Throw and Throws Keywords

Lecture 28

Java Programming (4343203)
Diploma in ICT - Semester IV
Gujarat Technological University

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Learning Objectives

By the end of this lecture, you will be able to:

🎯 Understand the difference between throw and throws keywords
🚀 Use the throw keyword to create and throw exceptions manually
📋 Implement the throws keyword for method exception declarations
⚡ Create custom validation logic using throw statements
🔧 Handle both checked and unchecked exception declarations
🎭 Apply exception chaining for better error tracing
🏗️ Design methods with proper exception handling contracts

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The 'throw' Keyword

Basic 'throw' Syntax

The throw keyword is used to explicitly throw an exception from within a method or block.

// Basic syntax
throw new ExceptionType("Error message");

// Examples
throw new IllegalArgumentException("Age cannot be negative");
throw new ArithmeticException("Division by zero attempted");
throw new NullPointerException("Object reference is null");

Simple throw Example

public class ThrowExample {
    public static void validateAge(int age) {
        System.out.println("Validating age: " + age);
        
        if (age < 0) {
            // Manually throw an exception
            throw new IllegalArgumentException(
                "Age cannot be negative. Received: " + age);
        }
        
        if (age > 150) {
            throw new IllegalArgumentException(
                "Age cannot exceed 150. Received: " + age);
        }
        
        System.out.println("Age validation successful: " + age);
    }
    
    public static void main(String[] args) {
        try {
            validateAge(25);    // Valid age
            validateAge(-5);    // Will throw exception
            validateAge(100);   // This won't execute
            
        } catch (IllegalArgumentException e) {
            System.out.println("Validation Error: " + e.getMessage());
        }
    }
}

/* Output:
Validating age: 25
Age validation successful: 25
Validating age: -5
Validation Error: Age cannot be negative. Received: -5
*/

When to Use 'throw'

1. Input Validation

public class BankAccount {
    private double balance;
    
    public void withdraw(double amount) {
        // Validate input parameters
        if (amount <= 0) {
            throw new IllegalArgumentException(
                "Withdrawal amount must be positive: " + amount);
        }
        
        if (amount > balance) {
            throw new IllegalArgumentException(
                "Insufficient funds. Balance: " + balance + 
                ", Requested: " + amount);
        }
        
        balance -= amount;
        System.out.println("Withdrawal successful. New balance: " + balance);
    }
    
    public void deposit(double amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException(
                "Deposit amount must be positive: " + amount);
        }
        
        if (Double.isInfinite(amount) || Double.isNaN(amount)) {
            throw new IllegalArgumentException(
                "Invalid deposit amount: " + amount);
        }
        
        balance += amount;
        System.out.println("Deposit successful. New balance: " + balance);
    }
}

2. State Validation

public class DatabaseConnection {
    private boolean isConnected = false;
    
    public void executeQuery(String query) {
        // Check object state before operation
        if (!isConnected) {
            throw new IllegalStateException(
                "Cannot execute query: Database not connected");
        }
        
        if (query == null || query.trim().isEmpty()) {
            throw new IllegalArgumentException(
                "Query cannot be null or empty");
        }
        
        System.out.println("Executing query: " + query);
        // Query execution logic here
    }
    
    public void connect() {
        if (isConnected) {
            throw new IllegalStateException(
                "Already connected to database");
        }
        
        isConnected = true;
        System.out.println("Database connection established");
    }
}

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The 'throws' Keyword

Basic 'throws' Syntax

The throws keyword is used in method signatures to declare that a method might throw certain exceptions.

// Method that declares it might throw exceptions
public void methodName() throws ExceptionType1, ExceptionType2 {
    // Method implementation
}

// Examples
public void readFile(String filename) throws IOException {
    // File reading code that might throw IOException
}

public void connectToDatabase() throws SQLException, ClassNotFoundException {
    // Database connection code
}

Simple throws Example

public class ThrowsExample {
    // Method declares it might throw IOException
    public static String readFileContent(String filename) throws IOException {
        System.out.println("Attempting to read file: " + filename);
        
        // This might throw FileNotFoundException (subclass of IOException)
        FileReader file = new FileReader(filename);
        BufferedReader reader = new BufferedReader(file);
        
        StringBuilder content = new StringBuilder();
        String line;
        
        // This might throw IOException
        while ((line = reader.readLine()) != null) {
            content.append(line).append("\n");
        }
        
        reader.close();
        file.close();
        
        return content.toString();
    }
    
    public static void main(String[] args) {
        try {
            // Must handle the declared exception
            String content = readFileContent("data.txt");
            System.out.println("File content:\n" + content);
            
        } catch (IOException e) {
            System.out.println("File reading failed: " + e.getMessage());
        }
    }
}

Checked vs Unchecked Exceptions with throws

Checked Exceptions (Must Declare)

public class CheckedExceptionExample {
    // Must declare IOException - it's checked
    public void writeToFile(String filename, String data) throws IOException {
        FileWriter writer = new FileWriter(filename);
        writer.write(data);
        writer.close();
    }
    
    // Must declare SQLException - it's checked
    public void queryDatabase(String query) throws SQLException {
        Connection conn = DriverManager.getConnection("jdbc:mysql://...");
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery(query);
        // Process results
    }
    
    // Must declare ClassNotFoundException - it's checked
    public Object createInstance(String className) throws ClassNotFoundException {
        Class<?> clazz = Class.forName(className);
        try {
            return clazz.newInstance();
        } catch (InstantiationException | IllegalAccessException e) {
            // Convert to unchecked exception
            throw new RuntimeException("Cannot create instance", e);
        }
    }
}

Unchecked Exceptions (Optional to Declare)

public class UncheckedExceptionExample {
    // Optional to declare RuntimeException - it's unchecked
    public void validateInput(String input) throws IllegalArgumentException {
        if (input == null) {
            throw new IllegalArgumentException("Input cannot be null");
        }
        
        if (input.trim().isEmpty()) {
            throw new IllegalArgumentException("Input cannot be empty");
        }
    }
    
    // Can throw RuntimeException without declaring
    public int divide(int a, int b) {
        if (b == 0) {
            throw new ArithmeticException("Division by zero");
        }
        return a / b;
    }
    
    // Optional to declare - but good practice for documentation
    public void processArray(int[] arr, int index) throws ArrayIndexOutOfBoundsException {
        if (index < 0 || index >= arr.length) {
            throw new ArrayIndexOutOfBoundsException(
                "Index " + index + " out of bounds for array of length " + arr.length);
        }
        
