Java.Core

1. Steps to Create a Custom Exception

1. Create a new class that extends either Exception (for checked exceptions) or RuntimeException (for unchecked exceptions).
2. Provide a constructor to initialize the exception message.
3. (Optional) Override methods or add additional functionality.

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2. Checked vs. Unchecked Custom Exceptions

Type	Extend	Handling Required?	When to Use
Checked Exception	Exception	✅ Must handle (try-catch or throws)	External failures (I/O, DB)
Unchecked Exception	RuntimeException	❌ Not required	Programming logic errors

3. Creating a Checked Exception (Extending Exception)

✅ Use case: If an invalid age is provided, throw InvalidAgeException.

// Custom Checked Exception
class InvalidAgeException extends Exception {
    public InvalidAgeException(String message) {
        super(message);
    }
}

public class CheckedExceptionExample {
    // Method that throws our custom checked exception
    public static void checkAge(int age) throws InvalidAgeException {
        if (age < 18) {
            throw new InvalidAgeException("Age must be 18 or older.");
        }
    }

    public static void main(String[] args) {
        try {
            checkAge(16);
        } catch (InvalidAgeException e) {
            System.out.println("Caught Exception: " + e.getMessage());
        }
    }
}

✅ Output:

Caught Exception: Age must be 18 or older.

📌 Why extend Exception?

• Forces the caller to handle or declare the exception using try-catch or throws.

4. Creating an Unchecked Exception (Extending RuntimeException)

✅ Use case: If a number is negative, throw NegativeNumberException.

// Custom Unchecked Exception
class NegativeNumberException extends RuntimeException {
    public NegativeNumberException(String message) {
        super(message);
    }
}

public class UncheckedExceptionExample {
    // Method that throws our custom unchecked exception
    public static void validateNumber(int num) {
        if (num < 0) {
            throw new NegativeNumberException("Number cannot be negative.");
        }
    }

    public static void main(String[] args) {
        validateNumber(-5); // No try-catch required, but program will crash if not handled
    }
}

🛑 Output (Uncaught Exception):

Exception in thread "main" NegativeNumberException: Number cannot be negative.

📌 Why extend RuntimeException?

• No need for throws or forced try-catch.
• Useful for developer mistakes (e.g., invalid method arguments).

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5. Custom Exception with Additional Fields

You can add extra fields like an error code for more information.

class CustomExceptionWithCode extends Exception {
    private int errorCode;

    public CustomExceptionWithCode(String message, int errorCode) {
        super(message);
        this.errorCode = errorCode;
    }

    public int getErrorCode() {
        return errorCode;
    }
}

public class ExceptionWithCodeExample {
    public static void checkValue(int value) throws CustomExceptionWithCode {
        if (value < 0) {
            throw new CustomExceptionWithCode("Negative values not allowed!", 1001);
        }
    }

    public static void main(String[] args) {
        try {
            checkValue(-10);
        } catch (CustomExceptionWithCode e) {
            System.out.println("Error Code: " + e.getErrorCode() + ", Message: " + e.getMessage());
        }
    }
}

✅ Output:

Error Code: 1001, Message: Negative values not allowed!

6. Best Practices for Custom Exceptions

✔ Use checked exceptions for recoverable situations (e.g., invalid input, business rules).
✔ Use unchecked exceptions for developer errors (e.g., null values, invalid method calls).
✔ Provide meaningful exception messages for easier debugging.
✔ Avoid overusing custom exceptions—use built-in Java exceptions when possible.