Java Queue, Deque & ArrayDeque: FIFO Buffers & Priority Queues

The Queue interface models a collection designed for holding elements prior to processing, typically following FIFO (First-In, First-Out) ordering. The Deque (Double-Ended Queue) interface extends Queue to permit insertion, deletion, and inspection from both ends.

The 2 Sets of Queue Methods: Exceptions vs. Special Values

Operation	Throws Exception on Failure	Returns Special Value (`false` / `null`)
Insert	`add(e)` (Throws `IllegalStateException` if full)	`offer(e)` (Returns `false` if full)
Remove	`remove()` (Throws `NoSuchElementException` if empty)	`poll()` (Returns `null` if empty)
Examine	`element()` (Throws `NoSuchElementException` if empty)	`peek()` (Returns `null` if empty)

1. `PriorityQueue` (Binary Min-Heap)

import java.util.PriorityQueue;

public class PriorityQueueDemo {
    public static void main(String[] args) {
        // Natural ordering: Min-Heap (Lowest integer has highest priority)
        PriorityQueue<Integer> pq = new PriorityQueue<>();
        pq.offer(40);
        pq.offer(10);
        pq.offer(30);
        pq.offer(5);

        // Elements polled strictly in priority order:
        while (!pq.isEmpty()) {
            System.out.print(pq.poll() + " "); // Output: 5 10 30 40
        }
    }
}

2. `ArrayDeque`: High-Speed Stack & Queue

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

public class ArrayDequeDemo {
    public static void main(String[] args) {
        // Using ArrayDeque as a Modern, High-Speed LIFO Stack:
        Deque<String> stack = new ArrayDeque<>();
        stack.push("Bottom Plate");
        stack.push("Middle Plate");
        stack.push("Top Plate");

        System.out.println("Popped: " + stack.pop()); // Top Plate
    }
}

Next Step

Now let us explore Iterators, ListIterator, and Fail-Fast vs Fail-Safe mechanics. Proceed to: Java Iterators →

Frequently Asked Questions (FAQ)

Q1. What is the difference between Queue and Deque in Java?

A Queue is a First-In-First-Out (FIFO) structure where elements are added at the tail and removed from the head. A Deque (Double-Ended Queue) permits insertion, inspection, and removal at BOTH ends (head and tail), functioning seamlessly as both a FIFO queue and a LIFO stack.

Q2. How does PriorityQueue order elements in Java?

PriorityQueue orders elements according to priority using a Binary Min-Heap (the smallest element is always at the head). Custom priority orders can be defined using a Comparator.

Q3. Why is ArrayDeque preferred over Stack and LinkedList?

ArrayDeque is backed by a circular resizable array. It has no synchronization locks (unlike legacy Stack) and creates zero node wrapper objects (unlike LinkedList), providing the fastest Stack and Queue performance in the JDK.