        System.out.println("Processing element: " + arr[index]);
    }
}

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Combining throw and throws

Method with Both throw and throws

public class ValidationService {
    // Method declares what it might throw
    public void validateUser(User user) throws ValidationException {
        System.out.println("Validating user: " + user.getName());
        
        // Check name
        if (user.getName() == null || user.getName().trim().isEmpty()) {
            // Explicitly throw the declared exception
            throw new ValidationException("User name cannot be empty");
        }
        
        // Check email
        if (user.getEmail() == null || !user.getEmail().contains("@")) {
            throw new ValidationException("Invalid email format: " + user.getEmail());
        }
        
        // Check age
        if (user.getAge() < 0 || user.getAge() > 150) {
            throw new ValidationException("Age must be between 0 and 150: " + user.getAge());
        }
        
        System.out.println("User validation successful");
    }
    
    // Method that calls another method with throws
    public boolean processUserRegistration(User user) {
        try {
            validateUser(user); // Must handle ValidationException
            
            // Additional processing
            saveUserToDatabase(user);  // Might throw SQLException
            sendWelcomeEmail(user);     // Might throw MailException
            
            System.out.println("User registration completed successfully");
            return true;
            
        } catch (ValidationException e) {
            System.err.println("Validation failed: " + e.getMessage());
            return false;
            
        } catch (SQLException e) {
            System.err.println("Database error: " + e.getMessage());
            return false;
            
        } catch (MailException e) {
            System.err.println("Email sending failed: " + e.getMessage());
            // User is registered but email failed - still success
            return true;
            
        } catch (Exception e) {
            System.err.println("Unexpected error: " + e.getMessage());
            return false;
        }
    }
    
    private void saveUserToDatabase(User user) throws SQLException {
        // Database save logic that might throw SQLException
        System.out.println("Saving user to database: " + user.getName());
    }
    
    private void sendWelcomeEmail(User user) throws MailException {
        // Email sending logic that might throw MailException
        System.out.println("Sending welcome email to: " + user.getEmail());
    }
}

Exception Propagation Chain

public class FileProcessor {
    // Low-level method that detects and throws
    public String readConfigValue(String filename, String key) 
            throws IOException, ConfigurationException {
        
        try {
            // This might throw IOException
            Properties props = new Properties();
            FileInputStream fis = new FileInputStream(filename);
            props.load(fis);
            fis.close();
            
            String value = props.getProperty(key);
            if (value == null) {
                // Throw custom exception with context
                throw new ConfigurationException(
                    "Configuration key '" + key + "' not found in " + filename);
            }
            
            return value;
            
        } catch (IOException e) {
            // Re-throw IOException with more context
            throw new IOException("Failed to read configuration file: " + filename, e);
        }
    }
    
    // Mid-level method that propagates
    public DatabaseConfig loadDatabaseConfig() throws ConfigurationException {
        try {
            String host = readConfigValue("db.properties", "db.host");
            String port = readConfigValue("db.properties", "db.port");
            String database = readConfigValue("db.properties", "db.name");
            
            return new DatabaseConfig(host, Integer.parseInt(port), database);
            
        } catch (IOException e) {
            // Convert IOException to ConfigurationException
            throw new ConfigurationException("Database configuration loading failed", e);
            
        } catch (NumberFormatException e) {
            throw new ConfigurationException("Invalid port number in database configuration", e);
        }
    }
    
    // High-level method that handles
    public void initializeApplication() {
        try {
            DatabaseConfig config = loadDatabaseConfig();
            System.out.println("Application initialized with config: " + config);
            
        } catch (ConfigurationException e) {
            System.err.println("Configuration Error: " + e.getMessage());
            
            // Print the full exception chain
            Throwable cause = e.getCause();
            while (cause != null) {
                System.err.println("Caused by: " + cause.getMessage());
                cause = cause.getCause();
            }
            
            // Use default configuration
            useDefaultConfiguration();
        }
    }
    
    private void useDefaultConfiguration() {
        System.out.println("Using default database configuration");
        // Initialize with defaults
    }
}

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Exception Chaining and Re-throwing

Exception Chaining

Exception chaining allows you to preserve the original exception while throwing a new one:

public class ExceptionChainingExample {
    // Method that chains exceptions
    public void processData(String data) throws DataProcessingException {
        try {
            // Step 1: Parse data
            parseData(data);
            
            // Step 2: Validate parsed data
            validateData(data);
            
            // Step 3: Transform data
            transformData(data);
            
        } catch (ParseException e) {
            // Chain the original exception
            throw new DataProcessingException(
                "Failed to process data due to parsing error", e);
                
        } catch (ValidationException e) {
            // Chain the original exception
            throw new DataProcessingException(
                "Failed to process data due to validation error", e);
                
        } catch (TransformationException e) {
            // Chain the original exception
            throw new DataProcessingException(
                "Failed to process data due to transformation error", e);
        }
    }
    
    private void parseData(String data) throws ParseException {
        if (data == null) {
            throw new ParseException("Data cannot be null", 0);
        }
        
        if (!data.startsWith("{")) {
            throw new ParseException("Invalid data format: must start with '{'", 0);
        }
        
        System.out.println("Data parsing completed");
    }
    
    private void validateData(String data) throws ValidationException {
        if (data.length() < 10) {
            throw new ValidationException("Data too short: minimum 10 characters required");
        }
        
        System.out.println("Data validation completed");
    }
    
    private void transformData(String data) throws TransformationException {
        try {
            // Simulate complex transformation that might fail
            if (data.contains("ERROR")) {
                throw new RuntimeException("Transformation failed on ERROR keyword");
            }
            
            System.out.println("Data transformation completed");
            
        } catch (RuntimeException e) {
            // Wrap runtime exception in checked exception
            throw new TransformationException("Data transformation failed", e);
        }
    }
}

Re-throwing Exceptions

Sometimes you want to catch an exception, do some processing, then re-throw:

public class ReThrowingExample {
    private static final Logger logger = Logger.getLogger(ReThrowingExample.class.getName());
    
    public void processFile(String filename) throws IOException {
        try {
            System.out.println("Starting file processing: " + filename);
            
            // Risky file operation
            FileReader reader = new FileReader(filename);
            BufferedReader bufferedReader = new BufferedReader(reader);
            
            String line;
            int lineCount = 0;
            while ((line = bufferedReader.readLine()) != null) {
                lineCount++;
                processLine(line, lineCount);
            }
            
            bufferedReader.close();
            reader.close();
            
            System.out.println("File processing completed successfully");
            
