Problem : Create a dynamic Set that contains only unique elements.

Program :

Step 1 - Coding

Create a text file c:\sunilos\TestHashSet.java  and copy below contents.

import java.util.*;
/*
 * A program to implement HashSet.
 */


public class TestHashSet {

    public static void main(String[] args) {

        HashSet hSet = new HashSet();

        // You can insert any object in the HashSet. Here it is string

        hSet.add("Jay");
        hSet.add("Viru");
        hSet.add("Basanti");

        System.out.println("Some Importent Methods");
        System.out.println(" Set size : " + hSet.size());
        System.out.println(" Contains Jay : " + hSet.contains("Jay"));

        /*
         * Primitive data type need to be converted into Objects before
         * inserting. Get all elements and print with help of Iterator interface
         */
        Iterator it = hSet.iterator(); // Get an iterator

        System.out.println("\nPrint All Elements with help of Iterator ");

        while (it.hasNext()) { // Checks if any element in list
            Object oo = it.next(); // Get next available element
            System.out.println(" From Iterator -- " + oo);
        }

        // Clear all elements
        hSet.clear();
        System.out.println(" Is Empty " + hSet.isEmpty());

    }
}

Step 2 - Deployment

1. Create a folder 'c:\sunilos'. 
2. Create or copy TestHashSet.java  into 'c:\sunilos' folder.
3. Open your command prompt and go to  'c:\sunilos'
   
4. Compile TestHashSet.java with help of 
• javac TestHashSet.java command. Compiled class file will be created in the same folder
  
5. Congratulations!! your Java program is ready to serve.

Step 3 - Testing

1. Make sure you are on Command Prompt under c:\sunilos directory
   
2. Now start your java program from command prompt with help of 
• java TestHashSet command.

Output

C:\sunilos>java TestHashSet
Some Importent Methods
 Set size : 3
 Contains Jay : true

Print All Elements with help of Iterator
 From Iterator -- Jay
 From Iterator -- Viru
 From Iterator -- Basanti
 Is Empty true

FAQ

What is HashSet ?

This class implements the Set interface, backed by a hash table (actually a HashMap instance).  
This class permits the null element.
When you gets elements through Iterator, elements order may be changed.

Is HashSet thread-safe?

No it is not synchronized, if you want to make it thread safe then synchronize it with help of below command

    Set s = Collections.synchronizedSet(new HashSet());

Most Interesting methods in HashSet?

• add() - Adds the specified element to this set if it is not already present.
• clear() - Removes all of the elements from this set.
• clone() - Returns a shallow copy of this HashSet instance: the elements themselves are not cloned.
• contains() - Returns true if this set contains the specified element.
• isEmpty() -  Returns true if this set contains no elements.
• iterator() - Returns an iterator over the elements in this set.
• remove() - Removes the specified element from this set if it is present.
• size() -  Returns the number of elements in this set (its cardinality).

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