feat(ComparatorDemo4): add demo of custom Comparator reversing integer order

What
- Added custom comparator class My:
  - Implements Comparator<Integer>.
  - Overrides compare(Integer i1, Integer i2).
  - Returns positive if i1 < i2, negative if i1 > i2, 0 if equal.
  - Effectively reverses natural ordering (descending sort).
- Added ComparatorDemo4 class:
  - Defines Integer array [2,4,6,8,1,3,5,7].
  - Calls Arrays.sort(a, new My()) to sort using custom comparator.
  - Prints sorted array.

Why
- Demonstrates how to override default natural ordering (ascending) by supplying custom comparator.
- Shows usage of Arrays.sort with a comparator.
- Educational example contrasting Comparable (default) vs Comparator (custom).

How
- Arrays.sort(a, new My()):
  - Delegates element comparison to My.compare().
  - Since My reverses the usual order, result is descending sort.
- Iterated through sorted array and printed elements.

Logic
- Input array: [2,4,6,8,1,3,5,7].
- My.compare(i1,i2):
  - If i1 < i2 → return 1 → i1 placed after i2.
  - If i1 > i2 → return -1 → i1 placed before i2.
- Output: [8,7,6,5,4,3,2,1].
- Edge cases:
  - Equal integers → comparator returns 0 → order remains stable.
  - Comparator must define total order; here logic is consistent with descending natural order.
- Complexity / performance: O(n log n) sorting.
- Concurrency / thread-safety: Not thread-safe; demo is safe with local array.
- Error handling: None needed.

Real-life applications
- Sorting numbers in descending order (leaderboards, scores, rankings).
- Custom ordering when natural order is not desired.
- Comparator is flexible: can sort by arbitrary criteria (length, priority, timestamps).

Notes
- Without passing new My(), Arrays.sort() uses natural ordering (Comparable).
- Comparator.comparingInt(i -> i).reversed() is a cleaner functional alternative.
- Demonstrates old-style comparator implementation via class; modern style often uses lambdas.

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

Author: Someshdiwan

Section 25 Collections Frameworks/List Interface/ArrayList/Comparator/src/ComparatorDemo4.java

@@ -0,0 +1,31 @@
+import java.util.*;
+
+/**
+ * This class demonstrates the usage of Arrays and the Comparator interface in Java.
+ *
+ * @see <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html">Comparator Interface </a>
+ */
+
+class My implements Comparator<Integer> {
+    //Implementing the compare method inside the comparator interface by reversing the order.
+    public int compare(Integer i1,Integer i2) {
+        if(i1<i2)
+            return 1;
+        if(i1>i2)
+            return -1;
+        return 0;
+    }
+}
+
+public class ComparatorDemo4 {
+    public static void main(String[] args) {
+        Integer a[] = {2, 4, 6, 8, 1, 3, 5, 7}; // Passing comparator and name of comparator class.
+
+        Arrays.sort(a, new My());
+        // Arrays.sort(a.new My());
+        // you can use without My() then comparable interface will be used.
+
+        for (Integer x : a)
+            System.out.print(" " + x);
+    }
+}