        } catch (FileNotFoundException e) {
            // Log the error but re-throw
            logger.error("File not found: " + filename, e);
            
            // Re-throw the same exception
            throw e;
            
        } catch (IOException e) {
            // Log the error and re-throw with additional context
            logger.error("IO error while processing file: " + filename, e);
            
            // Re-throw with additional message
            throw new IOException("Failed to process file: " + filename, e);
        }
    }
    
    private void processLine(String line, int lineNumber) throws IOException {
        try {
            // Process individual line
            if (line.trim().isEmpty()) {
                return; // Skip empty lines
            }
            
            // Simulate line processing that might fail
            if (line.contains("CORRUPT")) {
                throw new RuntimeException("Corrupted data found in line");
            }
            
            System.out.println("Processed line " + lineNumber + ": " + line.substring(0, Math.min(20, line.length())));
            
        } catch (RuntimeException e) {
            // Convert runtime exception to checked IOException with context
            throw new IOException("Error processing line " + lineNumber + ": " + line, e);
        }
    }
    
    // Method that handles the re-thrown exceptions
    public void handleFileProcessing(String filename) {
        try {
            processFile(filename);
            
        } catch (IOException e) {
            System.err.println("File processing failed: " + e.getMessage());
            
            // Print the full exception chain
            System.err.println("Exception chain:");
            Throwable current = e;
            int level = 0;
            while (current != null) {
                System.err.println("  " + "  ".repeat(level) + current.getClass().getSimpleName() + 
                                 ": " + current.getMessage());
                current = current.getCause();
                level++;
            }
        }
    }
}

Exception Suppression (Java 7+)

public class ExceptionSuppressionExample {
    public void demonstrateSuppression() throws IOException {
        FileInputStream input = null;
        
        try {
            input = new FileInputStream("data.txt");
            
            // This might throw an exception
            processFile(input);
            
        } finally {
            if (input != null) {
                try {
                    input.close(); // This might also throw an exception
                } catch (IOException closeException) {
                    // If an exception was already thrown in try block,
                    // this close exception will be suppressed
                    System.err.println("Error closing file: " + closeException.getMessage());
                }
            }
        }
    }
    
    // Better approach with try-with-resources (automatic suppression)
    public void demonstrateAutomaticSuppression() throws IOException {
        try (FileInputStream input = new FileInputStream("data.txt")) {
            processFile(input);
            // If both processFile() and close() throw exceptions,
            // the close() exception will be automatically suppressed
        }
    }
    
    private void processFile(FileInputStream input) throws IOException {
        // File processing logic that might throw IOException
    }
}

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Practical Validation Examples

1. User Registration System

public class UserRegistrationService {
    
    public void registerUser(String username, String password, String email, int age) 
            throws RegistrationException {
        
        try {
            // Validate each field
            validateUsername(username);
            validatePassword(password);
            validateEmail(email);
            validateAge(age);
            
            // Check if user already exists
            checkUserExists(username, email);
            
            // Create and save user
            User user = new User(username, password, email, age);
            saveUser(user);
            
            System.out.println("User registered successfully: " + username);
            
        } catch (ValidationException e) {
            // Convert validation exception to registration exception
            throw new RegistrationException("Registration failed: " + e.getMessage(), e);
            
        } catch (UserAlreadyExistsException e) {
            throw new RegistrationException("Registration failed: " + e.getMessage(), e);
            
        } catch (DatabaseException e) {
            throw new RegistrationException("Registration failed due to system error", e);
        }
    }
    
    private void validateUsername(String username) throws ValidationException {
        if (username == null || username.trim().isEmpty()) {
            throw new ValidationException("Username cannot be empty");
        }
        
        if (username.length() < 3) {
            throw new ValidationException("Username must be at least 3 characters long");
        }
        
        if (username.length() > 20) {
            throw new ValidationException("Username cannot exceed 20 characters");
        }
        
        if (!username.matches("[a-zA-Z0-9_]+")) {
            throw new ValidationException("Username can only contain letters, numbers, and underscores");
        }
    }
    
    private void validatePassword(String password) throws ValidationException {
        if (password == null || password.isEmpty()) {
            throw new ValidationException("Password cannot be empty");
        }
        
        if (password.length() < 8) {
            throw new ValidationException("Password must be at least 8 characters long");
        }
        
        boolean hasUpper = password.chars().anyMatch(Character::isUpperCase);
        boolean hasLower = password.chars().anyMatch(Character::isLowerCase);
        boolean hasDigit = password.chars().anyMatch(Character::isDigit);
        
        if (!hasUpper || !hasLower || !hasDigit) {
            throw new ValidationException(
                "Password must contain at least one uppercase letter, one lowercase letter, and one digit");
        }
    }
    
    private void validateEmail(String email) throws ValidationException {
        if (email == null || email.trim().isEmpty()) {
            throw new ValidationException("Email cannot be empty");
        }
        
        String emailRegex = "^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$";
        if (!email.matches(emailRegex)) {
            throw new ValidationException("Invalid email format: " + email);
        }
    }
    
    private void validateAge(int age) throws ValidationException {
        if (age < 13) {
            throw new ValidationException("Age must be at least 13 years");
        }
        
        if (age > 120) {
            throw new ValidationException("Age cannot exceed 120 years");
        }
    }
    
    private void checkUserExists(String username, String email) 
            throws UserAlreadyExistsException, DatabaseException {
        // Simulate database check
        if (isUsernameTaken(username)) {
            throw new UserAlreadyExistsException("Username already exists: " + username);
        }
        
        if (isEmailTaken(email)) {
            throw new UserAlreadyExistsException("Email already registered: " + email);
        }
    }
    
    private boolean isUsernameTaken(String username) throws DatabaseException {
        // Simulate database query
        return false; // Simplified
    }
    
    private boolean isEmailTaken(String email) throws DatabaseException {
        // Simulate database query
        return false; // Simplified
    }
    
    private void saveUser(User user) throws DatabaseException {
        // Simulate user saving
        System.out.println("Saving user to database: " + user.getUsername());
    }
}

2. File Upload System

public class FileUploadService {
    private static final long MAX_FILE_SIZE = 10 * 1024 * 1024; // 10MB
    private static final Set<String> ALLOWED_EXTENSIONS = 
        Set.of("jpg", "jpeg", "png", "gif", "pdf", "doc", "docx");
    
    public String uploadFile(MultipartFile file, String uploadDir) 
            throws FileUploadException {
        
        try {
            // Validate file
            validateFile(file);
            
            // Create upload directory
            createUploadDirectory(uploadDir);
            
