java.util.Deque – Deque Java Example

In this example, we are going to explain the java.util.Deque Deque Java interface. The interface name is an abbreviation of “Double Ended Queue”, and it is essentially a queue that implements methods which allows the developer to add elements to both ends of the queue (head and tail). We are going to show the most important methods of this interface, as well as explain their usage of java deque implementation.

1. Deque Java Example

Deque is an interface, so we cannot instantiate it by itself. We can use any of the following implementations,

• java.util.LinkedList – it is quite common to use Linked List for implementing Queue and the implementation of Deque in Java. This internally uses the Linked List. A new instance can be created as Deque deque = new LinkedList()
• java.util.ArrayDeque – it internally uses a dynamically resizable array. A new instance can be created as Deque deque = new ArrayDeque()

Below diagram shows the class hierarchy of java.util.Deque,

We are using a LinkedList implementation of java deque in the example. Let’s see how this works.

DequeExample.java

/**
 * @author: Santsoh Balgar Sachchidananda
 */

import java.util.Deque;
import java.util.Iterator;
import java.util.LinkedList;

public class DequeExample {

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

        // We can add elements to the queue in various ways
        deque.add("Element 1 (Tail)"); // add to tail
        deque.addFirst("Element 2 (Head)");
        deque.addLast("Element 3 (Tail)");
        deque.push("Element 4 (Head)"); //add to head
        deque.offer("Element 5 (Tail)");
        deque.offerFirst("Element 6 (Head)");
        deque.offerLast("Element 7 (Tail)");

        System.out.println("Deque elements: ");
        System.out.println(deque);
        System.out.println("*********************************************");
        // Iterate through the queue elements.
        System.out.println("Iterating over Deque with Standard Iterator");
        Iterator iterator = deque.iterator();
        while (iterator.hasNext()) {
            System.out.println("\t" + iterator.next());
        }
        System.out.println("*********************************************");

        // Reverse order iterator
        Iterator reverse = deque.descendingIterator();
        System.out.println("Iterating over Deque with Reverse Iterator");
        while (reverse.hasNext()) {
            System.out.println("\t" + reverse.next());
        }
        System.out.println("*********************************************");

        // Peek returns the head, without deleting it from the deque
        System.out.println("Peek into the Deque" + deque.peek());
        System.out.println("After peek: \n");
        System.out.println(deque);
        System.out.println("*********************************************");

        // Pop returns the head, and removes it from the deque
        System.out.println("Pop from Deque" + deque.pop());
        System.out.println("After pop: \n");
        System.out.println(deque);
        System.out.println("*********************************************");

        // We can check if a specific element exists in the deque
        System.out.println("Contains element 3: " + deque.contains("Element 3 (Tail)"));
        System.out.println("*********************************************");
        // We can remove the first / last element.
        deque.removeFirst();
        deque.removeLast();
        System.out.println("Deque after removing first and last: " + deque);
    }
}

Output

Follow below instructions to run the program,

• Copy java code and save it as DequeExample.java in a directory of your choice
• Open command prompt, navigate to the directory where java file is saved and run the command javac DequeExample.java
• The previous step generates a .class file. To run the program, run command java DequeExample (Note that, no extension is specified)

2. Method explanation

Now we will explain the usage of the methods presented in the example above. Some of them exist in the standard LinkedList implementation, so we mostly used Deque-specific methods, that have to do with the element insertion/removal from the head and the tail of the Deque.

Deque methods can be summarized as,

Each of the Deque method explanation is given below,

Apart from these methods, java.util.Deque inherits a number of methods from java.util.Collection interface.

3. Use cases for Deque

Below are some use cases to use java.util.Deque,

• Deque can be used to implement Stack, Queue, and List.
• Deque can be used to implement priority queues.
• implement undo or history – each new item is inserted at the front of the queue and older item can be removed from the tail of the queue.
• Can be used to implement recursive processes.

4. Considerations

• Deque is not threadsafe. Hence, can’t be used in concurrent processing.
• Null items can’t be inserted to Deque.

5. Download the Source Code

In this section, I have provided a link to download the example program.

Last updated on Sept. 12, 2019

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