175 Java HashSet Class

Introduction

The Java HashSet class implements the Set interface, backed by a hash table.Following are the important points about HashSet −

• This class makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.

• This class permits the null element.

Class declaration

Following is the declaration for java.util.HashSet class −

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

Parameters

Following is the parameter for java.util.HashSet class −

E − This is the type of elements maintained by this set.

Class constructors

Sr.No.	Constructor & Description
1	HashSet() This constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75).
2	HashSet(Collection<? extends E> c) This constructs a new set containing the elements in the specified collection.
3	HashSet(int initialCapacity) This constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).
4	HashSet(int initialCapacity, float loadFactor) This constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.

Class methods

Sr.No.	Method & Description
1	boolean add(E e) This method adds the specified element to this set if it is not already present.
2	void clear() This method removes all of the elements from this set.
3	Object clone() This method returns a shallow copy of this HashSet instance, the elements themselves are not cloned.
4	boolean contains(Object o) This method returns true if this set contains the specified element.
5	boolean isEmpty() This method returns true if this set contains no elements.
6	Iterator<E> iterator() This method returns an iterator over the elements in this set.
7	boolean remove(Object o) This method removes the specified element from this set if it is present.
8	int size() This method returns returns the number of elements in this set(its cardinality).
9	Spliterator<E> spliterator() This method returns a late-binding and fail-fast Spliterator over the elements in this set.

Methods inherited

This class inherits methods from the following classes −

• java.util.AbstractSet
• java.util.AbstractCollection
• java.util.Object
• java.util.Set

Adding element to a HashSet Example

The following example shows the usage of Java HashSet add() method to add entries to the HashSet. We've created a HashSet object of Integer. Then few entries are added using add() method and then set is printed.

package com.codeheap;

import java.util.HashSet;

public class HashSetDemo {
   public static void main(String args[]) {
      
      // create hash set
      HashSet <Integer> newset = new HashSet <>();

      // populate hash set
      newset.add(1); 
      newset.add(2);
      newset.add(3);  

      // checking elements in hash set
      System.out.println("Hash set values: "+ newset);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Hash set values: [1, 2, 3]