            // Generate unique filename
            String filename = generateUniqueFilename(file.getOriginalFilename());
            String fullPath = uploadDir + "/" + filename;
            
            // Save file
            saveFile(file, fullPath);
            
            // Verify file was saved correctly
            verifyFileSaved(fullPath, file.getSize());
            
            System.out.println("File uploaded successfully: " + filename);
            return filename;
            
        } catch (ValidationException e) {
            throw new FileUploadException("File validation failed: " + e.getMessage(), e);
            
        } catch (IOException e) {
            throw new FileUploadException("File save failed: " + e.getMessage(), e);
            
        } catch (SecurityException e) {
            throw new FileUploadException("Permission denied: " + e.getMessage(), e);
        }
    }
    
    private void validateFile(MultipartFile file) throws ValidationException {
        if (file == null || file.isEmpty()) {
            throw new ValidationException("File is empty or null");
        }
        
        // Check file size
        if (file.getSize() > MAX_FILE_SIZE) {
            throw new ValidationException(
                "File size (" + file.getSize() + " bytes) exceeds maximum allowed (" + 
                MAX_FILE_SIZE + " bytes)");
        }
        
        // Check filename
        String originalFilename = file.getOriginalFilename();
        if (originalFilename == null || originalFilename.trim().isEmpty()) {
            throw new ValidationException("Invalid filename");
        }
        
        // Check file extension
        String extension = getFileExtension(originalFilename);
        if (!ALLOWED_EXTENSIONS.contains(extension.toLowerCase())) {
            throw new ValidationException(
                "File type not allowed: " + extension + ". Allowed types: " + ALLOWED_EXTENSIONS);
        }
        
        // Check for potentially dangerous filenames
        if (originalFilename.contains("..") || originalFilename.contains("/") || originalFilename.contains("\\")) {
            throw new ValidationException("Filename contains invalid characters: " + originalFilename);
        }
    }
    
    private void createUploadDirectory(String uploadDir) throws IOException {
        File directory = new File(uploadDir);
        
        if (!directory.exists()) {
            boolean created = directory.mkdirs();
            if (!created) {
                throw new IOException("Failed to create upload directory: " + uploadDir);
            }
        }
        
        if (!directory.canWrite()) {
            throw new SecurityException("Cannot write to upload directory: " + uploadDir);
        }
    }
    
    private String generateUniqueFilename(String originalFilename) {
        String extension = getFileExtension(originalFilename);
        String baseName = getBaseName(originalFilename);
        String timestamp = String.valueOf(System.currentTimeMillis());
        String random = String.valueOf((int)(Math.random() * 1000));
        
        return baseName + "_" + timestamp + "_" + random + "." + extension;
    }
    
    private void saveFile(MultipartFile file, String fullPath) throws IOException {
        try (InputStream inputStream = file.getInputStream();
             FileOutputStream outputStream = new FileOutputStream(fullPath)) {
            
            byte[] buffer = new byte[8192];
            int bytesRead;
            long totalBytesRead = 0;
            long fileSize = file.getSize();
            
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
                totalBytesRead += bytesRead;
                
                // Show progress for large files
                if (fileSize > 1024 * 1024) { // Files larger than 1MB
                    int progress = (int) ((totalBytesRead * 100) / fileSize);
                    if (progress % 10 == 0) {
                        System.out.println("Upload progress: " + progress + "%");
                    }
                }
            }
        }
    }
    
    private void verifyFileSaved(String fullPath, long expectedSize) throws IOException {
        File savedFile = new File(fullPath);
        
        if (!savedFile.exists()) {
            throw new IOException("File was not saved: " + fullPath);
        }
        
        if (savedFile.length() != expectedSize) {
            // Delete incomplete file
            savedFile.delete();
            throw new IOException(
                "File size mismatch. Expected: " + expectedSize + ", Actual: " + savedFile.length());
        }
    }
    
    private String getFileExtension(String filename) {
        int lastDotIndex = filename.lastIndexOf('.');
        if (lastDotIndex == -1) {
            throw new IllegalArgumentException("File has no extension: " + filename);
        }
        return filename.substring(lastDotIndex + 1);
    }
    
    private String getBaseName(String filename) {
        int lastDotIndex = filename.lastIndexOf('.');
        if (lastDotIndex == -1) {
            return filename;
        }
        return filename.substring(0, lastDotIndex);
    }
}

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Method Signature Best Practices

Exception Declaration Guidelines

1. Be Specific with Exception Types

// BAD: Too generic
public void processData(String data) throws Exception {
    // What could go wrong? Hard to tell!
}

// GOOD: Specific exception types
public void processData(String data) 
        throws ParseException, ValidationException, TransformationException {
    // Clear what exceptions to expect and handle
}

// EVEN BETTER: Custom exception hierarchy
public void processData(String data) throws DataProcessingException {
    // Single exception type that wraps specific causes
}

2. Document Exception Conditions

public class OrderService {
    /**
     * Processes a customer order
     * 
     * @param order The order to process
     * @throws IllegalArgumentException if order is null or invalid
     * @throws InsufficientInventoryException if items are out of stock
     * @throws PaymentProcessingException if payment fails
     * @throws OrderProcessingException if order cannot be processed
     */
    public void processOrder(Order order) 
            throws IllegalArgumentException, 
                   InsufficientInventoryException,
                   PaymentProcessingException, 
                   OrderProcessingException {
        
        // Validate order
        if (order == null) {
            throw new IllegalArgumentException("Order cannot be null");
        }
        
        if (order.getItems().isEmpty()) {
            throw new IllegalArgumentException("Order must contain at least one item");
        }
        
        // Check inventory
        for (OrderItem item : order.getItems()) {
            if (!isInStock(item)) {
                throw new InsufficientInventoryException(
                    "Item out of stock: " + item.getProductName());
            }
        }
        
        // Process payment
        try {
            processPayment(order.getPaymentInfo(), order.getTotalAmount());
        } catch (PaymentException e) {
            throw new PaymentProcessingException("Payment failed", e);
        }
        
        // Save order
        try {
            saveOrder(order);
        } catch (DatabaseException e) {
            throw new OrderProcessingException("Failed to save order", e);
        }
    }
    
    private boolean isInStock(OrderItem item) {
        // Check inventory logic
        return true; // Simplified
    }
    
    private void processPayment(PaymentInfo paymentInfo, BigDecimal amount) 
            throws PaymentException {
        // Payment processing logic
    }
    
    private void saveOrder(Order order) throws DatabaseException {
        // Database save logic
    }
}

