Deque in Java

Deque in java collections with Example

Deque is short for “double-ended queue.” It is a linear collection that extends the Queue interface and supports insertion and deletion of the element at both the ends.

Deque is also used as a queue or as a stack because it supports both First In First Out(FIFO) and Last In First Out(LIFO). And its performance is faster than the queue or stack.

Deque provides the method to insert and delete the elements at both the ends. Each of the following methods is present in two forms; one throws an exception and other returns a value depending on the operation (either null or false).

	Elements at Head	Elements at Tail
	Throws exception	Special Value	Throws exception	Special Value
Insert	addFirst(e)	offerFirst(e)	addLast(e)	offerLast(e)
Remove	removeFirst()	pollFirst()	removeLast()	pollLast()
Examine	getFirst()	peekFirst()	getLast()	peekLast

Deque method and its description:

Modifier and Data Type	Methods	Description
void	addFirst(E e)	It pushes the specified element at the head of the deque if it is possible to do so without violating capacity restrictions.
void	addLast(E e)	It pushes the specified element at the tail of the deque if it is possible to do so without violating capacity restrictions.
Iterator<E>	descendingIterator()	It results in an iterator over the elements in the deque in reverse sequential order.
E	getFirst()	It retrieves the element from the head of the queue, but it does not remove that element from the Deque.
E	getLast()	It retrieves the element from the tail of the queue, but it does not remove that element from the Deque.
boolean	offerFirst(E e)	It inserts the specified element at the head of the Deque unless it violates capacity restrictions.
boolean	offerLast(E e)	It inserts the specified element at the tail of the Deque unless it violates capacity restrictions.
E	peekFirst()	It retrieves the elements from the head of the Deque, but does not remove that element, or returns null if the Deque is empty.
E	peekLast()	It retrieves the elements from the tail of the Deque but does not remove that element, or it returns null if the deque is empty.
E	pollFirst()	It retrieves the element from the head of the Deque and also removes that element, or returns null if the Deque is empty.
E	pollLast()	It retrieves and removes the element from the tail of the Deque, or returns null if this Deque is empty.
E	removeFirst()	It retrieves as well as removes the element from the head of the Deque.
boolean	removeFirstOccurrence()	This removes the first occurrence of the argumented element from the Deque.
E	removeLast()	It retrieves and removes the element from the tail of the Deque.
boolean	removeLastOccurance(Object o)	This removes the last occurrence of the specified element from the Deque.

Below is an example showing the working of some methods in the Deque.

import Java.util.*;
class DequeDemo
{
    public static void main(String[] args)
    {
        Deque<String> d = new LinkedList<String>();
        //adding elements to the queue using different ways
        d.add("Java"); // add to tail
        d.addFirst("Pyhton"); //add to head
        d.addLast("C#"); //add to tail
        d.push("CCNA"); //add to head
        d.offer("Jquery"); //add to tail
        d.offerFirst("HTML"); //add to head
        d.offerLast("CSS"); //add to tail
        //printing the deque
        System.out.println("Elements in the Deque is \n"+ d + "\n");  
        // displaying iterator in reverse order
        Iterator rev = d.descendingIterator();
        System.out.println("Iterator in Reverse Order:");
        while (rev.hasNext())
            System.out.println("\t" + rev.next());
        // Peek method returns the head, without deleting it from the deque
        System.out.println("Peek " + d.peek());
        System.out.println("After peek: " + d);
        // returning last element without removing
        System.out.println("PeekLast"+ d.peekLast());
        // Poll returns the head of the queue, and removes it from the deque
        System.out.println("Poll " + d.poll());
        System.out.println("After poll: " + d);
        //checking if a specific element exists in the deque
        System.out.println("Contains element 3: " + d.contains("Jquery"));
        //removing the first and last element.
        d.removeFirst();
        d.removeLast();
        System.out.println("Deque after removing first and last elements: " + d);
    }
} 

Output:

Elements in the Deque is
[HTML, CCNA, Pyhton, Java, C#, Jquery, CSS]
Iterator in Reverse Order:
       CSS
       Jquery
       C#
       Java
       Pyhton
       CCNA
       HTML
Peek HTML
After peek: [HTML, CCNA, Pyhton, Java, C#, Jquery, CSS]
PeekLastCSS
Poll HTML