Queue in Java: Data Structure, Methods and Implementation

Queue is a Data Structure. it stores the data in ordered from and delete data in First In First Out.

In java, Queue is an Interface to implement this functionality. Queue interface is inside the java.util package. It extends Collection Interface.

If we need to use this data structure in our program, then we need class like LinkedList or PriorityQueue to declare  Queue.

Queue interface has certain methods to achieve inserting, removing functionality.

List of Methods

To add element in the Queue

• add(element) -> It simply add element to the queue. It returns IllegalStateException if Queue size can not be increased.
• offer(element) -> It is same as add() method but it does not return Exception.

To retrieve element from the Queue

• peek() -> It retrieve first(HEAD) element from the Queue but it does not remove element from the Queue.It returns null if Queue is empty. 
• element() -> It is same as peek() method.

To remove element from the Queue

• remove() -> It returns and remove the HEAD element from the Queue. It returns null if Queue is empty.
• poll() -> It returns and remove the HEAD from the Queue.

To access additional feature of the Queue

• size() -> To find the size of Queue.
• contains(element) -> To find Whether element is present in the Queue or not.It has return type boolean.
• isEmpty() -> To check Queue is empty or not. it returns true it it is empty.

Example

import java.util.*;

public class QueueDemo{

     public static void main(String []args){

         

        Queue<String> queue = new LinkedList<>();

         
        // add element in the Queue

        queue.add("Year 2020");

        System.out.println("Elements in the queue :-> "+queue);

         

        // add element in the Queue using method offer()

        queue.offer("January");

        System.out.println("Elements in the queue :-> "+ queue);      

      
        // size of the Queue 

        int size = queue.size();

        System.out.println("Size of queue :-> "+ size );

        

        queue.offer("February");

        queue.offer("Corona");

        System.out.println("Elements in the queue :-> "+ queue);     

        
         // To check element is present in the Queue or not

        boolean isContains = queue.contains("Corona");

        if(isContains){

            
            // remove element from the Queue

            String element = queue.remove();

            System.out.println(element + " element is removed from the history");   

            System.out.println("Elements in the queue :-> "+ queue);  

        }

        
        // fetch element from the Queue but it will not remove

        String head = queue.peek();

        System.out.println("Now HEAD element in the queue :-> "+ head);   

        
        // remove element from the Queue using poll()

        String element = queue.poll();

        System.out.println(element + " element is removed from the history");   

        System.out.println("Elements in the queue :-> "+ queue);  

        

        head = queue.peek();

        System.out.println("Now HEAD element in the queue :-> "+ head); 

        

        element = queue.poll();

        System.out.println(element + " element is removed from the history");   

        System.out.println("Elements in the queue :-> "+ queue);  

        

        head = queue.element();

        System.out.println("Now HEAD element in the queue :-> "+ head);

        

        queue.poll();

        
        // check whether Queue is empty or not

        boolean isEmpty = queue.isEmpty();

        if(isEmpty)

            System.out.println("Congratulations!! Corona is gone from the earth "); 

        else

            System.out.println("Oh!! there is still some viruses "); 

        

        System.out.println("Elements in the queue :-> "+ queue); 

        

     }

}

Output

Output Of Queue Code

Thanks!!!

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Posted by : Denes Vaghani May 16, 2020 Tags : Data Structure

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