Java – Hashset class

Set interface: Implementations are

1. HashSet
2. LinkedHashSet
3. TreeSet

Note : HashSet & LinkedHashSet stores the data into collection Object according to HashTable algorithm.

How the elements store into HashTable ?

• Every Hash table has a fixed size by default.
• The size may varies depends on number of elements stored in the Collection.
• Hash table implementation is bit advanced to linked list representation.
• In Hashtable also, elements will be stored in the form of nodes only.
• Searching of element is faster than Linkedlist in Hashtable.

HashSet :

• available in util package.
• since from jdk 1.2
• not ordered
• not allowed duplicates.
• initial capacity  is 16
• default load factor is 0.75
• HashSet is not Synchronized by default, can be synchronized explicitly as follows…
  • Collections.synchronizedSet(new HashSet(….));     

ArrayList is an ordered collection. Hence the elements will be displayed in the insertion order only.

import java.util.*;
class Demo 
{
	public static void main(String[] args) 
	{
		ArrayList list = new ArrayList();
		for (int i=10 ; i<=50 ; i=i+10)
		{
			list.add(i);
		}
		System.out.println("List is : " + list);
	}
}

HashSet is not ordered. The following code snippet explains clearly.

import java.util.*;
class Demo 
{
	public static void main(String[] args) 
	{
		HashSet set = new HashSet();
		for (int i=10 ; i<=50 ; i=i+10)
		{
			set.add(i);
		}
		System.out.println("Set is : " + set);
	}
}

• ArrayList allow duplicates.
• Random class generates the random values from 0 to specified bound.
• The following code generates duplicates values as we have given low bound value and store those elements into ArrayList object.

import java.util.*;
class Demo 
{
	public static void main(String[] args) 
	{
		Random rand = new Random();
		ArrayList list = new ArrayList();
		for (int i=1 ; i<=10 ; i++)
		{
			int ele = rand.nextInt(5); // generate value from 0 - 4
			list.add(ele);
		}
		System.out.println("List is : " + list);
	}
}

• Set doesn’t allow duplicates.
• We can construct one collection from another collection object.
• If we construct Set from List, all the duplicates will be removed automatically.

import java.util.*;
class Demo 
{
	public static void main(String[] args) 
	{
		Random rand = new Random();
		ArrayList list = new ArrayList();
		for (int i=1 ; i<=10 ; i++)
		{
			int ele = rand.nextInt(5);
			list.add(ele);
		}
		System.out.println("List is : " + list);

		HashSet set = new HashSet(list);
		System.out.println("Set is : " + set);
	}
}