Queue and De-queue (Nikhil)

Java Queues and Deques: In-Depth Overview

What Is a Queue?

A Queue is a collection designed to hold elements prior to processing. It typically follows the FIFO (First-In-First-Out) principle—elements that are inserted first are the ones that are removed first. Queues are especially useful for tasks such as job scheduling, handling requests in order, or managing resources.

Key Characteristics of a Queue:

  • FIFO Order: Items are processed in the order they were added.
  • No Random Access: Unlike lists, queues do not offer methods to access elements by an index.
  • Basic Operations: Include adding, peeking at, and removing elements.

What Is a Deque?

A Deque (double-ended queue) extends the functionality of a standard queue by allowing insertion and removal of elements at both the front and the back. This versatile data structure supports both FIFO (queue behavior) and LIFO (stack behavior) operations.

Key Characteristics of a Deque:

  • Double-Ended Operations: You can add or remove elements from both ends.
  • Flexible Order: Can be used as a regular queue (FIFO) or a stack (LIFO) based on the operations you choose.
  • No Indexed Access: Similar to queues, deques do not support random access via indexes.

Types of Queue and Deque Implementations in Java

  1. LinkedList
    • Description: Implements both the Queue and Deque interfaces.
    • Usage: Suitable for basic FIFO queue operations and for scenarios that require double-ended operations.
    • Behavior: Maintains the order of insertion, but does not provide random access.
  2. PriorityQueue
    • Description: Implements the Queue interface, but orders its elements according to their natural order or a provided Comparator, rather than in FIFO order.
    • Usage: Ideal for situations where elements need to be processed based on priority (e.g., shortest job first).
    • Limitation: Does not support null values and the iteration order is not guaranteed to reflect the priority order.
  3. ArrayDeque
    • Description: A resizable array implementation of the Deque interface.
    • Usage: Often recommended for deque implementations because of its efficiency. It can be used to implement both stack and queue behaviors.
    • Behavior: Provides constant-time performance for adding and removing elements at both ends.
    • Limitation: Not thread-safe by default; external synchronization or concurrent alternatives are needed in multi-threaded environments.
  4. ConcurrentLinkedQueue (and ConcurrentLinkedDeque)
    • Description: Thread-safe implementations of the Queue and Deque interfaces found in the java.util.concurrent package.
    • Usage: Suitable when multiple threads need to access and modify the queue concurrently.
    • Behavior: Designed for high throughput in concurrent settings without locking the entire data structure.

Common Operations on Queues and Deques

For a Queue:

  • Add Elements:
    • boolean offer(E e): Inserts the specified element into the queue, returning false if the queue is bounded and there is no space.
    • boolean add(E e): Inserts the specified element; throws an exception if the element cannot be added.
  • Inspect Elements (without removal):
    • E peek(): Retrieves, but does not remove, the head of the queue; returns null if empty.
    • E element(): Similar to peek() but throws an exception if the queue is empty.
  • Remove Elements:
    • E poll(): Retrieves and removes the head of the queue; returns null if the queue is empty.
    • E remove(): Retrieves and removes the head; throws an exception if the queue is empty.

For a Deque:

  • Add Elements at Both Ends:
    • void addFirst(E e) / boolean offerFirst(E e): Inserts the specified element at the front.
    • void addLast(E e) / boolean offerLast(E e): Inserts the specified element at the end.
  • Inspect Elements at Both Ends:
    • E peekFirst(): Retrieves but does not remove the first element; returns null if empty.
    • E peekLast(): Retrieves but does not remove the last element; returns null if empty.
  • Remove Elements from Both Ends:
    • E removeFirst(): Removes and returns the first element; throws an exception if empty.
    • E removeLast(): Removes and returns the last element; throws an exception if empty.
    • E pollFirst(): Removes and returns the first element; returns null if empty.
    • E pollLast(): Removes and returns the last element; returns null if empty.

What You Can and Cannot Do with Queues and Deques

You Can:

  • Maintain Order: Use queues for FIFO processing of elements.
  • Process by Priority: Use PriorityQueue to process elements based on priority instead of insertion order.
  • Add or Remove from Both Ends: Use deques to flexibly operate at both the beginning and the end.
  • Efficiently Manage Large Data: Use concurrent versions for thread-safe operations in a multi-threaded environment.

You Cannot:

  • Randomly Access Elements: Neither queues nor deques provide methods for accessing elements by index.
  • Assume Ordering: With standard queues (like PriorityQueue), iteration order is based on priority criteria, not necessarily the order of insertion.
  • Store Nulls (in Some Implementations): Implementations such as PriorityQueue do not permit null elements.

Mini Example: Using ArrayDeque as a Queue and Deque

Below is a short Java program that demonstrates basic operations on an ArrayDeque, which implements the Deque interface:

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

public class QueueDequeExample {
    public static void main(String[] args) {
        // Create an ArrayDeque to use as a double-ended queue
        Deque<String> deque = new ArrayDeque<>();

        // Adding elements at the tail (end) - typical queue behavior (FIFO)
        deque.offer("First");
        deque.offer("Second");
        deque.offer("Third");
        System.out.println("Deque after offer operations: " + deque);

        // Peeking at the head of the queue
        String head = deque.peek();
        System.out.println("Head element (without removal): " + head);

        // Removing the head element using poll (FIFO removal)
        String removedHead = deque.poll();
        System.out.println("Removed head element: " + removedHead);
        System.out.println("Deque now: " + deque);

        // Using deque-specific methods: adding at the front
        deque.offerFirst("New First");
        System.out.println("Deque after offerFirst: " + deque);

        // Removing an element from the tail
        String removedTail = deque.pollLast();
        System.out.println("Removed tail element: " + removedTail);
        System.out.println("Final deque: " + deque);
    }
}

QueueDequeExample.main(null);

Deque after offer operations: [First, Second, Third]
Head element (without removal): First
Removed head element: First
Deque now: [Second, Third]
Deque after offerFirst: [New First, Second, Third]
Removed tail element: Third
Final deque: [New First, Second]

A simple demonstration of a priority queue (ordering by natural order):

import java.util.PriorityQueue;
import java.util.Queue;

public class PriorityQueueDemo {
    public static void main(String[] args) {
        Queue<Integer> priorities = new PriorityQueue<>();
        priorities.offer(50);
        priorities.offer(10);
        priorities.offer(30);
        priorities.offer(20);
        
        System.out.println("Priority queue elements (in natural order):");
        while (!priorities.isEmpty()) {
            System.out.println(priorities.poll());
        }
    }
}

PriorityQueueDemo.main(null);

Priority queue elements (in natural order):
10
20
30
50

POPCORN HACK:

Task: Use an ArrayDeque to implement a basic double-ended queue (deque) where you can add or remove items from both ends.

  1. Add several strings using both addFirst() and addLast().
  2. Then remove one element from each end and print the contents of the deque.

Optional:

  • Convert the deque into a list and print it.
  • Use a loop to empty the deque while printing each removed element.
import java.util.ArrayDeque;
import java.util.Deque;

public class DequeExperiment {
    public static void main(String[] args) {
        // code
    }
}

DequeExperiment.main(null);

Takeaways:

  • FIFO Behavior: The initial offer() and poll() operations illustrate standard queue behavior.
  • Double-Ended Operations: Methods like offerFirst() and pollLast() showcase deque functionality.
  • No Random Access: Notice that no methods allow you to access elements by index.
  • Null Handling: Be aware that some implementations (e.g., PriorityQueue) do not permit null values.