What is the Difference Between HashMap and HashSet

The main difference between HashMap and HashSet is that the HashMap belongs to the Map Interface Hierarchy and there is no association with the Collection interface while the HashSet belongs to the Collection Interface Hierarchy.

Programming languages such as Java provides a feature called ‘collections’ to store data dynamically. It is possible to store multiple objects as a single unit using collections. Furthermore, it is also possible to perform operations such as adding, deleting, removing, sorting and searching on a collection. The base interface to implement collections in Java is called Collection. Besides, Set is an interface that extends Collection interface. HashSet comes under this hierarchy. On the other hand, Map is a separate interface, and it does not extend Collection interface. HashMap comes under Map hierarchy.

Key Areas Covered

1. What is HashMap
     – Definition, Functionality
2. What is HashSet
     – Definition, Functionality
3. What is the Difference Between HashMap and HashSet
     – Comparison of Key Differences

Key Terms

HashMap, HashSet, Java, Map

What is HashMap

HashMap is a class that implements the Map interface using a hash table. It extends the AbstractMap while AbstractMap implements the Map interface. Moreover, it contains key-value pairs, and there can only be unique keys. It can have one null key and multiple null values. Furthermore, HashMap is not synchronized, and it does not maintain a specific order. An example program is as follows.

Figure 1: Program with HashMap

The ‘students’ is an object of HashMap. It can store multiple items that have an integer as a key and a string as a value. The put method helps to insert new items to the HashMap. The for loop with the Map.Entry allows going through all the items in students. The getKey() method prints the keys while the getValues() method prints the values. Furthermore, HashMap does not maintain a specific order. Therefore, it does not print the elements according to the inserted order. Besides, there are two items with the key 2, but it only prints a single item. Therefore, it only contains unique keys.

What is HashSet

HashSet is a class that helps to create a collection by using a hash table for storage. HashSet extends AbstractSet; AbstractSet implements Set and Set extends Collection. It can contain null values, and it is not synchronized. Furthermore, HashSet does not maintain the insertion order, and it only contains unique elements. An example is as follows.

Figure 2: Program with HashSet

The ‘fruits’ is an object of HashSet; it can store String. The add method helps to insert new elements into the HashSet. The iterator method takes the number of items in the HashSet and stores it on the variable i. Then, the while loop goes through each item to print them. First, added item was Orange, but the first item in the output is Apple. Consequently, HashSet does not maintain the insertion order. Furthermore, there are two items as Apple, but it only prints one of them. Therefore, it only contains unique items.

Difference Between HashMap and HashSet

Definition

HashMap is a class that helps to create a collection which inherits the AbstractMap class and implements the Map interface. HashSet is a class that helps to create a set of elements as a single unit which inherits the AbstractSet class and implements Set interface. Thus, this is the fundamental difference between HashMap and HashSet.

Association with Collection Interface

Furthermore, HashMap is in the Map Hierarchy. It does not belong to the Collection interface. On the other hand, HashSet is in the Collection Hierarchy. Hence, this is one main difference between HashMap and HashSet.

Key, Values

Another difference between HashMap and HashSet is that while HashMap contains keys and values, HashSet contains values.

Functionality

One other difference between HashMap and HashSet is that the HashMap stores values based on keys using Hashtable while the HashSet stores elements using the hashing mechanism.

Conclusion

The main difference between HashMap and HashSet is that the HashMap belongs to the Map Interface Hierarchy and there is no association with the Collection interface while the HashSet belongs to the Collection Interface Hierarchy.

Reference:

1. “HashMap in Java – Javatpoint.” Www.javatpoint.com, Available here.
2. “HashSet in Java – Javatpoint.” Www.javatpoint.com, Available here.