feat: Implement custom checked exception LowBalanceException with try–catch handling

WHAT the code does:
- Defines LowBalanceException:
  - Extends Exception (checked exception).
  - Overrides toString() to return a custom message: "Balance Should not be less than 5000".
- Defines LowBankBalanceTrycatch class with three static methods:
  - fun1():
    - Explicitly throws new LowBalanceException.
    - Immediately catches it in catch block.
    - Prints exception using overridden toString().
  - fun2(): calls fun1().
  - fun3(): calls fun2().
- main():
  - Calls fun3(), which cascades down to fun1() where exception is thrown and handled.

WHY this matters:
- Demonstrates creation and usage of a **custom exception class** for domain-specific error handling.
- Shows how checked exceptions require explicit handling, either with try–catch or throws.
- Illustrates exception propagation and how exceptions can be thrown and caught within the same method.

HOW it works:
1. main() calls fun3() → fun2() → fun1().
2. fun1():
   - Executes `throw new LowBalanceException()`.
   - Immediately caught in catch block.
   - System.out.println(e) → calls LowBalanceException.toString() → prints "Balance Should not be less than 5000".
3. Program ends gracefully.

Tips and gotchas:
- Typically, user-defined exceptions are **thrown up the call stack** to be handled by calling code, rather than caught immediately where they are thrown. This demo keeps it local for simplicity.
- Best practice: include a constructor in custom exceptions that accepts a message or cause, so error context can be passed.
- Extending RuntimeException instead of Exception would make it unchecked (no need for try–catch at compile time).
- Method naming conventions: fun1(), fun2(), fun3() should be replaced with descriptive names in production.

Use-cases / analogies:
- LowBalanceException models banking rules: accounts must maintain a minimum balance (e.g., 5000).
- Real-world: throwing InsufficientFundsException when a withdrawal would drop balance below threshold.

Short key: java custom-exception checked low-balance banking-rules exception-handling.

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

Author: Someshdiwan

Section18ExceptionHandling/src/LinksOpen.cmd

@@ -1,38 +1,29 @@
-//This is the user defined exceptions
-
-class LowBalanceException extends Exception
-{
-    public String toString()
-    {
+class LowBalanceException extends Exception {
+    //This is the user-defined exceptions
+    public String toString() {
         return "Balance Should not be less than 5000";
     }
 }
-public class LowBankBalanceTrycatch
-{
-    static void fun1()
-    {
-        try
-        {
+
+public class LowBankBalanceTrycatch {
+    static void fun1() {
+        try {
             throw new LowBalanceException();
         }
-        catch(LowBalanceException e)
-        {
+        catch(LowBalanceException e) {
             System.out.println(e);
         }
     }
 
-    static void fun2()
-    {
+    static void fun2() {
         fun1();
     }
 
-    static void fun3()
-    {
+    static void fun3() {
         fun2();
     }
 
-    public static void main(String[] args)
-    {
+    public static void main(String[] args) {
         fun3();
     }
-}
+}