HashSet in Java With Examples

This class implements the Set interface, backed by a hash table (actually a HashMap instance). It does not guarantee the iteration order of the set. This means that the iteration order of Java HashSet elements doesn’t remain constant. This class permits the null element.

Points to Note about HashSet:

1. HashSet internally uses Hashtable data structure.
2. HashSet doesn’t maintain any order, the elements would be returned in any random order.
3. HashSet doesn’t allow duplicates. If you try to add a duplicate element in HashSet, the old value would be overwritten.
4. HashSet allows null values, however if you insert more than one nulls, it would override the previous null value.
5. HashSet is non-synchronized. However it can be synchronized explicitly like this: Set s = Collections.synchronizedSet(new HashSet(...));
6. The iterator returned by this class is fail-fast which means iterator would throw ConcurrentModificationException, if HashSet has been modified after creation of iterator, by any means except iterator’s own remove method.

A Simple Example of HashSet in Java

Let’s see a simple HashSet example, where we are adding few string elements to HashSet and then iterating the HashSet to print the elements.

import java.util.HashSet;
public class JavaExample {
  public static void main(String args[]) {
    // HashSet declaration
    HashSet<String> hSet = new HashSet<>();

    // Adding elements to the HashSet
    hSet.add("Cricket");
    hSet.add("Hockey");
    hSet.add("Basketball");

    System.out.println("HashSet Elements: ");
    // Iterating HashSet
    for(String s: hSet){
      System.out.println(s);
    }
  }
}

Output:

HashSet class Hierarchy

HashSet class extends AbstractSet class. The AbstractSet class implements Set interface, which extends Collection interface. This hierarchy can be represented as follows:

HashSet -> AbstractSet -> Set -> Collection -> Iterable

HashSet Declaration

HashSet class belongs to java.util package. The declaration of HashSet in Java is:

public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable

Initial Capacity and Load Factor:

Initial capacity represents the initial data buckets allocated to HashSet, this automatically increases when HashSet gets full.

Load factor measures the load of HashSet, it represents how much the HashSet is full. A load factor of .60 means that when HashSet is 60% full, the capacity of HashSet is automatically increased.

                         Number of element in HashSet
Load Factor Of HashSet = ----------------------------
                         Size of the HashSet

Constructors of Java HashSet Class

Constructor	Description
HashSet()	It builds an empty HashSet with initial capacity of 16 and load factor of .75. Example: HashSet<String> hSet = HashSet<>();
HashSet(int initialCapacity)	It builds an empty HashSet with the specified initial capacity. The default load factor remains .75.
HashSet(int initialCapacity, float loadFactor)	It builds an empty HashSet with the specified initial capacity and load factor.
HashSet(Collection)	This doesn’t create an empty HashSet. It creates the HashSet with the elements copied from the passed Collection.

Java HashSet Examples

Let’s see few examples of HashSet in Java.

1. Adding duplicate elements

HashSet overrides duplicate values.

import java.util.HashSet;
public class HashSetExample {
   public static void main(String args[]) {
      // HashSet declaration
      HashSet<String> hset = 
               new HashSet<String>();

      // Adding elements to the HashSet
      hset.add("Apple");
      hset.add("Mango");
      hset.add("Grapes");
      hset.add("Orange");
      hset.add("Fig");
      //Addition of duplicate elements
      hset.add("Apple");
      hset.add("Mango");
      //Addition of null values
      hset.add(null);
      hset.add(null);

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

Output:

[null, Mango, Grapes, Apple, Orange, Fig]

As you can see there all the duplicate values are not present in the output including the duplicate null value.

2. Removing elements

import java.util.HashSet;
public class JavaExample {
  public static void main(String args[]) {
    // HashSet declaration
    HashSet<String> hSet = new HashSet<>();

    // Adding elements to the HashSet
    hSet.add("AA");
    hSet.add("BB");
    hSet.add("CC");
    hSet.add("DD");
    hSet.add("EE");

    //removing elements
    hSet.remove("EE");
    hSet.remove("CC");

    System.out.println("HashSet Elements: ");
    // Iterating HashSet
    for(String s: hSet){
      System.out.println(s);
    }
  }
}

Output:

HashSet Elements: 
AA
BB
DD

3. Adding elements from other Collection

Here, we are adding ArrayList elements to HashSet

import java.util.*;
public class JavaExample {
  public static void main(String args[]) {
    //ArrayList declaration and and adding elements
    ArrayList<String> arrList=new ArrayList<>();
    arrList.add("AA");
    arrList.add("BB");
    arrList.add("CC");

    //copying ArrayList elements to HashSet
    HashSet<String> hSet=new HashSet(arrList);
    //adding another element to HashSet after copy
    hSet.add("DD");

    System.out.println("HashSet elements: ");
    Iterator<String> it= hSet.iterator();
    while(it.hasNext())
    {
      System.out.println(it.next());
    }
  }
}

Output:

HashSet elements: 
AA
BB
CC
DD

HashSet Methods:

1. boolean add(Element  e): It adds the element e to the list.
2. void clear(): It removes all the elements from the list.
3. Object clone(): This method returns a shallow copy of the HashSet.
4. boolean contains(Object o): It checks whether the specified Object o is present in the list or not. If the object has been found it returns true else false.
5. boolean isEmpty(): Returns true if there is no element present in the Set.
6. int size(): It gives the number of elements of a Set.
7. boolean(Object o): It removes the specified Object o from the Set.

More HashSet Guides:

• Delete all elements from HashSet
• How to iterate through a HashSet
• Convert a HashSet to an array
• Convert a HashSet to a TreeSet
• Convert HashSet to a List/ArrayList
• HashSet vs HashMap
• HashSet vs TreeSet