feat: Add NegativeValueExceptionDemo to showcase custom exception handling in static method call

WHAT the code does:
- Defines NegativeValueExceptionDemo with a main() method.
- Declares length = -5 and width = 3.
- Calls Rectangle.calculateArea(length, width) directly using the class name (static method call).
- Catches NegativeValueException if thrown, printing its message.

WHY this matters:
- Demonstrates how custom exceptions (NegativeValueException) can be triggered and caught in client code.
- Reinforces that static methods can directly throw exceptions to the caller without requiring an object.
- Provides a realistic case where invalid input (negative dimensions) triggers exception handling.

HOW it works:
1. main() defines invalid dimensions (length = -5).
2. Calls Rectangle.calculateArea(), which checks input.
   - If negative, throws NegativeValueException with a descriptive message.
3. The try–catch block in NegativeValueExceptionDemo catches the thrown exception.
4. Prints the error message instead of crashing.

Tips and gotchas:
- Always validate method arguments before processing to avoid logical errors.
- Exception handling separates error-handling logic from normal flow, improving readability.
- Since calculateArea() is static, it should be called with Rectangle.calculateArea(), not via an instance.

Use-cases / analogies:
- Validating user input in forms (e.g., age, salary must be non-negative).
- Preventing invalid dimensions in geometry-related applications.
- Guard clauses in business rules (e.g., minimum balance, positive quantities).

Short key: java custom-exception demo static-method input-validation.

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

Author: Someshdiwan

Section18ExceptionHandling/src/NegativeValueExceptionDemo.java

@@ -1,7 +1,5 @@
-class NegativeValueExceptionDemo
-{
-    public static void main (String[] args)
-    {
+class NegativeValueExceptionDemo {
+    public static void main (String[] args) {
         try {
             int length = -5;
             int width = 3;