Deque Interface in Java

The Deque interface is 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.

• Allows insertion and deletion from both front and rear
• Can be used as both stack (LIFO) and queue (FIFO)
• Common implementations are ArrayDeque and LinkedList

Declaration

public interface Deque<E> extends Queue<E>

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

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.

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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. 

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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. 

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Methods of Deque Interface

Below are some common list of methods of Deque Interface