Exception Hierarchy Design

1. Create Meaningful Exception Hierarchies

// Base application exception
public class ApplicationException extends Exception {
    public ApplicationException(String message) {
        super(message);
    }
    
    public ApplicationException(String message, Throwable cause) {
        super(message, cause);
    }
}

// Business logic exceptions
public class BusinessException extends ApplicationException {
    public BusinessException(String message) {
        super(message);
    }
    
    public BusinessException(String message, Throwable cause) {
        super(message, cause);
    }
}

// Specific business exceptions
public class ValidationException extends BusinessException {
    private String fieldName;
    private Object invalidValue;
    
    public ValidationException(String message, String fieldName, Object invalidValue) {
        super(message);
        this.fieldName = fieldName;
        this.invalidValue = invalidValue;
    }
    
    public String getFieldName() { return fieldName; }
    public Object getInvalidValue() { return invalidValue; }
}

public class InsufficientFundsException extends BusinessException {
    private BigDecimal requestedAmount;
    private BigDecimal availableAmount;
    
    public InsufficientFundsException(BigDecimal requested, BigDecimal available) {
        super("Insufficient funds. Requested: " + requested + ", Available: " + available);
        this.requestedAmount = requested;
        this.availableAmount = available;
    }
    
    public BigDecimal getRequestedAmount() { return requestedAmount; }
    public BigDecimal getAvailableAmount() { return availableAmount; }
}

// System-level exceptions
public class SystemException extends ApplicationException {
    public SystemException(String message) {
        super(message);
    }
    
    public SystemException(String message, Throwable cause) {
        super(message, cause);
    }
}

public class DatabaseException extends SystemException {
    private String operation;
    
    public DatabaseException(String operation, String message, Throwable cause) {
        super("Database operation failed: " + operation + " - " + message, cause);
        this.operation = operation;
    }
    
    public String getOperation() { return operation; }
}

2. Exception Handling Strategy

public class ServiceLayer {
    // Service methods catch and convert exceptions appropriately
    public void transferMoney(String fromAccount, String toAccount, BigDecimal amount) 
            throws BusinessException {
        try {
            // Call DAO layer
            Account from = accountDao.findById(fromAccount);
            Account to = accountDao.findById(toAccount);
            
            // Business validation
            if (from.getBalance().compareTo(amount) < 0) {
                throw new InsufficientFundsException(amount, from.getBalance());
            }
            
            // Perform transfer
            from.debit(amount);
            to.credit(amount);
            
            accountDao.update(from);
            accountDao.update(to);
            
        } catch (DatabaseException e) {
            // Convert system exception to business exception
            throw new BusinessException("Transfer failed due to system error", e);
            
        } catch (BusinessException e) {
            // Re-throw business exceptions as-is
            throw e;
            
        } catch (Exception e) {
            // Convert unexpected exceptions
            throw new BusinessException("Unexpected error during transfer", e);
        }
    }
}

public class ControllerLayer {
    // Controllers handle business exceptions and return appropriate responses
    public ResponseEntity<?> transferMoney(TransferRequest request) {
        try {
            serviceLayer.transferMoney(
                request.getFromAccount(), 
                request.getToAccount(), 
                request.getAmount()
            );
            
            return ResponseEntity.ok("Transfer completed successfully");
            
        } catch (InsufficientFundsException e) {
            return ResponseEntity.badRequest()
                .body("Insufficient funds: " + e.getMessage());
                
        } catch (ValidationException e) {
            return ResponseEntity.badRequest()
                .body("Validation error: " + e.getMessage());
                
        } catch (BusinessException e) {
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
                .body("Service temporarily unavailable");
                
        } catch (Exception e) {
            // Log unexpected exceptions
            logger.error("Unexpected error in transferMoney", e);
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
                .body("An unexpected error occurred");
        }
    }
}

layout: default

Hands-on Exercise 1: Employee Management System

Task: Build Employee Validation System

Create a comprehensive employee management system with proper exception handling:

public class EmployeeService {
    private List<Employee> employees = new ArrayList<>();
    
    // TODO: Implement with proper throw/throws
    public void addEmployee(Employee employee) throws EmployeeException {
        // Validate employee data
        // Check for duplicates
        // Add to collection
    }
    
    // TODO: Implement with proper validation
    public void validateEmployee(Employee employee) throws ValidationException {
        // Validate name (not null, not empty, reasonable length)
        // Validate employee ID (format: EMP-XXXX)
        // Validate email (proper format)
        // Validate age (18-65 years)
        // Validate salary (positive, reasonable range)
        // Validate department (not null, valid department)
    }
    
    // TODO: Implement with proper exception handling
    public Employee findEmployeeById(String employeeId) throws EmployeeNotFoundException {
        // Search for employee
        // Throw exception if not found
    }
    
    // TODO: Implement with proper validation
    public void updateEmployee(String employeeId, Employee updatedEmployee) 
            throws EmployeeNotFoundException, ValidationException {
        // Find existing employee
        // Validate updated data
        // Update employee
    }
    
    // TODO: Implement with business rules
    public void promoteEmployee(String employeeId, String newDepartment, 
                               BigDecimal newSalary) 
            throws EmployeeNotFoundException, PromotionException {
        // Find employee
        // Validate promotion rules
        // Apply promotion
    }
    
    // TODO: Implement batch operation with error handling
    public EmployeeImportResult importEmployees(List<Employee> employeesToImport) {
        // Import employees one by one
        // Collect successes and failures
        // Return detailed result
    }
}

class Employee {
    private String employeeId;
    private String name;
    private String email;
    private int age;
    private BigDecimal salary;
    private String department;
    
    // Constructor, getters, setters
}

class EmployeeImportResult {
    private int successCount;
    private int failureCount;
    private List<String> errors;
    
    // Constructor, getters, setters
}

Requirements:

Create custom exception hierarchy
Implement comprehensive validation
Use throw for validation failures
Use throws for method declarations
Handle batch operations gracefully

Solution Framework

// Exception hierarchy
public abstract class EmployeeException extends Exception {
    public EmployeeException(String message) {
        super(message);
    }
    
    public EmployeeException(String message, Throwable cause) {
        super(message, cause);
    }
}

public class ValidationException extends EmployeeException {
    private String fieldName;
    private Object invalidValue;
    
    public ValidationException(String fieldName, Object invalidValue, String message) {
        super("Validation failed for " + fieldName + ": " + message);
        this.fieldName = fieldName;
        this.invalidValue = invalidValue;
    }
    
    public String getFieldName() { return fieldName; }
    public Object getInvalidValue() { return invalidValue; }
}

public class EmployeeNotFoundException extends EmployeeException {
    private String employeeId;
    
    public EmployeeNotFoundException(String employeeId) {
        super("Employee not found: " + employeeId);
        this.employeeId = employeeId;
    }
    
    public String getEmployeeId() { return employeeId; }
}

public class DuplicateEmployeeException extends EmployeeException {
    public DuplicateEmployeeException(String employeeId) {
        super("Employee already exists: " + employeeId);
    }
}

public class PromotionException extends EmployeeException {
    public PromotionException(String message) {
        super(message);
    }
}

