Queue and Deque

Definition

A Queue hands items out in the order they arrive (first in, first out). A PriorityQueue hands out the smallest item first. A Deque works at both ends, so ArrayDeque can act as a queue or as a stack.

Example code

FIFO queue, priority queue and stack

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.PriorityQueue;
import java.util.Queue;

class Demo {
    public static void main(String[] args) {
        Queue<String> line = new ArrayDeque<>();           // first in, first out
        line.offer("Asha");
        line.offer("Ravi");
        System.out.println(line.poll() + " is served first");

        PriorityQueue<Integer> tasks = new PriorityQueue<>();   // smallest number first
        tasks.add(5);
        tasks.add(1);
        tasks.add(3);
        System.out.println(tasks.poll() + " then " + tasks.poll());

        Deque<String> stack = new ArrayDeque<>();          // last in, first out
        stack.push("page 1");
        stack.push("page 2");
        System.out.println(stack.pop());
    }
}

// Output:
// Asha is served first
// 1 then 3
// page 2