Difference between HashSet and HashMap

In this article we are gonna discuss the differences between HashSet and HashMap classes.

HashSet vs HashMap

Differences:

HashSet	HashMap
HashSet class implements the Set interface	HashMap class implements the Map 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”}	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”}
HashSet does not allow duplicate elements that means you can not store duplicate values in HashSet.	HashMap does not allow duplicate keys however it allows to have duplicate values.
HashSet permits to have a single null value.	HashMap permits single null key and any number of null values.

Similarities:
1) Both HashMap and HashSet are not synchronized which means they are not suitable for thread-safe operations unitl unless synchronized explicitly. This is how you can synchronize them explicitly:
HashSet:

Set s = Collections.synchronizedSet(new HashSet(...));

HashMap:

 Map m = Collections.synchronizedMap(new HashMap(...));

2) Both of these classes do not guarantee that the order of their elements will remain constant over time.

3) If you look at the source code of HashSet then you may find that it is backed up by a HashMap. So basically it internally uses a HashMap for all of its operations.

4) They both provide constant time performance for basic operations such as adding, removing element etc.

HashSet example

import java.util.HashSet;
class HashSetDemo{ 
  public static void main(String[] args) {
     // Create a HashSet
     HashSet<String> hset = new HashSet<String>();
 
     //add elements to HashSet
     hset.add("AA");
     hset.add("BB");
     hset.add("CC");
     hset.add("DD");
 
     // Displaying HashSet elements
     System.out.println("HashSet contains: ");
     for(String temp : hset){
        System.out.println(temp);
     }
  }
}

Output:

HashSet contains: 
AA
BB
CC
DD

HashMap example

import java.util.HashMap;
class HashMapDemo{ 
  public static void main(String[] args) {
     // Create a HashMap
     HashMap<Integer, String> hmap = new HashMap<Integer, String>();
 
     //add elements to HashMap
     hmap.put(1, "AA");
     hmap.put(2, "BB");
     hmap.put(3, "CC");
     hmap.put(4, "DD");
 
     // Displaying HashMap elements
     System.out.println("HashMap contains: "+hmap);
  }
}

Output:

HashMap contains: {1=AA, 2=BB, 3=CC, 4=DD}

References:

HashSet javadoc
HashMap javadoc