feat(collections): demonstrate sorting custom objects with lambda-based Comparator

What
- Added `Main.java` in `Package3` to illustrate sorting of custom objects (`Student`) using `Collections.sort` with a comparator.
- Defined `Student` as a static inner class with fields:
  - `id` (Integer)
  - `name` (String)
- Created a list of students and applied a **custom comparator** to sort them in descending order of `id`.
- Overrode `toString()` in `Student` for readable output (e.g., `33: Donald`).

Why
- By default, Java’s `Collections.sort()` relies on natural ordering (like integers, strings).
- For custom objects, we must explicitly define sorting rules via:
  - Implementing `Comparable<T>` (natural order), or
  - Providing a `Comparator<T>` (custom order).
- This example shows how to sort domain-specific objects (like students) with a comparator inline via lambda.

Logic
1. Constructed 3 students with different IDs and names.
2. Added them to a `List<Student>`.
3. Used `Collections.sort(li, (a, b) -> b.id - a.id)`:
   - Comparator subtracts `id` values → sorts descending by ID.
   - Demonstrates custom ordering beyond natural sorting.
4. Printed the sorted list, relying on overridden `toString()` for formatted display.

Key Takeaways
✔ Custom objects don’t have inherent ordering — you must define it.
✔ Comparator lambdas provide concise, inline sorting logic.
✔ `(a, b) -> b.id - a.id` → descending sort by ID.
✔ For ascending order: `(a, b) -> a.id - b.id`.
✔ `toString()` improves debugging/logging by showing meaningful output.
✔ Collections framework integrates seamlessly with lambdas introduced in Java 8.

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

Author: Someshdiwan

Java 8 Crash Course/Lambda Expression/Comparator Using Lambda Expression/src/Package3/Main.java

@@ -0,0 +1,39 @@
+package Package3;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class Main {
+    public static void main(String[] args) {
+        Student s1 = new Student(2, "Jackson");
+        Student s2 = new Student(3, "Stark");
+        Student s3 = new Student(33, "Donald");
+
+        List<Student> li = new ArrayList<Student>();
+        li.add(s1);
+        li.add(s2);
+        li.add(s3);
+
+        // custom objects ko sort kar rahe hai haam.
+        // We need to create our own Comparator.
+        // We need to define how to sort custom objects or data it is not Integers that sort Natural order.
+        Collections.sort(li, (a,b) -> b.id - a.id); // .id is custom for Objects.
+        System.out.println(li);
+    }
+
+    static class Student {
+        public Integer id;
+
+        public String name;
+
+        public Student(Integer id, String name) {
+            this.id = id;
+            this.name = name;
+        }
+
+        public String toString(){
+            return this.id + ": "+ this.name;
+        }
+    }
+}