HashSet Class in Java Tutorial with Example

Java HashSet class implements Set interface. It has the following properties.

1. HashSet does not allow duplicates, means it contains unique elements.
2. HashSet is unordered meaning it does not guarantee that the order will remain constant over time.
3. HashSet is backed by Hashtable (actually a HashMap instance).
4. HashSet permits the null elements.
5. HashSet implementation is not synchronized. It can be synchronized externally.
    Set s = Collections.synchronizedSet(new HashSet(...));
6. Iterators returned by HashSet class is fail- fast.

HashSet Example

import java.util.*;

public class HashSetExample {

   public static void main(String args[]) {

      // Declare HashSet 
      HashSet<String> hashset = 
             new HashSet<String>();

      // Adding elements to the HashSet
      hashset.add("Apple");
      hashset.add("Pear");
      hashset.add("Mango");
      hashset.add("Papaya");
      hashset.add("Orange");
      //Addition of duplicate elements
      hashset.add("Apple");
      hashset.add("Mango");
      //Addition of null values
      hashset.add(null);
      hashset.add(null);

      //Displaying HashSet elements
      System.out.println(hashset);
   }
}

Output :
[null, Apple, Pear, Papaya, Mango, Orange]

HashSet Methods

1. boolean add(Element e) : it adds the element e to the list.
2. public void clear() : it removes all of the elements from the set.
3. public Object clone() : It returns the shallow copy of the HashSet instance.
4. public boolean contains(Object o) : it returns true if the set contains the specified element.
5. public boolean isEmpty() : This method returns true if the set  contains no elements.
6. public boolean remove(Object o) : it removes the specified element from the set if it is present.
7. public int size() : it returns the number of elements in the set.

HashSet Tutorials

Basics

How HashSet works in Java

HashSet Iterator Example

Conversions

Convert HashSet to List

Convert HashSet to TreeSet

Convert HashSet to Array

Differences

HashSet vs TreeSet

HashSet vs HashMap

HashSet vs CopyOnWriteArraySet

HashSet vs ArrayList 

More Interview Questions

HashSet Interview Questions

References : HashSet Oracle docs