Difference between HashSet and HashMap?

Question

HashSet is a set, e.g. {1,2,3,4,5}

HashMap is a key -> value (key to value) map, e.g. {a -> 1, b -> 2, c -> 2, d -> 1}

Notice in my example above that in the HashMap there must not be duplicate keys, but it may have duplicate values.

In the HashSet, there must be no duplicate elements.

They are entirely different constructs. A HashMap is an implementation of Map. A Map maps keys to values. The key look up occurs using the hash.

On the other hand, a HashSet is an implementation of Set. A Set is designed to match the mathematical model of a set. A HashSet does use a HashMap to back its implementation, as you noted. However, it implements an entirely different interface.

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HashSet

1. HashSet class implements the Set interface
2. In HashSet, we store objects(elements or values) e.g. If we have a HashSet of string elements then it could depict a set of HashSet elements: {“Hello”, “Hi”, “Bye”, “Run”}
3. HashSet does not allow duplicate elements that mean you can not store duplicate values in HashSet.
4. HashSet permits to have a single null value.

5. HashSet is not synchronized which means they are not suitable for thread-safe operations until unless synchronized explicitly.[similarity]

                         add      contains next     notes
   HashSet               O(1)     O(1)     O(h/n)   h is the table 
   

HashMap

1. HashMap class implements the Map interface
2. HashMap is used for storing key & value pairs. In short, it maintains the mapping of key & value (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This is how you could represent HashMap elements if it has integer key and value of String type: e.g. {1->”Hello”, 2->”Hi”, 3->”Bye”, 4->”Run”}
3. HashMap does not allow duplicate keys however it allows having duplicate values.
4. HashMap permits single null key and any number of null values.

5. HashMap is not synchronized which means they are not suitable for thread-safe operations until unless synchronized explicitly.[similarity]

                          get      containsKey next     Notes
    HashMap               O(1)     O(1)        O(h/n)   h is the table 
   

Please refer this article to find more information.

It's really a shame that both their names start with Hash. That's the least important part of them. The important parts come after the Hash - the Set and Map, as others have pointed out. What they are, respectively, are a Set - an unordered collection - and a Map - a collection with keyed access. They happen to be implemented with hashes - that's where the names come from - but their essence is hidden behind that part of their names.

Don't be confused by their names; they are deeply different things.