feat(generics): implement custom generic Pair<K,V> class with usage demo

WHAT:
- Added a generic `Pair<K,V>` class that can hold a key-value pair of any types.
  - Type parameters:
    - `K` → Key
    - `V` → Value
  - Provides constructor and getter methods (`getKey()`, `getValue()`).
- Created `Main` class to demonstrate usage of the `Pair` class.
  - Example with `Pair<String, Integer>` storing ("Age", 50) and ("Age", 88).
  - Printed keys and values to validate correctness.

WHY:
- Showcases how to define and use custom generic classes in Java.
- Demonstrates type-safety and reusability compared to raw types.
- Eliminates need for casting, reducing runtime errors.

HOW:
- Defined `class Pair<K,V>` with private fields `K key` and `V value`.
- Provided constructor to initialize both fields.
- Getter methods return strongly-typed key and value.
- In `Main`, created multiple `Pair<String,Integer>` objects to simulate real-world key-value mapping.

REAL-WORLD APPLICATIONS:
- **Data Storage**: Represent entries similar to `Map.Entry<K,V>` in Java Collections.
- **Configurations**: Store property-value or attribute-value pairs.
- **Database Results**: Represent column-value pairs for rows.
- **Networking**: Key-value headers or parameters in API requests.

NOTES:
- Java Collections Framework provides a built-in `Map.Entry<K,V>` interface that works like this `Pair`.
- This example builds intuition for how generics power frameworks like `HashMap`, `ConcurrentHashMap`, etc.
- Enhancements could include:
  - `setKey()` and `setValue()` methods for mutability.
  - Overriding `toString()`, `equals()`, and `hashCode()` for better usability in collections.

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

Author: Someshdiwan

Section24JavaGenerics/src/GenericClass/Main.java

@@ -2,8 +2,12 @@
 
 public class Main {
     public static void main(String[] args) {
-        Pair<String, Integer> pair = new Pair<>("Age", 30);
-        System.out.println("Key: " + pair.getKey());   // Prints: Key: Age
-        System.out.println("Value: " + pair.getValue()); // Prints: Value: 30
+        Pair<String, Integer> pair = new Pair<>("Age", 50);
+        Pair<String, Integer> pair2 = new Pair<>("Age", 88);
+
+        System.out.println("Key: " + pair.getKey());
+        System.out.println("Key: " + pair2.getKey());
+        System.out.println("Value: " + pair.getValue());
+        System.out.println("Value: " + pair2.getValue());
     }
-}
+}

Section24JavaGenerics/src/GenericClass/Pair.java

@@ -22,4 +22,4 @@ public V getValue() {
 Map<String, Integer> map = new HashMap<>();
 map.put("One", 1);
 map.put("Two", 2);
-*/
+*/