HashSet in java

Let’s learn hashset in java.

HashSet in java

  • underlying data structure for hashset is Hash table.
  • HashSet implements Set interface.
  • Hashset do not allow duplicate elements. If any duplicate object is added, then add() method returns “false” without adding any duplicate object.
  • Object are inserted based on hash code.
  • Hashset insertion order is not preserved.
  • In hashset all objects will be inserted based on hashcode of objects.
  • In hashset heterogeneous elements are allowed.
  • Hashset allow “null” value.
  • Hashset implements Serializable and Cloneable interface.
  • In hashset, searching objects is easier. Because objects are stored based on hashcode.
  • HashSet do not implement RandomAccess interface.

Let’s see simple hashset in java example,

import java.util.HashSet;
import java.util.Iterator;

public class HashSetDemo 
{
   public static void main(String[] args) 
   {
      HashSet<String> hs = new HashSet<String>();
      // add elements using add() method 
      hs.add("Ilama"); 
      hs.add("Apple"); 
      hs.add("Strawberry");
      hs.add("Banana");
      // add duplicate elements
      hs.add("Ilama");
      // print HashSet elements 
      System.out.println(hs); 
      System.out.println("Does hashset contain Ilama?: " + hs.contains("Ilama"));
      // remove item from HashSet using remove() 
      hs.remove("Apple"); 
      System.out.println("List after removing Apple: " + hs);
      // iterating over hashset 
      System.out.println("Iterating over list: "); 
      Iterator<String> iterate = hs.iterator(); 
      while(iterate.hasNext())
      {
         System.out.println(iterate.next());
      }
   }
}



Output:

[Apple, Ilama, Strawberry, Banana]
Does hashset contain Ilama?: true
List after removing Apple: [Ilama, Strawberry, Banana]
Iterating over list:
Ilama
Strawberry
Banana


Initial capacity : is initial number of buckets that a Hashset object can hold.

Also read – treemap in java

Load factor/ fill ratio: is a measure to calculate after how much load a new hashset object is created.

                                no. of elements in the table
load factor   =    ————————————————-
                                       size of hash table

NOTE : best load factor would be 0.75 with respect to time and space complexity, according to java.

hashset in java

Here you can see hierarchy of hashset class. Hashset class extends abstractset class. AbstractSet class implements set interface.

And set interface extends collection interface. Collection interface extends iterable interface.


Also read – treeset in java


Hashset methods

MethodsDescription
boolean add(E e)adds the specified element to this set if it is not already present
void clear()removes all of the elements from this set.
Object clone()returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
boolean contains(Object o)returns true if this set contains the specified element.
boolean isEmpty()method used to check whether the set is empty or not. Returns true if this set contains no elements and false for non-empty condition.
Iterator iterator()returns an iterator over the elements in this set.
boolean remove(Object o)removes the specified element from this set if it is present.
int size()returns the number of elements in this set(its cardinality).

Constructors in hashset

  • HashSet h = new HashSet();

This constructor creates an empty hashset object with default initial capacity 16 and default fill ratio or load factor 0.75

  • HashSet h = new HashSet(int initialCapacity);

this constructor creates hashset object with specified initialCapacity with load factor 0.75. Nothing but customized initial capacity and default load factor.

  • HashSet h = new HashSet(int initialCapacity, float loadFactor);

Here we can customize load factor.

  • HashSet h = new HashSet(Collection C);

If you want Interconversion between Collection object then use this constructor.


Let’s see hashset implementation in java,

import java.util.HashSet;
import java.util.Iterator;

public class HashsetExample
{
   public static void main(String[] args)
   {
      // HashSet declaration
      HashSet<String> hs = new HashSet<String>();
      // add to hashset java
      hs.add("orange");
      hs.add("mango");
      hs.add("strawberry");
      // now adding duplicate values to HashSet
      hs.add("orange");

      // display the HashSet 
      System.out.println(hs); 
      System.out.println("HashSet contains orange or not : " + hs.contains("orange")); 

      // hashset remove java 
      hs.remove("mango"); 
      System.out.println("HashSet after removing - mango : " + hs); 

      // iterate through hashset 
      System.out.println("Iterating over list : "); 
      Iterator<String> iterate = hs.iterator(); 
      while(iterate.hasNext())
      {
         System.out.println(iterate.next());
      }
   }
}



Output:

[orange, strawberry, mango]

HashSet contains orange or not : true
HashSet after removing – mango : [orange, strawberry]
Iterating over list :
orange
strawberry


linkedhashset in java

Linkedhashset was introduced in java 1.4 version. Underlying data structure for linkedhashset is combination of hashtable and linked list.

Insertion order in linkedhashset is preserved and will not allow duplicate elements.

