feat(IteratorDemo): add demo of traversing list with Iterator

What
- Added IteratorDemo class.
- Created a List<Integer> with values [5, 10, 50, 60, 70, 80, 90].
- Obtained Iterator<Integer> from list.iterator().
- Used while loop with hasNext() / next() to traverse and print elements.

Why
- Demonstrates basic usage of Iterator for one-directional traversal of collections.
- Shows explicit iteration API instead of enhanced for-loop or streams.
- Useful for understanding underlying iteration mechanism and safe element removal.

How
- list.iterator() returns an Iterator positioned before the first element.
- hasNext() checks if more elements exist.
- next() advances the iterator and returns current element.
- Printed each element sequentially.

Logic
- Inputs: list of integers.
- Outputs:
  Iterating with Iterator:
  5
  10
  50
  60
  70
  80
  90
- Flow:
  1. Create list.
  2. Get iterator.
  3. Traverse while hasNext() true.
  4. Print each next().
- Edge cases:
  - Calling next() without hasNext() throws NoSuchElementException.
  - Iterator is fail-fast: structural modification of list during iteration throws ConcurrentModificationException (except via iterator.remove()).
- Complexity / performance: O(n) traversal.
- Concurrency / thread-safety: Iterator not thread-safe; concurrent modification needs synchronized collections or CopyOnWriteArrayList.
- Error handling: Not needed in demo.

Real-life applications
- Iterating over collections when you need explicit control of traversal.
- Removing elements safely during iteration via iterator.remove().
- Forms basis for custom traversal algorithms in collections.

Notes
- Enhanced for-loop (for-each) and streams are syntactic sugar built on top of Iterator.
- ListIterator provides bi-directional traversal and element modification, unlike plain Iterator.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section 25 Collections Frameworks/List Interface/ArrayList/Iterator/src/IteratorDemo.java

@@ -0,0 +1,17 @@
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+/* Demonstrates using Iterator to traverse elements in a single direction. */
+
+public class IteratorDemo {
+    public static void main(String[] args) {
+        List<Integer> list = new ArrayList<>(List.of(5, 10, 50, 60, 70, 80, 90));
+
+        Iterator<Integer> it = list.iterator();
+        System.out.println("Iterating with Iterator:");
+        while (it.hasNext()) {
+            System.out.println(it.next());
+        }
+    }
+}