Stacks and Queues

Queue Enqueue/Dequeue

Enqueue values at the back and dequeue them from the front in first-in, first-out order.

Algorithm

The replay uses the same three values in every language, so this Java DSA implementation can be compared directly with the rest of the DSA track.

front The front is the oldest value still waiting in the queue.
FIFO A queue removes values in first-in, first-out order.

Visual walkthrough

The queue keeps the oldest value at the front and adds new values at the back.

Step 1 - Enqueue 10, 20, 30

New values join at the back. The oldest value, 10, stays at the front.

Queue after three enqueues: front 10, then 20, then back 30.nextnext10front2030back

Step 2 - Dequeue removes 10

Removing from the front returns 10 and makes 20 the new front.

After one dequeue: removed is 10; front moves to 20.next10removed20front30back

Basic Implementation

Basic.java 
import java.util.*;

public class Basic {
    static String render(List<Integer> values) {
        StringBuilder out = new StringBuilder();
        for (int i = 0; i < values.size(); i++) {
            if (i > 0) out.append(" -> ");
            out.append(values.get(i));
        }
        return out.toString();
    }
    public static void main(String[] args) {
        Queue<Integer> queue = new ArrayDeque<>();
        for (int value : new int[] {10, 20, 30}) queue.add(value);
        List<Integer> removed = new ArrayList<>();
        while (!queue.isEmpty()) removed.add(queue.remove());
        System.out.println(render(removed));
    }
}

Complexity

  • Time: O(1) per operation with a real queue
  • Space: O(n)

Implementation notes

  • Java declares the queue as Queue<Integer> and backs it with new ArrayDeque<>(), so operations use the queue interface while storage is an array-backed deque.
  • The enhanced for loop reads primitive values from new int[] {10, 20, 30}; each queue.add(value) appends at the back and boxes the int into an Integer for the generic container. These small boxed values may come from the Integer cache.
  • Dequeue uses while (!queue.isEmpty()) removed.add(queue.remove()). remove() takes from the front and would throw on an empty queue, so the guard is the checked empty behavior in this implementation.
  • The replay-visible FIFO states move from [10, 20, 30] to [20, 30] and then [], while removed grows to [10, 20, 30]. The ArrayDeque, ArrayList, and StringBuilder output are normal JVM heap objects managed while referenced.