Chapter 68: Queue in Java

A Queue is a Collection shaped for hand-offs: things join at the tail and leave from the head (FIFO) unless you pick a LIFO stack view. Java splits the idea across Queue, Deque (double-ended), and later blocking variants for threads.

PriorityQueue (Chapter 69) is not a strict FIFO—order comes from priority. Iterator details land in Chapter 70.

1. Topic title

Queue: polite lines—offer joins, poll serves, peek looks without removing

2. What it means

Common Queue methods: add / offer (enqueue, offer fails softly when bounded), remove / poll (dequeue, poll returns null when empty), element / peek (front without remove).

Deque adds push/pop stack names plus addFirst/removeLast—ArrayDeque implements Deque with a growable ring buffer, usually faster than LinkedList for both ends when you do not need List index access.

3. Why it is used

Work queues, breadth-first search layers, UI event pipes, rate smoothing—anywhere order of arrival matters more than random access.

4. Mental hook

Picture a cafeteria tray rail: new trays offer at the back, hungry poll from the front. peek is looking at the next tray without stealing it.

5. Java code example

import java.util.ArrayDeque;
import java.util.Queue;
 
public class QueueDemo {
    public static void main(String[] args) {
        Queue<String> line = new ArrayDeque<>();
        line.offer("Ada");
        line.offer("Bob");
        line.offer("Chen");
 
        while (!line.isEmpty()) {
            System.out.println(line.poll());
        }
 
        System.out.println("empty peek=" + line.peek());
    }
}

Prints Ada, Bob, Chen—classic FIFO. peek on empty returns null here; element would throw instead.

6. Explanation of code

ArrayDeque as Queue<String> keeps the field type flexible—you could swap implementations later for specialized blocking queues when you study concurrency.

7. Common mistakes

Iterating a queue expecting FIFO order—iterator() is not guaranteed to follow poll order on all implementations; poll in a loop is the reliable story.

Using LinkedList as default queue out of habit—measure; ArrayDeque is often the better default.

Calling remove() on empty queue—throws NoSuchElementException; prefer poll.

8. Best practices

Prefer isEmpty() over size() == 0 for readability.

When teaching APIs, pair offer/poll/peek—they form a consistent failure style with null instead of exceptions on emptiness.

9. Small practice task

Simulate a ticket counter: offer five names, poll two, peek the next without removing, then drain the rest with poll. Print each step.

Beginner tip

Stack class exists but is considered legacy—modern code leans on Deque (ArrayDeque) for stack operations (push/pop/peek).