Throwing custom exceptions in Java - Step-by-Step Execution

Concept Flow - Throwing custom exceptions

Start

↓

Check condition

Yes↓

Throw custom exception

↓

Catch exception?

No→Program crashes

Yes↓

Handle exception

↓

End

The program checks a condition, throws a custom exception if needed, then optionally catches and handles it.

Execution Sample

Java

class MyException extends Exception {
  MyException(String msg) { super(msg); }
}

void test(int x) throws MyException {
  if (x < 0) throw new MyException("Negative not allowed");
}

Defines a custom exception and throws it when input is negative.

Execution Table

Step	Action	Condition	Result	Exception Thrown	Output
1	Call test(5)	5 < 0?	False	No	No exception
2	Call test(-3)	-3 < 0?	True	Yes: MyException("Negative not allowed")	Exception thrown
3	Catch exception?	Is there a catch block?	Yes	Handled	Print message: Negative not allowed
4	Program continues	-	-	No	Program ends normally

💡 Execution stops throwing exception if no catch; otherwise continues after handling.

Variable Tracker

Variable	Start	After test(5)	After test(-3)	Final
x	undefined	5	-3	-3
exceptionThrown	false	false	true	true

Key Moments - 3 Insights

Why does the program stop when the exception is thrown?

Because at step 2 in the execution_table, the exception is thrown and if not caught, it stops normal flow.

What happens if there is no catch block for the custom exception?

Without a catch, the program crashes and does not continue normally.

How do you create a custom exception?

By extending Exception class and defining a constructor, as shown in the execution_sample code.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'exceptionThrown' after step 2?

A false

B undefined

C true

D null

💡 Hint

Check the variable_tracker row for 'exceptionThrown' after test(-3)

At which step does the program handle the exception?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Exception Thrown' and 'Output' columns in execution_table

If the condition 'x < 0' was changed to 'x <= 0', what would happen at step 1 when x=0?

A Exception thrown

B No exception

C Program crashes immediately

D Exception caught at step 3

💡 Hint

Refer to the condition column in execution_table and how it controls throwing exception

Concept Snapshot

Throwing custom exceptions in Java:
- Create a class extending Exception
- Use 'throw new MyException(message)' to throw
- Declare 'throws MyException' in method signature
- Catch with try-catch to handle
- If uncaught, program stops with error

Full Transcript

This visual trace shows how Java throws and handles custom exceptions. First, a method checks a condition. If the condition is true, it throws a custom exception object. The program then looks for a catch block to handle this exception. If found, it runs the catch code and continues. If not, the program stops with an error. Variables like 'x' and 'exceptionThrown' track the input and whether an exception occurred. This helps beginners see step-by-step how exceptions interrupt normal flow and how to manage them.