Difference Between HashSet and HashMap- Scaler Topics

Overview

The Java Collection Framework in the java.util package simplifies data storage and manipulation for programmers. Within this framework, HashSet and HashMap play key roles.

HashSet is an unsorted and unordered Set. It places elements based on their hashcode values, making a well-designed hashCode() method important for efficient searches. Know more about HashSet here: HashSet in Java

HashMap: A collection adhering to the Map interface, organizing data into (Key, Value) pairs. Unlike HashTable, HashMap operates without synchronization, enhancing speed and conserving memory. Know more about HashMap HashMap in Java

HashSet Vs HashMap

Let's understand the distinction between a HashSet Vs HashMap using the following table:

Basis	HashSet	HashMap
Definition	A HashSet is a type of Set that establishes a collection utilizing a hash table for storage.	The Java HashMap represents an implementation of the Map interface built upon a hash table.
Internal Implementation	Hashset internally uses Hashmap for its implementation.	Hashmap internally do not implement hashset or any set for its implementation
Implementation	HashSet implements the Set, Cloneable, Serializable, Iterable, and Collection interfaces.	HashMap implements the Map, Cloneable, and Serializable interfaces.
Data storage	HashSet solely stores objects without maintaining corresponding key-value pairs.	HashMap organizes elements as key-value pairs, wherein each element possesses a key crucial for its retrieval during iteration.
Duplicate values	It does not allow duplicate values.	It does not allow duplicate keys, but duplicate values are allowed.
Null values	It can accommodate a singular null value.	It can hold a lone null key and numerous null values.
Method of insertion	HashSet utilizes the add() method to append elements to the set.	HashMap employs the put() method to insert elements into the map.
Performance	HashSet tends to be slower than HashMap as it relies on the member object to compute hashcode values, which might result in identical hashcodes for two objects.	HashMap outperforms HashSet due to its association of values with distinct keys
The Number of objects	Two objects are created during the put operation, one for key and one for value.	Only one object is created during the add operation.
Storing Mechanism	Internally, HashSet utilizes a HashMap object to store its elements internally.	Internally, HashMap employs hashing as a mechanism for object storage.
Complexity	O(n)	O(1)
Uses	To maintain the uniqueness of data.	To organize the data in key and value pairs.
Example	{6, 43, 2, 90, 4} This set is characterized by its elements: 6, 43, 2, 90, and 4.	{a->4, b->9, c->5} This represents a mapping where a, b, and c serve as keys, and 4, 9, and 5 are their corresponding associated values.

Example of HashSet

Let's see how HashSet can be implemented in Java

Code:

Output:

Explanation: We can see that the insertion order is not maintained in the HashSet. Also, duplicate values are not allowed.

Example of HashMap

Let's see how HashMap can be implemented in Java

Code:

Output:

Explanation:

Once a hashmap is created and the key-value pairs are inserted, we observe that the insertion order is maintained. Let's see how it happens step-by-step.

Afterwards, a duplicate key 4 is inserted with a different value "Topics Article". Since we know that keys must be unique in a HashMap, when we try to insert a duplicate key, it replaces the old value associated with that key. So, the old value "Topics" associated with key 4 is replaced with the new value "Topics Article." Here's the updated content of the HashMap:

Thus, we can see that HashMap in Java maintains the insertion order in which elements are added to the HashMap is present. When we insert a duplicate key, it replaces the old value associated with that key. This behaviour ensures that each key remains unique in the HashMap.

FAQs

Q: What's the key difference between HashMap and HashSet in Java?

A: HashMap stores key-value pairs, while HashSet stores unique elements without key-value associations.

Q: How is order handled differently in HashMap and HashSet?

A: HashMap doesn't guarantee any specific order of elements, whereas HashSet doesn't maintain any particular order.

Q: Can both HashMap and HashSet contain null values?

A: Both HashMap and HashSet can hold null values, but HashMap can also contain null keys.

Q: When would you choose HashMap over HashSet or vice versa?

A: Use HashMap when you need to store key-value pairs and perform efficient lookups; use HashSet for storing unique elements without concern for order or key-value associations.

Conclusion

HashSet stands as an unsorted and unordered Set
HashMap operates without synchronization, which enhances its speed and conserves memory. This unsynchronized behaviour signifies that HashMap doesn't ensure any particular element arrangement.
HashSet tends to be slower than HashMap as it relies on the member object for computing hashcode values, which might result in identical hashcodes for two different objects
HashMap is used when we store key-value pairs and perform efficient lookups. In contrast, HashSet is used for storing unique elements without concern for order or key-value associations.