feat(generics): add generic Pair<K,V> class with demo in Main

WHAT:
- Implemented a generic class `Pair<K,V>` that stores a key-value pair.
- Added constructor for initialization and getter methods (`getKey()`, `getValue()`).
- Created `Main` class to demonstrate usage of `Pair<String,Integer>`.
- Printed keys and values to verify functionality.

WHY:
- To illustrate how generics allow type-safe and reusable classes in Java.
- Demonstrates how multiple type parameters (`K`, `V`) can represent different roles.
- Provides a custom example similar to `Map.Entry<K,V>` in Java Collections Framework.

HOW:
- Defined `class Pair<K,V>` with private fields `K key` and `V value`.
- Used type parameters instead of fixed types to ensure flexibility.
- In `Main`, instantiated `Pair<String,Integer>` objects for demonstration.

TYPE PARAMETER NAMING CONVENTIONS:
- By convention, single-letter type names are used:
  - `T` → Type (general-purpose)
  - `E` → Element (commonly used in collections)
  - `K` → Key (commonly used in maps)
  - `V` → Value (commonly used in maps)
  - `N` → Number
- Example: `Map<K,V>` in Java
  - `K` → type of keys
  - `V` → type of values

REAL-WORLD USE CASES:
- **Maps/Key-Value Stores**: Represent entries like `HashMap.Entry<K,V>`.
- **Database Results**: Represent column-value pairs.
- **Configuration/Properties**: Store attribute-value mappings.
- **Networking**: Store headers or query parameters.

NOTES:
- Enhancements could include `toString()`, `equals()`, and `hashCode()` for better debugging and collection support.
- This example builds the foundation for understanding advanced generic classes in Java.

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

Author: Someshdiwan

Section24JavaGenerics/src/GenericClass/Type Parameter

@@ -1,16 +1,17 @@
-Here, the Pair<K, V> class has two type parameters K and V, and you can specify the types when you create an instance of Pair.
+Here, the Pair<K, V> class has two type parameters K and V, and
+you can specify the types when you create an instance of Pair.
 
-Type Parameter Naming Conventions
-
-While you can name type parameters anything, the convention is to use single letters that describe the purpose of the type parameter:
+Type Parameter Naming Conventions:
+While you can name type parameters anything, the convention is to use single letters that describe the
+purpose of the type parameter.
 
 T: Type
 E: Element (used in collections)
 K: Key (used in maps)
 V: Value (used in maps)
 N: Number
+
 For example, in the java.util.Map<K, V> interface:
 
 K stands for the key type
 V stands for the value type.
-