Stacks, Queues and Priority Queues

Stack: last in, first out

Think of it like this: A stack of plates: you put new plates on top and take from the top. The last plate added is the first one used.

import java.util.ArrayDeque;

public class Main {
    public static void main(String[] args) {
        ArrayDeque<String> stack = new ArrayDeque<>();
        stack.push("page1");
        stack.push("page2");
        stack.push("page3");
        System.out.println("Top: " + stack.peek());
        System.out.println("Back to: " + stack.pop());
        System.out.println("Back to: " + stack.pop());
        System.out.println("Left: " + stack);
    }
}

Output

Top: page3
Back to: page3
Back to: page2
Left: [page1]

Your browser's Back button works like a stack of pages you visited.

Queue: first in, first out

Think of it like this: A queue at a ticket counter: the first person to arrive is the first one served.

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

public class Main {
    public static void main(String[] args) {
        Queue<String> line = new ArrayDeque<>();
        line.offer("Asha");
        line.offer("Ravi");
        line.offer("Meera");
        while (!line.isEmpty()) {
            System.out.println("Serving " + line.poll());
        }
    }
}

Output

Serving Asha
Serving Ravi
Serving Meera

PriorityQueue: the most important first

A PriorityQueue always gives you the smallest item first (or whatever order you choose), no matter when it was added:

import java.util.Collections;
import java.util.PriorityQueue;

public class Main {
    public static void main(String[] args) {
        PriorityQueue<Integer> pq = new PriorityQueue<>();
        pq.add(30);
        pq.add(5);
        pq.add(18);
        while (!pq.isEmpty()) {
            System.out.print(pq.poll() + " ");
        }
        System.out.println();

        PriorityQueue<Integer> biggestFirst = new PriorityQueue<>(Collections.reverseOrder());
        biggestFirst.add(30);
        biggestFirst.add(5);
        biggestFirst.add(18);
        System.out.println(biggestFirst.poll());
    }
}

Output

5 18 30 
30

Tip: Java also has an old Stack class and a LinkedList that can act as a queue. ArrayDeque is the modern, faster choice for both.