Java Collections by going through our other Java blogs.

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