feat(ListDemo2): add demo of custom sorting with lambdas

What
- Added ListDemo2 class.
- Created List<String> with values ["banana","apple","date"].
- Sorted strings by length in descending order using lambda comparator (b.length() - a.length()).
- Created List<Integer> with values [2,1,3].
- Sorted integers in descending natural order using lambda comparator (b - a).
- Printed sorted results.

Why
- Demonstrates custom sorting logic using lambda expressions.
- Shows how to sort strings by length instead of natural alphabetical order.
- Shows how to sort integers descending without using Comparator.reverseOrder().
- Useful example of how lambdas simplify comparator creation.

How
- words = ["banana","apple","date"].
- words.sort((a,b) -> b.length() - a.length()) → ["banana","apple","date"] (lengths: 6,5,4).
- ints = [2,1,3].
- ints.sort((a,b) -> b - a) → [3,2,1].
- Printed both results.

Logic
- Inputs: strings and integers.
- Outputs:
  - Strings sorted by descending length.
  - Integers sorted descending.
- Flow:
  1. Initialize string and integer lists.
  2. Apply sort() with lambda comparator.
  3. Print final results.
- Edge cases:
  - Comparator using subtraction can overflow for large integers; safer to use Integer.compare(b,a).
  - Stable sort: equal lengths keep original order.
- Complexity: O(n log n) sorting.
- Concurrency: not thread-safe if lists modified concurrently.

Real-life applications
- Sorting strings by length for UI display (e.g., longest words first).
- Sorting integers descending for leaderboards, rankings, or high-score lists.
- Custom comparator lambdas eliminate boilerplate comparator classes.

Notes
- words.sort(null) would sort alphabetically; custom comparator overrides natural order.
- Comparator.comparingInt(String::length).reversed() is a more readable alternative to b.length() - a.length().
- For integers, Comparator.reverseOrder() achieves the same descending effect.

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

Author: Someshdiwan

Section 25 Collections Frameworks/List Interface/ArrayList/src/ListDemo2.java

@@ -0,0 +1,25 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class ListDemo2 {
+    public static void main(String[] args) {
+        // Sort strings by length descending.
+        List<String> words = Arrays.asList("banana", "apple", "date");
+
+        // The original example used (a, b) -> b.length() - a.length()
+        words.sort((a, b) -> b.length() - a.length());
+
+        System.out.println("Words sorted by length (desc): " + words);
+
+
+        // Sort integers in descending natural order
+        List<Integer> ints = new ArrayList<>();
+        ints.add(2);
+        ints.add(1);
+        ints.add(3);
+
+        ints.sort((a, b) -> b - a);
+        System.out.println("Integers sorted desc: " + ints);
+    }
+}