QUEUE.md

Queue:

Queue: A First-In-First-Out (FIFO) data structure where elements are added at the back and removed from the front, like a line of people waiting. It is useful for processing items in the order they arrive. Common operations include:

Methods of Queue:

• .offer(Object x): boolean - Adds an element to the back of the queue. Returns true if the element was added successfully.
• .peek(): Object - Returns the front element without removing it. Returns null if the queue is empty.
• .poll(): Object - Removes and returns the front element. Returns null if the queue is empty.

Additional Methods (Not Recommended for General Use):

• .add(Object x): boolean - Similar to .offer(), but throws an exception if the queue is full.
• .remove(): Object - Similar to .poll(), but throws an exception if the queue is empty.
• .element(): Object - Similar to .peek(), but throws an exception if the queue is empty.

Collection Class Methods

Since Queue is part of the Collection framework, it also supports methods inherited from the Collection interface:

• .isEmpty(): boolean - Checks if the queue is empty.
• .size(): int - Returns the number of elements in the queue.
• .contains(Object x): boolean - Checks if a specific element exists in the queue.

Common Uses of Queue:

• Keyboard Buffering - Ensures keystrokes are processed in the order they are typed.
• Printer Task Scheduling - Manages print jobs so they are printed in the order they were received.
• Breadth-First Search (BFS) - Used in graph traversal algorithms.
• Task Processing - Manages events or tasks that need to be processed in the order they were added.

Code Example:

Example Usage:

package DataStructures.queues;

import java.util.LinkedList;
import java.util.Queue;

public class Main {
    
    public static void main(String[] args) {
        // We cannot create an instance of Queue directly as it is an interface.
        // Instead, we can instantiate it using a class that implements Queue, like LinkedList or PriorityQueue.
        // Here, we use LinkedList to implement the Queue interface.
        Queue<String> queue = new LinkedList<>();

        // Adding items to the queue using .offer()
        queue.offer("Karen");
        queue.offer("Chad");
        queue.offer("Steve");
        queue.offer("Herold");

        // Displaying the queue
        System.out.println(queue);

        // Getting the head of the queue using .peek()
        String head = queue.peek();
        System.out.println("Head of the Queue: " + head);

        System.out.println();

        // Removing the head of the queue using .poll() (new head will be the next item)
        String karen = queue.poll();
        System.out.println("Dequeue: " + karen);
        System.out.println(queue);

        System.out.println();

        String chad = queue.poll();
        System.out.println("Dequeue: " + chad);
        System.out.println(queue);

        System.out.println();

        // Using Collection class methods:
        // Checking if the queue is empty
        System.out.println("Is Queue empty? " + queue.isEmpty());

        System.out.println();

        // Checking the size of the queue
        System.out.println("Size of the Queue: " + queue.size());

        System.out.println();

        // Checking if the queue contains a specific object
        System.out.println("Queue contains 'Steve'? " + queue.contains("Steve"));
    }
}

Output:

[Karen, Chad, Steve, Herold]
Head of the Queue: Karen

Dequeue: Karen
[Chad, Steve, Herold]

Dequeue: Chad
[Steve, Herold]

Is Queue empty? false

Size of the Queue: 2

Queue contains 'Steve'? true