Deque interface in Java with Example

Deque Interface present in java.util package is a subtype of the queue interface. The Deque is related to the double-ended queue that supports adding or removing elements from either end of the data structure. It can either be used as a queue(first-in-first-out/FIFO) or as a stack(last-in-first-out/LIFO). Deque is the acronym for double-ended queue.

• Null Handling: Most implementations do not allow null elements, as null is used as a special return value to indicate the absence of elements.
• Thread-Safe Alternatives: Use ConcurrentLinkedDeque or LinkedBlockingDeque for thread-safe operations and avoid ArrayDeque in concurrent environments as it is not thread-safe.

Example :

import java.util.ArrayDeque;
import java.util.Deque;

public class Example 
{
    public static void main(String[] args)
    {

        // Create a Deque of Strings
        Deque<String> deque = new ArrayDeque<>();

        deque.addFirst(1);
        deque.addLast(2);

        int first = deque.removeFirst();
        int last = deque.removeLast();

        // Displaying the Deque
        System.out.println("First: " + first + ", Last: " + last);
    }
}

Deque elements: []

Hierarchy Diagram of Queue and Deque Interface in Java

Below is the hierarchy diagram of Queue Interface, where Deque Interface extends the Queue Interface. Go through the below diagram to know high level ideas on Deque Interface.

Syntax: The deque interface is declared as,

public interface Deque extends Queue

Creating Deque Objects

Since Deque is an interface, objects cannot be created of the type deque. We always need a class that extends this list in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Deque. This type-safe queue can be defined as:

// Obj is the type of the object to be stored in Deque Deque<Obj> deque = new ArrayDeque<Obj> ();

Example: Deque

// Java program to demonstrate the working
// of a Deque in Java

import java.util.*;

public class DequeExample {
    public static void main(String[] args)
    {
        Deque<String> deque
            = new LinkedList<String>();

        // We can add elements to the queue
        // in various ways

        // Add at the last
        deque.add("Element 1 (Tail)");

        // Add at the first
        deque.addFirst("Element 2 (Head)");

        // Add at the last
        deque.addLast("Element 3 (Tail)");

        // Add at the first
        deque.push("Element 4 (Head)");

        // Add at the last
        deque.offer("Element 5 (Tail)");

        // Add at the first
        deque.offerFirst("Element 6 (Head)");

        System.out.println(deque + "\n");

        // We can remove the first element
        // or the last element.
        deque.removeFirst();
        deque.removeLast();
        System.out.println("Deque after removing "
                        + "first and last: "
                        + deque);
    }
}

[Element 6 (Head), Element 4 (Head), Element 2 (Head), Element 1 (Tail), Element 3 (Tail), Element 5 (Tail)]

Deque after removing first and last: [Element 4 (Head), Element 2 (Head), Element 1 (Tail), Element 3 (Tail)]

Operations Using the Deque Interface and the ArrayDeque Class

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

1. Adding Elements:

In order to add an element in a deque, we can use the add() method. The difference between a queue and a deque is that in deque, the addition is possible from any direction. Therefore, there are other two methods available named addFirst() and addLast() which are used to add the elements at either end. 

// Java program to demonstrate the
// addition of elements in deque

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:

In order to remove an element from a deque, there are various methods available. Since we can also remove from both ends, the deque interface provides us with removeFirst(), removeLast() methods. Apart from that, this interface also provides us with the poll(), pop(), pollFirst(), pollLast() methods where pop() is used to remove and return the head of the deque. However, poll() is used because this offers the same functionality as pop() and doesn’t return an exception when the deque is empty. 

// Java program to demonstrate the
// removal of elements in deque
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. 

// Java program to demonstrate the
// iteration of elements in deque

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 

Note: The class which implements the Deque interface is ArrayDeque. 

ArrayDeque

ArrayDeque class which is implemented in the collection framework provides us with a way to apply resizable-array. This is a special kind of array that grows and allows users to add or remove an element from both sides of the queue. Array deques have no capacity restrictions and they grow as necessary to support usage. They are not thread-safe which means that in the absence of external synchronization, ArrayDeque does not support concurrent access by multiple threads. ArrayDeque class is likely to be faster than Stack when used as a stack. ArrayDeque class is likely to be faster than LinkedList when used as a queue. Let’s see how to create a queue object using this class. 

// Java program to demonstrate the
// creation of deque object using the
// ArrayDeque class in Java

import java.util.*;

public class ArrayDequeDemo {
  
    public static void main(String[] args)
    {
        // Initializing an deque
        Deque<Integer> de_que = new ArrayDeque<Integer>(10);

        // add() method to insert
        de_que.add(10);
        de_que.add(20);
        de_que.add(30);
        de_que.add(40);
        de_que.add(50);

        System.out.println(de_que);

        // clear() method
        de_que.clear();

        // addFirst() method to insert the
        // elements at the head
        de_que.addFirst(564);
        de_que.addFirst(291);

        // addLast() method to insert the
        // elements at the tail
        de_que.addLast(24);
        de_que.addLast(14);

        System.out.println(de_que);
    }
}

[10, 20, 30, 40, 50]
[291, 564, 24, 14]

Methods of Deque Interface

The following are the methods present in the deque interface:  

Advantages of Using Deque

• Double-Ended: The main advantage of the Deque interface is that it provides a double-ended queue, which allows elements to be added and removed from both ends of the queue. This makes it a good choice for scenarios where you need to insert or remove elements at both the front and end of the queue.
• Flexibility: The Deque interface provides a number of methods for adding, removing, and retrieving elements from both ends of the queue, giving you a great deal of flexibility in how you use it.
• Blocking Operations: The Deque interface provides blocking methods, such as takeFirst and takeLast, that allow you to wait for elements to become available or for space to become available in the queue. This makes it a good choice for concurrent and multithreaded applications.

Disadvantages of Using Deque

• Performance: The performance of a Deque can be slower than other data structures, such as a linked list or an array, because it provides more functionality.
• Implementation Dependent: The behavior of a Deque can depend on the implementation you use. For example, some implementations may provide thread-safe operations, while others may not. It’s important to choose an appropriate implementation and understand its behavior before using a Deque.

Reference Book:

“Java Generics and Collections” by Maurice Naftalin and Philip Wadler is a comprehensive guide to the Java Collections Framework and generics, including the Deque interface. This book covers the basics of the Collections Framework, explains how to use collections and generics in practice, and provides tips and best practices for writing correct and efficient code. If you’re looking to learn more about the Deque interface and the Java Collections Framework in general, this book is an excellent resource.

GeeksforGeeks

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