// Validation implementation
public void validateEmployee(Employee employee) throws ValidationException {
    // Name validation
    if (employee.getName() == null || employee.getName().trim().isEmpty()) {
        throw new ValidationException("name", employee.getName(), "Name cannot be empty");
    }
    
    if (employee.getName().length() > 100) {
        throw new ValidationException("name", employee.getName(), "Name too long (max 100 characters)");
    }
    
    // Employee ID validation
    String employeeId = employee.getEmployeeId();
    if (employeeId == null || !employeeId.matches("EMP-\\d{4}")) {
        throw new ValidationException("employeeId", employeeId, "Invalid format (expected: EMP-XXXX)");
    }
    
    // Email validation
    String email = employee.getEmail();
    if (email == null || !email.matches("^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$")) {
        throw new ValidationException("email", email, "Invalid email format");
    }
    
    // Age validation
    int age = employee.getAge();
    if (age < 18 || age > 65) {
        throw new ValidationException("age", age, "Age must be between 18 and 65");
    }
    
    // Salary validation
    BigDecimal salary = employee.getSalary();
    if (salary == null || salary.compareTo(BigDecimal.ZERO) <= 0) {
        throw new ValidationException("salary", salary, "Salary must be positive");
    }
    
    if (salary.compareTo(new BigDecimal("1000000")) > 0) {
        throw new ValidationException("salary", salary, "Salary exceeds maximum limit");
    }
    
    // Department validation
    String department = employee.getDepartment();
    Set<String> validDepartments = Set.of("HR", "IT", "FINANCE", "MARKETING", "OPERATIONS");
    if (department == null || !validDepartments.contains(department.toUpperCase())) {
        throw new ValidationException("department", department, "Invalid department");
    }
}

layout: default

Hands-on Exercise 2: Configuration Manager

Task: Build Configuration System

Create a robust configuration manager with comprehensive error handling:

public class ConfigurationManager {
    private Properties properties = new Properties();
    private boolean isLoaded = false;
    
    // TODO: Implement with multiple exception types
    public void loadConfiguration(String... configFiles) 
            throws ConfigurationException {
        // Load configuration from multiple files
        // Handle missing files gracefully
        // Validate configuration after loading
        // Support property inheritance/override
    }
    
    // TODO: Implement with validation
    public void validateConfiguration() throws ConfigurationException {
        // Check required properties exist
        // Validate property formats and ranges
        // Check property dependencies
    }
    
    // TODO: Implement with type conversion and validation
    public String getStringProperty(String key) throws PropertyException {
        // Get string property
        // Throw exception if not found or invalid
    }
    
    public int getIntProperty(String key, int min, int max) throws PropertyException {
        // Get integer property with range validation
        // Convert string to int with error handling
    }
    
    public boolean getBooleanProperty(String key) throws PropertyException {
        // Get boolean property
        // Handle various boolean representations (true/false, yes/no, 1/0)
    }
    
    // TODO: Implement with fallback handling
    public DatabaseConfig getDatabaseConfig() throws ConfigurationException {
        // Extract database configuration
        // Validate database connection parameters
        // Test connection if requested
    }
    
    // TODO: Implement with encryption support
    public String getEncryptedProperty(String key) 
            throws PropertyException, DecryptionException {
        // Get encrypted property
        // Decrypt value
        // Handle decryption failures
    }
    
    // TODO: Implement configuration saving
    public void saveConfiguration(String filename) 
            throws IOException, SecurityException {
        // Save current configuration to file
        // Handle file permissions
        // Create backup of existing file
    }
}

// Supporting classes
class DatabaseConfig {
    private String host;
    private int port;
    private String database;
    private String username;
    private String password;
    
    // Constructor, getters, setters
}

Requirements:

Handle multiple configuration sources
Provide type-safe property access
Support property validation
Handle missing/corrupted configuration gracefully
Provide meaningful error messages

Solution Implementation

// Exception hierarchy for configuration
public abstract class ConfigurationException extends Exception {
    public ConfigurationException(String message) {
        super(message);
    }
    
    public ConfigurationException(String message, Throwable cause) {
        super(message, cause);
    }
}

public class PropertyException extends ConfigurationException {
    private String propertyKey;
    
    public PropertyException(String propertyKey, String message) {
        super("Property '" + propertyKey + "': " + message);
        this.propertyKey = propertyKey;
    }
    
    public PropertyException(String propertyKey, String message, Throwable cause) {
        super("Property '" + propertyKey + "': " + message, cause);
        this.propertyKey = propertyKey;
    }
    
    public String getPropertyKey() { return propertyKey; }
}

public class ConfigurationValidationException extends ConfigurationException {
    private List<String> validationErrors;
    
    public ConfigurationValidationException(List<String> errors) {
        super("Configuration validation failed: " + String.join(", ", errors));
        this.validationErrors = new ArrayList<>(errors);
    }
    
    public List<String> getValidationErrors() { return validationErrors; }
}

// Implementation
public void loadConfiguration(String... configFiles) throws ConfigurationException {
    if (configFiles == null || configFiles.length == 0) {
        throw new ConfigurationException("No configuration files specified");
    }
    
    List<String> loadErrors = new ArrayList<>();
    boolean anyLoaded = false;
    
    for (String configFile : configFiles) {
        try {
            loadSingleConfiguration(configFile);
            System.out.println("Loaded configuration from: " + configFile);
            anyLoaded = true;
            
        } catch (IOException e) {
            String error = "Failed to load " + configFile + ": " + e.getMessage();
            loadErrors.add(error);
            System.err.println("Warning: " + error);
        }
    }
    
    if (!anyLoaded) {
        throw new ConfigurationException(
            "No configuration files could be loaded. Errors: " + String.join("; ", loadErrors));
    }
    
    // Validate configuration after loading
    validateConfiguration();
    
    isLoaded = true;
    System.out.println("Configuration loaded and validated successfully");
}

private void loadSingleConfiguration(String configFile) throws IOException {
    try (FileInputStream fis = new FileInputStream(configFile)) {
        properties.load(fis);
    }
}

public void validateConfiguration() throws ConfigurationException {
    List<String> errors = new ArrayList<>();
    
