what is the difference between hastmap and hashset?

Question

please explain in detail

Answer 1

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.

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.

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}

Summary:
HashSet

HashSet class implements the Set interface

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”}

HashSet does not allow duplicate elements that means you can not store duplicate values in HashSet.

HashSet permits to have a single null value.

HashMap

HashMap class implements the Map interface

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”}

HashMap does not allow duplicate keys however it allows to have duplicate values.

HashMap permits single null key and any number of null values.

Answer 2

thank you clearing my concept