Syntax:

LinkedHashSet<String> lhs = new LinkedHashSet<String>();

Linkedhashset in java constructors

  • LinkedHashSet(): this constructor constructs a new, empty linked hash set with the default initial capacity (16) and load factor (0.75).
  • LinkedHashSet(Collection<? extends E> c): this constructor constructs a new linked hash set with the same elements as the specified collection.
  • LinkedHashSet(int initialCapacity): this constructor constructs a new, empty linked hash set with the specified initial capacity and the default load factor (0.75).
  • LinkedHashSet(int initialCapacity, float loadFactor): this constructor constructs a new, empty linked hash set with the specified initial capacity and load factor.

Let’s see LinkedHashSet in java with example

import java.util.LinkedHashSet;

public class LinkedHashsetExample 
{
   public static void main(String[] args) 
   {
      LinkedHashSet<String> lhs = new LinkedHashSet<String>();
      // add elements to LinkedHashSet
      lhs.add("Anteater");
      lhs.add("Bear");   
      lhs.add("Cheetah");   
      lhs.add("Deer");
      // will not add new element as Anteater already exists  
      lhs.add("Anteater");  
      lhs.add("Elephant");
      System.out.println("LinkedHashSet size: " + lhs.size());   
      System.out.println("Given LinkedHashSet: " + lhs);   
      System.out.println("Deer removed from LinkedHashSet: " + lhs.remove("Deer"));   
      System.out.println("Remove Zebra which is not present: " + lhs.remove("Zebra"));   
      System.out.println("Check if Anteater is present: " + lhs.contains("Anteater")); 
      System.out.println("New LinkedHashSet: " + lhs);
   }
}


Output:

LinkedHashSet size: 5
Given LinkedHashSet: [Anteater, Bear, Cheetah, Deer, Elephant]
Deer removed from LinkedHashSet: true
Remove Zebra which is not present: false
Check if Anteater is present: true
New LinkedHashSet: [Anteater, Bear, Cheetah, Elephant]

iterate hashset in java

To iterate hashset in java we have to use iterator() method, for-each loop and forEach() method (hashset java 8 method).

Because hashset class does not provide get() method fetch elements.

using iterator() method: this method returns an iterator over the elements in this set. The elements are returned in no particular order. Let’s see an example,

import java.util.HashSet;
import java.util.Iterator;

public class HashSetIteratorDemo 
{
   public static void main(String[] args) 
   {
      HashSet<String> hs = new HashSet<String>();
      // add elements using add() method 
      hs.add("Hello");
      hs.add("world"); 
      hs.add("Java");
      // Iterating over hashset 
      Iterator<String> iterate = hs.iterator(); 
      while(iterate.hasNext())
      {
         System.out.println(iterate.next());
      }
   }
}


Output:

Java
world
Hello

using enhanced for loop or for-each loop:

import java.util.HashSet;

public class HashSetForEach 
{
   public static void main(String[] args) 
   {
      HashSet<String> hs = new HashSet<String>();
      hs.add("Hello"); 
      hs.add("world"); 
      hs.add("Java"); 
      for(String str : hs)
      {
         System.out.println(str);
      }
   }
}


Output:

Java
world
Hello

using forEach() method:

import java.util.HashSet;

public class HashSetForEachMethod 
{
   public static void main(String[] args) 
   {
      HashSet hs = new HashSet();
      hs.add("Hello"); 
      hs.add("world"); 
      hs.add("Java");
      // iterate over hash set 
      hs.forEach(str -> System.out.println(str));
   }
}


Output:

Java
world
Hello

java hashset vs hashmap

Difference between hashset and hashmap

HashSetHashMap
introduced in java 1.2 version
implements Serializable and Cloneable interface
insertion order is not preserved. It is based on hashcodedoes not maintain insertion order. It is based on Hash function.
has add() methodhas put() method
implements Set interfaceimplements Map interface.
do not allow duplicate elements.allows duplicate values. Does not allow duplicate keys.
allows single null value.allows single null key and more than one null values.

java hashmap example

import java.util.HashMap;

public class HashMapExample 
{
   public static void main(String[] args) 
   {
      HashMap<Integer, String> hm = new HashMap<Integer, String>();
      // add elements
      hm.put(10,"Apple");
      hm.put(20,"Banana");
      hm.put(30,"Cherry");
      hm.put(40,"Dragonfruit");
      // print HashMap elements
      System.out.println("HashMap elements: " + hm);
   }
}


Output:

HashMap elements: {20=BB, 40=DD, 10=AA, 30=CC}

java hashset example

import java.util.HashSet;

public class HashSetExample 
{
   public static void main(String[] args) 
   {
      HashSet<String> hs = new HashSet<String>();
      hs.add("Anteater");
      hs.add("Bear");
      hs.add("Cheetah");
      hs.add("Deer");
      System.out.println("HashSet elements: ");
      for(String str : hs)
      {
         System.out.println(str);
      }
   }
}


Output:

HashSet elements:
Anteater
Bear
Cheetah
Deer

Reference – docs Oracle, oracle docs