    // Check required properties
    String[] requiredProperties = {
        "database.host", "database.port", "database.name",
        "app.name", "app.version", "logging.level"
    };
    
    for (String required : requiredProperties) {
        if (!properties.containsKey(required) || 
            properties.getProperty(required).trim().isEmpty()) {
            errors.add("Required property missing or empty: " + required);
        }
    }
    
    // Validate specific property formats
    try {
        int port = getIntProperty("database.port", 1, 65535);
    } catch (PropertyException e) {
        errors.add(e.getMessage());
    }
    
    // Validate logging level
    String logLevel = properties.getProperty("logging.level");
    if (logLevel != null) {
        Set<String> validLevels = Set.of("TRACE", "DEBUG", "INFO", "WARN", "ERROR");
        if (!validLevels.contains(logLevel.toUpperCase())) {
            errors.add("Invalid logging level: " + logLevel);
        }
    }
    
    if (!errors.isEmpty()) {
        throw new ConfigurationValidationException(errors);
    }
}

public int getIntProperty(String key, int min, int max) throws PropertyException {
    if (!isLoaded) {
        throw new PropertyException(key, "Configuration not loaded");
    }
    
    String value = properties.getProperty(key);
    if (value == null) {
        throw new PropertyException(key, "Property not found");
    }
    
    try {
        int intValue = Integer.parseInt(value.trim());
        
        if (intValue < min || intValue > max) {
            throw new PropertyException(key, 
                "Value " + intValue + " is out of range [" + min + ", " + max + "]");
        }
        
        return intValue;
        
    } catch (NumberFormatException e) {
        throw new PropertyException(key, "Invalid integer format: " + value, e);
    }
}

layout: default

Real-world Applications

1. Banking Transaction System

@Service
public class BankingTransactionService {
    
    /**
     * Transfers money between accounts
     * @throws InsufficientFundsException if source account lacks funds
     * @throws AccountNotFoundException if either account doesn't exist
     * @throws AccountFrozenException if either account is frozen
     * @throws TransferLimitException if transfer exceeds limits
     * @throws SystemException if system error occurs
     */
    @Transactional
    public TransactionResult transferMoney(String fromAccountId, String toAccountId, 
                                         BigDecimal amount, String description) 
            throws InsufficientFundsException, AccountNotFoundException, 
                   AccountFrozenException, TransferLimitException, SystemException {
        
        // Input validation
        if (amount == null || amount.compareTo(BigDecimal.ZERO) <= 0) {
            throw new IllegalArgumentException("Transfer amount must be positive");
        }
        
        if (fromAccountId == null || toAccountId == null) {
            throw new IllegalArgumentException("Account IDs cannot be null");
        }
        
        if (fromAccountId.equals(toAccountId)) {
            throw new IllegalArgumentException("Cannot transfer to same account");
        }
        
        try {
            // Get accounts with locking
            Account fromAccount = getAccountForUpdate(fromAccountId);
            Account toAccount = getAccountForUpdate(toAccountId);
            
            // Validate account states
            validateAccountForTransfer(fromAccount, amount, true);  // source account
            validateAccountForTransfer(toAccount, amount, false);   // destination account
            
            // Check transfer limits
            validateTransferLimits(fromAccount, amount);
            
            // Perform the transfer
            fromAccount.debit(amount);
            toAccount.credit(amount);
            
            // Save accounts
            accountRepository.save(fromAccount);
            accountRepository.save(toAccount);
            
            // Record transaction
            Transaction transaction = createTransaction(
                fromAccountId, toAccountId, amount, description);
            transactionRepository.save(transaction);
            
            // Send notifications
            notifyAccountHolders(fromAccount, toAccount, amount, transaction.getId());
            
            logger.info("Transfer completed: {} from {} to {}", 
                       amount, fromAccountId, toAccountId);
            
            return new TransactionResult(transaction.getId(), "Transfer completed successfully");
            
        } catch (AccountNotFoundException | InsufficientFundsException | 
                 AccountFrozenException | TransferLimitException e) {
            // Business exceptions - log and re-throw
            logger.warn("Transfer failed - business rule violation: {}", e.getMessage());
            throw e;
            
        } catch (DataAccessException e) {
            // Database exceptions
            logger.error("Transfer failed - database error", e);
            throw new SystemException("Transfer failed due to system error", e);
            
        } catch (Exception e) {
            // Unexpected exceptions
            logger.error("Transfer failed - unexpected error", e);
            throw new SystemException("Transfer failed due to unexpected system error", e);
        }
    }
    
    private Account getAccountForUpdate(String accountId) throws AccountNotFoundException {
        Account account = accountRepository.findByIdForUpdate(accountId);
        if (account == null) {
            throw new AccountNotFoundException("Account not found: " + accountId);
        }
        return account;
    }
    
    private void validateAccountForTransfer(Account account, BigDecimal amount, boolean isSource) 
            throws AccountFrozenException, InsufficientFundsException {
        
        if (account.getStatus() == AccountStatus.FROZEN) {
            throw new AccountFrozenException("Account is frozen: " + account.getAccountId());
        }
        
        if (account.getStatus() == AccountStatus.CLOSED) {
            throw new AccountNotFoundException("Account is closed: " + account.getAccountId());
        }
        
        if (isSource && account.getBalance().compareTo(amount) < 0) {
            throw new InsufficientFundsException(
                "Insufficient funds. Available: " + account.getBalance() + 
                ", Requested: " + amount);
        }
    }
    
    private void validateTransferLimits(Account account, BigDecimal amount) 
            throws TransferLimitException {
        
        // Check single transaction limit
        if (amount.compareTo(account.getSingleTransferLimit()) > 0) {
            throw new TransferLimitException(
                "Amount exceeds single transfer limit: " + account.getSingleTransferLimit());
        }
        
        // Check daily limit
        BigDecimal todayTotal = transactionRepository.getDailyTransferTotal(
            account.getAccountId(), LocalDate.now());
            
        if (todayTotal.add(amount).compareTo(account.getDailyTransferLimit()) > 0) {
            throw new TransferLimitException(
                "Transfer would exceed daily limit: " + account.getDailyTransferLimit());
        }
    }
}

2. E-commerce Order Processing

@Service
public class OrderProcessingService {
    
    /**
     * Processes customer order
     * @throws ValidationException if order data is invalid
     * @throws InventoryException if items are unavailable
     * @throws PaymentException if payment processing fails
     * @throws ShippingException if shipping calculation fails
     * @throws OrderException if order cannot be processed
     */
    public OrderResult processOrder(Order order) 
            throws ValidationException, InventoryException, PaymentException, 
                   ShippingException, OrderException {
        
