Error Handling and Exceptions

# CHAPTER 7

Error Handling and Exceptions #

1. Introduction #

2. Learning Objectives #

• Contrast legacy "Error Return Codes" with modern "Exceptions."

• Write clean try-catch-finally blocks that do not obscure business logic.

• Avoid the catastrophic anti-pattern of "Silent Failures."

• Create descriptive, custom Exception classes.

• Implement Defensive Programming to fail fast and fail loudly.

3. Use Exceptions, Not Return Codes #

$device = getDevice($id);
if ($device !== "DEVICE_NOT_FOUND") {
    $status = pauseDevice($device);
    if ($status !== "PAUSE_ERROR") {
        return "SUCCESS";
    }
}

*The Problem:* The caller is forced to check for errors immediately after every single function call. This creates deeply nested, unreadable code where the error handling dominates the business logic.

*Clean (Exceptions):*

try {
    $device = getDevice($id);
    pauseDevice($device);
    return "SUCCESS";
} catch (DeviceNotFoundException $e) {
    logger->error("Device not found.");
} catch (DevicePauseException $e) {
    logger->error("Failed to pause.");
}

*The Solution:* The business logic is clean and uninterrupted in the try block. The error processing is cleanly separated into the catch blocks.

4. The "Silent Failure" Anti-Pattern #

try {
    savePaymentToDatabase($payment);
} catch (Exception $e) {
    // Do nothing. The user won't see an error!
}

*The Result:* The payment failed, but the system continues executing as if it succeeded. The company loses money, and there is zero log data to investigate. Always log exceptions or throw them up the chain.

5. Provide Context with Exceptions #

• *Bad:* throw new Exception("Error in file upload.");

• *Clean:* throw new FileUploadException("Upload failed for User ID: {$userId}. File size exceeded the 5MB limit.");

6. Custom Exception Classes #

• Example: InsufficientFundsException, InvalidTokenException, DatabaseConnectionException.

7. Diagrams/Visual Suggestions #

[ The Controller Function ]
      |
[ TRY BLOCK ]
- Call Service A
- Call Service B (THROWS EXCEPTION!)  --\
- Call Service C (Skipped)              |
                                        |
[ CATCH BLOCK ] <-----------------------/
- Log the specific Error
- Return 500 API Response

8. Best Practices #

• Fail Fast: If a function receives invalid arguments, do not let it execute half its logic before failing. Check the arguments on Line 1 of the function, and throw new InvalidArgumentException() immediately. This is defensive programming.

• Don't Return Null: Returning null forces the caller to write if ($obj === null) everywhere. Instead, throw an Exception, or use the "Special Case Object" pattern.

9. Common Mistakes #

• Using Exceptions for Control Flow: Exceptions are for *exceptional*, unexpected errors (database down, file missing). Do not use them for standard logic routing (e.g., throwing an exception just because a user typed the wrong password; that is a normal business rule, not an application crash).

10. Mini Project: Refactor Error Handling #

function login($email, $password) {
    $user = db_getUser($email);
    if ($user == -1) { return "ERROR_USER"; }
    
    if (hash($password) != $user->password) { return "ERROR_PASS"; }
    
    return "SUCCESS";
}

*After (Clean Exceptions):*

function login(string $email, string $password): void {
    $user = $this->userRepository->findByEmail($email);
    
    if (!$user) {
        throw new UserNotFoundException("No user found for email: {$email}");
    }
    if (!$this->hasher->verify($password, $user->password)) {
        throw new InvalidPasswordException("Password mismatch for email: {$email}");
    }
    
    $this->session->create($user);
}

*(The controller calling login() will wrap it in a try-catch to handle the UI responses).*

11. Practice Exercises #

1. 1. Explain why returning -1 or false to indicate an error is an outdated and dangerous practice compared to throwing an Exception.

1. 2. What is a "Silent Failure"? Write a brief example of how it can destroy data integrity.

12. MCQs with Answers #

13. Interview Questions #

• Q: You are reviewing code and see an empty catch block (Catch and Ignore). Explain to the developer the catastrophic risk they are introducing to the production environment.

• Q: Explain the concept of "Fail Fast." Why is it better for an application to throw a massive error on startup than to run quietly with corrupt configuration data?

• Q: A developer has a function that searches for a user. If the user isn't found, they return null. Explain the maintenance issues this causes (NullPointerExceptions) and propose a cleaner alternative.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #