What is Deque in Java and how to implement its interface?

Deque in Java is a double-ended queue. It is used to add or remove data elements from either head or tail. Java.util.Dequeue interface is used by deque in Java which is a subtype of the java.util.Queue interface. Let us study the concept in detail.

Below mentioned pointers are going to be the agenda for this article:

1. What is Deque in Java?
2. Methods used in Deque
3. Java program to show the working of deque

Let’s begin!

What is Deque in Java?

The Deque is a double-ended queue. It helps in adding and removing data elements from a data structure from either head or tail. It can be used either as a FIFO or a LIFO. Examples of FIFO and LIFO are Queue and Stack respectively.

This is how the working diagrammatically looks like. Moving on, we have several methods included in the deque. Let’s take a look.

Methods used in the deque

Methods used in deque are as follows:

1. addFirst(element): this method adds an element to the head.
2. addLast(element): this method adds an element to the tail.
3. add(element): this method adds an element to the tail.
4. removeFirst(): this method removes the element from the head.
5. removeLast(): this method removes the element from the tail.
6. push(element): this method adds an element to the head.
7. pop(element): this method removes an element from the head.
8. offerFirst(element): this method adds an element to the head and will return a Boolean to depict if the insertion was successful.
9. offerLast(element): this method removes an element from the tail and will return a Boolean to depict if the insertion was successful.
10. descendingIterator(): this method returns an iterator that has the reverse order for this deque.
11. poll(): this method retrieves and removes the head of the deque or it will return null if the deque is empty.
12. pollFirst(): this method retrieves and removes the first element of this deque, or it will return null if this deque is empty.
13. pollLast(): this method retrieves and removes the last element of this deque, or will return null if this deque is empty.
14. iterator(): this method returns an iterator for the deque.

Well, these were a few methods. Let me show you the implementation process through a Java code.

Java program to show the working of deque

Take a look at the example program below:

import java.util.ArrayList;
import java.util.List;
public class DoubleEndedQueueImpl {
private List<Integer> deque = new ArrayList<Integer>();
public void insertFront(int item){
//add element at the beginning of the queue
System.out.println("element added at front: "+item);
deque.add(0,item);
System.out.println(deque);
}
public void insertRear(int item){
//add element at the end of the queue
System.out.println("element added at rear: "+item);
deque.add(item);
System.out.println(deque);
}
public void removeFront(){
if(deque.isEmpty()){
System.out.println("Deque underflow, unable to remove.");
return;
}

//remove an item from the beginning of the queue
int rem = deque.remove(0);
System.out.println("element removed from front: "+rem);
System.out.println(deque);
}

public void removeRear(){
if(deque.isEmpty()){
System.out.println("Deque underflow, unable to remove.");
return;
}

//remove an item from the beginning of the queue
int rem = deque.remove(deque.size()-1);
System.out.println("element removed from front: "+rem);
System.out.println(deque);
}



public int peakFront(){
//gets the element from the front without removing it
int item = deque.get(0);
System.out.println("Element at first: "+item);
return item;
}



public int peakRear(){
//gets the element from the rear without removing it
int item = deque.get(deque.size()-1);
System.out.println("Element at rear: "+item);
return item;
}

public static void main(String a[]){
DoubleEndedQueueImpl deq = new DoubleEndedQueueImpl();
deq.insertFront(34);
deq.insertRear(45);
deq.removeFront();
deq.removeFront();
deq.removeFront();
deq.insertFront(21);
deq.insertFront(98);
deq.insertRear(5);
deq.insertFront(43);
deq.removeRear();
}
}

Output

adding at front: 34

[34]

adding at rear: 45

[34, 45]

removed from front: 34

[45]

removed from front: 45

[]

Deque underflow!! unable to remove.

adding at front: 21

[21]

adding at front: 98

[98, 21]

adding at rear: 5

[98, 21, 5]

adding at front: 43

[43, 98, 21, 5]

removed from front: 5

[43, 98, 21]

With this, we have reached towards the end of this article. I hope the content explained above added value to your Java knowledge. Keep reading, keep exploring!

Check out the Java Training by Edureka, a trusted online learning company with a network of more than 250,000 satisfied learners spread across the globe. We are here to help you with every step on your journey, for becoming a besides this java interview questions, we come up with a curriculum which is designed for students and professionals who want to be a Java Developer. 

Got a question for us? Please mention it in the comments section of this “Deque in Java” blog and we will get back to you as soon as possible.