        String orderId = generateOrderId();
        
        try {
            logger.info("Processing order: {}", orderId);
            
            // Step 1: Validate order
            validateOrder(order);
            
            // Step 2: Check inventory and reserve items
            List<InventoryReservation> reservations = reserveInventory(order.getItems());
            
            try {
                // Step 3: Calculate totals
                OrderTotals totals = calculateOrderTotals(order);
                
                // Step 4: Process payment
                PaymentResult paymentResult = processPayment(order.getPayment(), totals);
                
                try {
                    // Step 5: Calculate shipping
                    ShippingInfo shipping = calculateShipping(order);
                    
                    // Step 6: Create order record
                    Order savedOrder = createOrderRecord(order, orderId, totals, 
                                                        paymentResult, shipping);
                    
                    // Step 7: Update inventory
                    updateInventory(reservations);
                    
                    // Step 8: Send confirmation
                    sendOrderConfirmation(savedOrder);
                    
                    logger.info("Order processed successfully: {}", orderId);
                    return new OrderResult(orderId, OrderStatus.CONFIRMED, savedOrder);
                    
                } catch (Exception e) {
                    // Rollback payment if order creation fails
                    try {
                        refundPayment(paymentResult);
                    } catch (Exception refundException) {
                        logger.error("Failed to refund payment for failed order", refundException);
                    }
                    throw e;
                }
                
            } catch (Exception e) {
                // Release inventory reservations if payment or later steps fail
                releaseInventoryReservations(reservations);
                throw e;
            }
            
        } catch (ValidationException | InventoryException e) {
            // Business validation failures - don't retry
            logger.warn("Order processing failed - validation error: {}", e.getMessage());
            throw e;
            
        } catch (PaymentException e) {
            // Payment failures - might be retryable
            logger.warn("Order processing failed - payment error: {}", e.getMessage());
            throw e;
            
        } catch (Exception e) {
            // System errors
            logger.error("Order processing failed - system error", e);
            throw new OrderException("Order processing failed due to system error", e);
        }
    }
    
    private void validateOrder(Order order) throws ValidationException {
        List<String> errors = new ArrayList<>();
        
        // Validate customer
        if (order.getCustomerId() == null) {
            errors.add("Customer ID is required");
        }
        
        // Validate items
        if (order.getItems() == null || order.getItems().isEmpty()) {
            errors.add("Order must contain at least one item");
        } else {
            for (int i = 0; i < order.getItems().size(); i++) {
                OrderItem item = order.getItems().get(i);
                if (item.getProductId() == null) {
                    errors.add("Item " + (i + 1) + ": Product ID is required");
                }
                if (item.getQuantity() <= 0) {
                    errors.add("Item " + (i + 1) + ": Quantity must be positive");
                }
                if (item.getPrice() == null || item.getPrice().compareTo(BigDecimal.ZERO) <= 0) {
                    errors.add("Item " + (i + 1) + ": Valid price is required");
                }
            }
        }
        
        // Validate shipping address
        if (order.getShippingAddress() == null) {
            errors.add("Shipping address is required");
        } else {
            validateAddress(order.getShippingAddress(), "Shipping address", errors);
        }
        
        // Validate payment info
        if (order.getPayment() == null) {
            errors.add("Payment information is required");
        } else {
            validatePaymentInfo(order.getPayment(), errors);
        }
        
        if (!errors.isEmpty()) {
            throw new ValidationException("Order validation failed", errors);
        }
    }
    
    private List<InventoryReservation> reserveInventory(List<OrderItem> items) 
            throws InventoryException {
        
        List<InventoryReservation> reservations = new ArrayList<>();
        List<String> unavailableItems = new ArrayList<>();
        
        for (OrderItem item : items) {
            try {
                InventoryReservation reservation = inventoryService.reserveItem(
                    item.getProductId(), item.getQuantity());
                reservations.add(reservation);
                
            } catch (InsufficientInventoryException e) {
                unavailableItems.add(item.getProductId() + " (requested: " + 
                                   item.getQuantity() + ", available: " + e.getAvailableQuantity() + ")");
            }
        }
        
        if (!unavailableItems.isEmpty()) {
            // Release any successful reservations
            releaseInventoryReservations(reservations);
            
            throw new InventoryException(
                "The following items are not available: " + String.join(", ", unavailableItems));
        }
        
        return reservations;
    }
}

layout: default

Summary and Key Takeaways

What We Learned

🎯 throw keyword: Manually creating and throwing exceptions
📋 throws keyword: Declaring exceptions in method signatures
⚡ Exception chaining: Preserving original exception information
🔧 Re-throwing exceptions: Catching, processing, and re-throwing
🎭 Custom validation: Creating business rule validation
🏗️ Exception hierarchy: Designing meaningful exception types
🚀 Best practices: Proper exception declaration and handling

Key Differences: throw vs throws

`throw` Statement

Used inside methods to create and throw exceptions
Followed by exception object (new ExceptionType())
Causes immediate termination of method execution
Used for validation and error detection

`throws` Declaration

Used in method signatures to declare possible exceptions
Followed by exception class names (ExceptionType1, ExceptionType2)
Documents what exceptions method might throw
Required for checked exceptions (optional for unchecked)

Exception Handling Patterns

Input Validation: Use throw for parameter validation
State Validation: Use throw to check object state
Resource Management: Use throws to declare resource exceptions
Exception Conversion: Catch and re-throw with different types
Exception Chaining: Preserve original exception information
Graceful Degradation: Handle errors without crashing
Meaningful Messages: Provide clear error descriptions

Best Practices Recap

Exception Declaration

Be specific about exception types
Document exception conditions clearly
Use custom exception hierarchies
Group related exceptions appropriately

Exception Creation

Provide meaningful error messages
Include relevant context information
Use exception chaining when appropriate
Validate inputs before processing

Real-world Impact

Without Proper Exception Handling

Silent failures that are hard to debug
Generic error messages that don't help users
Lost context about what went wrong
Difficult troubleshooting and maintenance

With Proper Exception Handling

Clear error reporting and logging
Meaningful error messages for users
Detailed context for debugging
Robust, maintainable applications

layout: center class: text-center

Thank You!

Throw and Throws Keywords Complete

Lecture 28 Successfully Completed!
You can now create and declare exceptions effectively in Java

Ready to build robust exception handling!