Deque Interface in Java

The Deque interface is a part of the java.util package and extends the Queue interface. It stands for Double-Ended Queue, and represents a linear collection that allows insertion, removal, and retrieval of elements from both ends

• Flexible Data Structure: Can be used as a stack (LIFO) or a queue (FIFO).
• Null Elements Not Allowed: Some implementations, like ArrayDeque, do not allow null.
• Resizable Implementations: Implementations like ArrayDeque grow dynamically as needed.
• Thread Safety: Most implementations are not thread-safe; synchronization is required for concurrent use.

Declaration of Deque

public interface Deque<E> extends Queue<E>

Here, E is the type of elements stored in the deque.

import java.util.ArrayDeque; import java.util.Deque; public class Geeks  {  public static void main(String[] args)  {  // Create a Deque of Strings  Deque<String> d = new ArrayDeque<>();  d.addFirst("1");  d.addLast("2");  String f = d.removeFirst();  String l = d.removeLast();  // Displaying the Deque  System.out.println("First: " + f + ", Last: " + l);  } } 

First: 1, Last: 2 

Hierarchy of Deque interface

It extends the Queue interface which extends the Collection Interface. It has implementation classes like ArrayDeque and LinkedList.

Common Implementations classes

• ArrayDeque: Resizable array implementation; fast for insertion/removal at both ends; not thread-safe.
• LinkedList: Doubly-linked list; supports all deque operations; allows nulls.
• ConcurrentLinkedDeque: Thread-safe deque based on linked nodes; suitable for concurrent use.

Deque Operations using ArrayDeque

Let’s see how to perform a few frequently used operations on the Deque using the ArrayDeque class. 

1. Adding Elements

To add elements in a Deque, you can use add() , addFirst(), or addLast(), allowing insertion at either end, unlike a standard Queue which only adds at the tail.

import java.util.*; public class ArrayDequeDemo {  public static void main(String[] args)  {  // Initializing an deque  Deque<String> dq = new ArrayDeque<String>();  // add() method to insert  dq.add("For");  dq.addFirst("Geeks");  dq.addLast("Geeks");  System.out.println(dq);  } } 

[Geeks, For, Geeks] 

2. Removing Elements

Elements can be removed from both ends using removeFirst(), removeLast(), pop(), or poll() variants; pop() throws an exception if empty, while poll() is safe. 

import java.util.*; public class ArrayDequeDemo {    public static void main(String[] args)  {  // Initializing an deque  Deque<String> dq = new ArrayDeque<String>();  // add() method to insert  dq.add("For");  dq.addFirst("Geeks");  dq.addLast("Geeks");  System.out.println(dq);    System.out.println(dq.pop());    System.out.println(dq.poll());    System.out.println(dq.pollFirst());    System.out.println(dq.pollLast());  } } 

[Geeks, For, Geeks] Geeks For Geeks null 

3. Iterating through the Deque

Since a Deque can be iterated from both directions, the iterator method of the Deque interface provides us two ways to iterate. One from the first and the other from the back. 

import java.util.*; public class ArrayDequeDemo {  public static void main(String[] args)  {  // Initializing an deque  Deque<String> dq = new ArrayDeque<String>();  // add() method to insert  dq.add("For");  dq.addFirst("Geeks");  dq.addLast("Geeks");  dq.add("is so good");  for (Iterator itr = dq.iterator(); itr.hasNext();) {  System.out.print(itr.next() + " ");  }  System.out.println();  for (Iterator itr = dq.descendingIterator(); itr.hasNext();) {  System.out.print(itr.next() + " ");  }  } } 

Geeks For Geeks is so good is so good Geeks For Geeks 

Methods of Deque Interface

Deque in Java

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