The Queue Interface

The Queue interface extends the Collection interface. It is used to hold elements about to be processed and typically orders elements in a FIFO (First-In-First-Out) manner.

However, Java also provides the Deque (Double Ended Queue) interface, which extends Queue and allows elements to be inserted and removed from both ends.

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Queue Operations

Unlike standard collections, Queues have specific methods for insertion, removal, and inspection. These methods exist in two forms: one throws an exception if the operation fails, and the other returns a special value (like null or false).

Operation	Throws Exception	Returns Special Value
Insert	add(e)	offer(e)
Remove	remove()	poll()
Examine	element()	peek()

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1. PriorityQueue

The PriorityQueue class is a popular implementation of the Queue interface. It is not a standard FIFO queue. Instead, elements are ordered based on their natural ordering (like TreeSet), or by a custom Comparator provided at queue construction time.

• The head of this queue is the "least" element with respect to the specified ordering.
• It does not allow null elements.

Example

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

public class LabQueue1 {

  public static void main(String args[]) {
    // A PriorityQueue of Integers
    Queue<Integer> pq = new PriorityQueue<>();

    // Adding elements out of order
    pq.offer(50);
    pq.offer(10);
    pq.offer(30);

    System.out.println("Head of queue (smallest element): " + pq.peek()); // 10

    // Removing elements. They will be removed in sorted order!
    System.out.println("Processing: " + pq.poll()); // 10
    System.out.println("Processing: " + pq.poll()); // 30
    System.out.println("Processing: " + pq.poll()); // 50
  }
}

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2. ArrayDeque

The ArrayDeque class implements the Deque interface. It uses a resizable array and allows you to operate on both the head and the tail of the queue.

• It is significantly faster than Stack when used as a stack (LIFO).
• It is significantly faster than LinkedList when used as a queue (FIFO).
• It does not allow null elements.

Example (Using ArrayDeque as a FIFO Queue)

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

public class LabQueue2 {

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

    // Adding to the tail
    deque.offer("Customer 1");
    deque.offer("Customer 2");
    deque.offer("Customer 3");

    // Removing from the head (FIFO behavior)
    System.out.println("Served: " + deque.poll()); // Customer 1
    System.out.println("Next in line: " + deque.peek()); // Customer 2
  }
}