feat: Implement custom checked exception for validating rectangle dimensions

WHAT the code does:
- Defines Rectangle class:
  - calculateArea(int length, int width) throws NegativeValueException.
  - Validates inputs: if length or width < 0, throws a custom checked exception.
  - Otherwise returns area = length * width.
- Defines NegativeValueException:
  - Extends Exception (checked exception).
  - Constructor accepts error message and passes it to super().
- Defines CustomExceptions main():
  - Initializes length = -5, width = 3.
  - Calls calculateArea() inside try–catch.
  - Catches NegativeValueException and prints error message.

WHY this matters:
- Demonstrates **custom checked exceptions**:
  - Extending Exception requires caller to handle or declare with throws.
  - Enforces validation logic at compile-time, ensuring input correctness.
- Validates real-world logic: a rectangle cannot have negative dimensions.
- Shows how to integrate exception handling into normal program flow.

HOW it works:
1. main() sets length = -5, width = 3.
2. calculateArea(-5, 3) detects invalid input.
3. Throws new NegativeValueException("Error: length or width cannot be negative").
4. Exception propagates to main() and is caught in catch block.
5. Prints "Error: length or width cannot be negative".

Tips and gotchas:
- Custom exceptions improve readability and domain-specific error handling.
- Checked vs unchecked:
  - Extending Exception (checked) forces handling at compile-time.
  - Extending RuntimeException (unchecked) would not require try–catch but shifts responsibility to runtime.
- Prefer meaningful class names: NegativeValueException is clear and domain-specific.
- Avoid unnecessary object instantiation: calculateArea is static, so no need for `new Rectangle()`.

Use-cases / analogies:
- Like form validation: you can’t submit a form with invalid values; exception is thrown.
- In banking: throwing InsufficientFundsException when withdrawal exceeds balance.

Short key: java custom-checked-exception input-validation rectangle-area.

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

Author: Someshdiwan

Section18ExceptionHandling/src/CustomExceptions.java

@@ -1,4 +1,3 @@
-// Submit to check the output
 import java.util.*;
 
 class Rectangle {
@@ -16,10 +15,8 @@ public NegativeValueException(String message) {
     }
 }
 
-class CustomExceptions
-{
-    public static void main (String[] args)
-    {
+class CustomExceptions {
+    public static void main (String[] args) {
         try {
             int length = -5;
             int width = 3;