HashSet vs HashMap in Java

In this post, we will learn the difference between HashSet and HashMap in Java. This is a frequently asked question in Java interviews for beginners. Let's dive into it. 

Difference between HashSet and HashMap in Java

Features	HashSet	HashMap
Interface	Implements Set interface	Implements Map interface
Data Storage	Stores data as objects	Stores data as key-value pairs
Internal Structure	Uses HashMap internally	Uses an array of Entry<K, V> objects internally
Duplicate Elements	Doesn't allow duplicates	Allows duplicate values but not duplicate keys
Null Elements	Allows only one null element	Allows one null key and multiple null values
Insertion Operation	Requires one object	Requires two objects - key and value
Performance	Slightly slower than HashMap	Slightly faster than HashSet

Example with HashSet:

import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        // Create a HashSet
        HashSet<String> hSet = new HashSet<>();

        // Add elements
        hSet.add("apple");
        hSet.add("banana");
        hSet.add("cherry");
        hSet.add("apple"); // Trying to add a duplicate

        // Display the HashSet
        System.out.println("HashSet:");
        for (String fruit : hSet) {
            System.out.println(fruit);
        }
    }
}

Output:

HashSet:
apple
banana
cherry

Example with HashMap:

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        // Create a HashMap
        HashMap<Integer, String> hMap = new HashMap<>();

        // Add elements (key-value pairs)
        hMap.put(1, "apple");
        hMap.put(2, "banana");
        hMap.put(3, "cherry");
        hMap.put(3, "date"); // Trying to use a duplicate key

        // Display the HashMap
        System.out.println("HashMap:");
        for (Integer key : hMap.keySet()) {
            System.out.println(key + " => " + hMap.get(key));
        }
    }
}

Output:

HashMap:
1 => apple
2 => banana
3 => date

Related Collections Interview QA

1. map() vs flatMap() in Java
2. Collections vs Streams
3. ArrayList vs Vector
4. Iterator vs ListIterator
5. HashMap vs HashTable
6. HashSet vs HashMap
7. Array vs ArrayList
8. Fail-Fast Iterators vs Fail-Safe Iterators
9. HashMap vs ConcurrentHashMap
10. LinkedList vs ArrayDeque
11. LinkedList vs Array
12. LinkedList vs Doubly LinkedList
13. Enum vs EnumSet in Java
14. HashMap vs. TreeMap in Java
15. Synchronized Collections vs. Concurrent Collections