The main difference between a HashSet and a HashMap are:

1. Base class: A HashSet class implements the Set interface.

Whereas a HashMap class implements the Map interface.

2. Storage: A HashSet is used to store distinct objects. A

HashMap is used for storing key & value pairs, so that

these can be retrieved by key later on.

3. Duplicate Elements: A HashSet does not allow storing

duplicate elements. A HashMap also does not allow

duplicate keys. But we can store duplicate values in a

HashMap.

4. Null Elements: In a HashSet we can store a single null

value. In a HashMap we can store single null key, but any

number of null values.

5. Element Type: A HashSet contains only values of objects

as its elements. Whereas a HashMap contains entries(key

value pairs).

6. Iteration: By using an Iterator we can iterate a HashSet.

But a HashMap has to be converted into Set for iteration.

https://www.geeksforgeeks.org/difference-between-hashmap-and-hashset/