Java Collections – Deque Example in Java

The java.util.Deque interface is a subtype of the java.util.Queue interface. In this type of queue, you can add and remove elements from both the ends and hence it is abbreviated as “Double Ended Queue” and pronounced as “deck” in short.

Lets see the basic operations using Dequeue

 Implementation

Since Deque is an interface,we need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following Deque implementations of Java Collections.

• java.util.LinkedList
• java.util.ArrayDeque

Examples of creating Queue Instance

Queue queue1 = new LinkedList();
Queue queue2 = new ArrayDeque();

The order in which the elements are stored internally depends upon the type of implementation that we choose.

Adding elements to the queue

To add element  the deque we can use any of the below methods.

add() method inserts element to the head of the deque. This throws IllegalStateException” if no space is currently available. When using a capacity-restricted deque

addFirst() method also inserts element to the head of the deque. This throws IllegalStateException” if no space is currently available. When using a capacity-restricted deque

addLast() method to insert element to the tail of the deque. These method throws IllegalStateException” if no space is currently available. When using a capacity-restricted deque. These method returns a boolean, if the insertion is successful it will return “true” else it will return “false”.

Deque deque = new LinkedList();
deque.add("Java");
deque.addFirst("jQuery");
deque.addLast("HTML5");

Other way of adding element to the queue is through offer() method. All the method will throw IllegalStateException” if no space is currently available. When using a capacity-restricted deque

offer() method inserts element to the head of the deque.

offerFirst() method also inserts element to the head of the deque.

offerLast() method to insert element to the tail of the deque.

 

 deque.offer("AngualarJS");
 deque.offerFirst("NodeJS");
 deque.offerLast("Javascript");

Accessing elements of the queue

Elements of the queue can be accessed in two ways.

Using getFirst() method / getLast() method, This method returns the head/ tail element of the deque without removing from the queue. It throws “NoSuchElementException”  when the queue is empty.

deque.getFirst()
deque.getLast()

Using peekFirst()/peekLast() method, This method also returns the head/tail element of the deque without removing from the queue. It returns “null”  when the queue is empty.

deque.peekFirst()
deque.peekLast()

Removing elements from the queue

Using removeFirst()/pop() method, This method removes and returns the first element of the deque, will throw “NoSuchElementException”  when the queue is empty.

deque.removeFirst()
deque.pop()

Using removeLast() method, This method removes and returns the last element of the deque, will throw “NoSuchElementException”  when the queue is empty.

deque.removeLast()

Code Snippet

import java.util.Deque;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.Queue;

public class DequeExample {

	public static void main(String[] args) 
	{
	    //Creating a queue object through LinkedList
		Deque deque = new LinkedList();
        
		//Adding elements to the deque
		deque.add("Java");
		//addFirst() adds the element to the head of the deque
		deque.addFirst("jQuery");
		//addFirst() adds the element to the tail of the deque
		deque.addLast("HTML5"); 
		
		//offer() adds the elements to the deque and returns a boolean
		deque.offer("AngualarJS");
		//offerFirst() adds the element to the head of the deque and returns a boolean
		deque.offerFirst("NodeJS");
		//offerFirst() adds the element to the tail of the deque and returns a boolean
		deque.offerLast("Javascript"); 
		
		System.out.println("Elements of the deque"+deque);
		
		//getFirst() Will retrive the head of the deque
		System.out.println("First element of the deque before removal:"+deque.getFirst());
		//The removeFirst() &pop() method will remove the first element of the queue
		deque.removeFirst();
		deque.pop();
		//peekFirst() Will retrive the head of the deque
		System.out.println("First element of the deque after removal:"+deque.peekFirst());
		
		//getLast() Will retrive the tail of the deque
		System.out.println("Last element of the deque before removal:"+deque.getLast());
		//The removeLast() method will remove the tail element of the queue
		deque.removeLast();
		//peekLast() Will retrive the tail of the deque
		System.out.println("Last element of the deque after removal:"+deque.peekLast());
		
		// Iterate through the queue elements.
        System.out.println("Normal Iteration");
        Iterator it1 = deque.iterator();
        while (it1.hasNext()) {
            System.out.println("    "+ it1.next());
        }
        
        // Reverse order iterator
        Iterator it2 = deque.descendingIterator();
        System.out.println("Reversed Iteration");
        while (it2.hasNext()) {
            System.out.println("    "+ it2.next());
        }
	}
}

Output

Elements of the deque[NodeJS, jQuery, Java, HTML5, AngualarJS, Javascript]
First element of the deque before removal:NodeJS
First element of the deque after removal:Java
Last element of the deque before removal:Javascript
Last element of the deque after removal:AngualarJS
Standard Iterator
    Java
    HTML5
    AngualarJS
Reverse Iterator
    AngualarJS
    HTML5
    Java