Queue and Deque

Learn queues with Queue<E> and Deque<E>.

The Queue<E> interface

Queue<E> is a first-in-first-out (FIFO) order. ArrayDeque is the fastest concrete implementation: Queue<String> todo = new ArrayDeque<>();.

offer(e) appends to the tail, poll() removes and returns the head, peek() returns the head without removing it. If the queue is empty, poll and peek return null; remove throws an exception.

Deque: a double-ended queue

Deque<E> (double-ended queue) allows add and remove at both ends. ArrayDeque implements both Queue and Deque. Methods: addFirst(e), addLast(e), removeFirst(), removeLast(), peekFirst(), peekLast().

For example a browser back/forward simulation uses addLast for forward and removeLast for back. When a stack is needed, push/pop also work on a Deque; the old Stack class is replaced by ArrayDeque.

Exercise Build a todo queue

Declare Queue<String> todo = new ArrayDeque<>();. Add two tasks with todo.offer("Send email"); and todo.offer("Meeting");. Pull the first task with String first = todo.poll(); and print it with System.out.println(first);. Print the remaining count with System.out.println(todo.size());.

Show example solution

import java.util.Queue;
import java.util.ArrayDeque;

public class Main {
    public static void main(String[] args) {
        Queue<String> todo = new ArrayDeque<>();
        todo.offer("Send email");
        todo.offer("Meeting");
        String first = todo.poll();
        System.out.println(first);
        System.out.println(todo.size());
    }
}

Test yourself

Complete the queue-add call

todo.____("Send email");

Show answer

todo.offer("Send email");

What does todo.poll() do?
- Removes and returns the head
- Only peeks at the head
- Appends an element to the tail
- Empties the queue
Show answer

Removes and returns the head — poll removes and returns the head; returns null if empty.
Match the Queue/Deque methods

offer(e)

Appends to tail
poll()

Removes the head
peek()

Shows the head
addFirst(e)

Adds to head

Next lesson: Collections class

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