Create QueueDemo.java

rahulXbarnwal · web-flow · commit b006f1ea9c9a · 2026-03-16T00:59:18.000+05:30
diff --git a/src/learnCollections/QueueDemo.java b/src/learnCollections/QueueDemo.java
@@ -0,0 +1,79 @@
+package learnCollections;
+
+import java.util.LinkedList;
+import java.util.Queue;
+import java.util.concurrent.ArrayBlockingQueue;
+
+/*
+                 Queue (Interface)
+
+                      FIFO
+               (First In First Out)
+
+        +----------------------------------+
+        |           Queue Ops              |
+        |                                  |
+        |  enqueue                         |
+        |  dequeue                         |
+        |  peek                            |
+        +----------------------------------+
+
+                ↓ Equivalent Methods
+
+        enqueue  →  add() / offer()
+        dequeue  →  remove() / poll()
+        peek     →  element() / peek()
+
+
+        +----------------------------------+
+        |        Queue Implementations     |
+        |                                  |
+        |  LinkedList                      |
+        |  PriorityQueue                   |
+        +----------------------------------+
+
+*/
+
+public class QueueDemo {
+    public static void main(String[] args) {
+        LinkedList<Integer> list = new LinkedList<>();
+        list.addLast(1); // enqueue
+        list.addLast(2); // enqueue
+        list.addLast(3); // enqueue
+        System.out.println(list);
+
+        Integer i = list.removeFirst(); // dequeue
+        System.out.println(list); // [2, 3]
+        list.getFirst(); // peek
+
+
+        Queue<Integer> queue = new LinkedList<>();
+        queue.add(1);
+        queue.add(2);
+        queue.add(3);
+        System.out.println(queue); // [1, 2, 3]
+
+        Integer j = queue.remove(); // dequeue
+        System.out.println(queue); // [2, 3]
+        queue.peek(); // peek
+
+
+        Queue<Integer> q = new LinkedList<>();
+        q.add(1);
+        System.out.println(q.size()); 
+
+        System.out.println(q.remove()); // throws exception if queue is empty
+        System.out.println(q.poll()); // returns null if queue is empty
+
+        // System.out.println(q.element()); // throws exception if queue is empty
+        System.out.println(q.peek()); // returns null if queue is empty
+
+
+        Queue <Integer> queue2 = new ArrayBlockingQueue<>(2); // capacity of 2
+        System.out.println(queue2.add(1)); // true
+        System.out.println(queue2.offer(2)); // true
+
+        System.out.println(queue2.add(3)); // throws IllegalStateException: Queue full
+        System.out.println(queue2.offer(3)); // returns false if queue is full
+    }
+}
