20. జావా ఎక్సెప్షన్ హ్యాండ్లింగ్ & Custom Exceptions (Robust Error Handling Mastery)

public class CheckedVsUncheckedExceptionDemo {

    // Simulating a calculation that might fail with Unchecked ArithmeticException
    public static int divide(int a, int b) {
        return a / b;
    }

    // Simulating a method that throws Checked Exception
    public static void validateAgeForLicense(int age) throws Exception {
        if (age < 18) {
            throw new Exception("Age " + age + " is below legal driving limit of 18!");
        }
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("⚡ CHECKED VS UNCHECKED EXCEPTION MULTI-CATCH DEMO");
        System.out.println("=================================================
");

        // Test Case 1: Unchecked Arithmetic Exception
        try {
            System.out.println("1. Attempting division 100 / 0...");
            int result = divide(100, 0);
            System.out.println("   Result: " + result);
        } catch (ArithmeticException e) {
            System.out.println("   🛡️ CAUGHT UNCHECKED: Division by zero caught! " + e.getMessage());
        }

        // Test Case 2: Checked Exception with Validation
        System.out.println("
2. Attempting Checked Age Validation (16 yrs)...");
        try {
            validateAgeForLicense(16);
        } catch (Exception e) {
            System.out.println("   🛡️ CAUGHT CHECKED: " + e.getMessage());
        } finally {
            System.out.println("   🔒 Finally Block: Security audit check log written.");
        }

        System.out.println("
=================================================");
        System.out.println("✅ Application survived all exceptions without crashing!");
        System.out.println("=================================================");
    }
}

public class TryWithResourcesAutoCloseableDemo {

    // Custom Cloud DB Socket implementing AutoCloseable
    public static class CloudDatabaseSocket implements AutoCloseable {
        private final String connectionUrl;

        public CloudDatabaseSocket(String url) {
            this.connectionUrl = url;
            System.out.println("  🔌 [Socket Opened] Connected to " + connectionUrl);
        }

        public void executeQuery(String sql) {
            System.out.println("  📊 [Executing Query] " + sql);
            if (sql.contains("FAIL")) {
                throw new IllegalStateException("Database query hardware timeout occurred!");
            }
        }

        @Override
        public void close() {
            System.out.println("  🛑 [Socket Closed] Safely released connection to " + connectionUrl);
        }
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("🔄 TRY-WITH-RESOURCES AUTOCLOSEABLE LIFECYCLE");
        System.out.println("=================================================
");

        System.out.println("1. NORMAL EXECUTION (Auto-close after success):");
        try (CloudDatabaseSocket db = new CloudDatabaseSocket("jdbc:aurora://primary-cluster:5432")) {
            db.executeQuery("SELECT * FROM customer_accounts LIMIT 10;");
        } catch (Exception e) {
            System.out.println("   Error: " + e.getMessage());
        }

        System.out.println("
2. EXCEPTION EXECUTION (Auto-close STILL GUARANTEED):");
        try (CloudDatabaseSocket db = new CloudDatabaseSocket("jdbc:aurora://failover-cluster:5432")) {
            db.executeQuery("SELECT * FROM FAIL_TRIGGER_TABLE;");
        } catch (Exception e) {
            System.out.println("   🛡️ CAUGHT EXCEPTION: " + e.getMessage());
        }

        System.out.println("
=================================================");
        System.out.println("✅ Zero Memory / Socket leaks achieved via AutoCloseable!");
        System.out.println("=================================================");
    }
}

public class ResilientPaymentGatewayTransactionEngine {

    // Domain Custom Exception 1: Insufficient Balance
    public static class InsufficientFundsException extends Exception {
        private final double shortfall;
        public InsufficientFundsException(double available, double required) {
            super(String.format("Insufficient balance: Available ₹%,.2f, Required ₹%,.2f", available, required));
            this.shortfall = required - available;
        }
        public double getShortfall() { return shortfall; }
    }

    // Domain Custom Exception 2: Fraud Alert (Unchecked Runtime)
    public static class FraudDetectedException extends RuntimeException {
        public FraudDetectedException(String message) {
            super(message);
        }
    }

    // Account Service with Transaction Simulation
    public static class BankPaymentProcessor {
        private double accountBalance = 50000.0;

        public void executeTransfer(String targetVpa, double amount) throws InsufficientFundsException {
            System.out.printf("💸 Initiating Transfer of ₹%,.2f to [%s]...%n", amount, targetVpa);

            // Fraud Check
            if (targetVpa.contains("blacklisted_hacker")) {
                throw new FraudDetectedException("🚨 Transaction blocked by AI Anti-Fraud Radar for recipient: " + targetVpa);
            }

            // Balance Check
            if (amount > accountBalance) {
                throw new InsufficientFundsException(accountBalance, amount);
            }

            // Deduct
            accountBalance -= amount;
            System.out.printf("✅ Transfer Successful! Remaining Account Balance: ₹%,.2f%n", accountBalance);
        }
    }

    public static void main(String[] args) {
        System.out.println("=================================================");
        System.out.println("💳 RESILIENT FINTECH TRANSACTION & ROLLBACK ENGINE");
        System.out.println("=================================================
");

        BankPaymentProcessor processor = new BankPaymentProcessor();

        // Transaction 1: Successful Transfer
        try {
            processor.executeTransfer("balaji@okhdfc", 15000.0);
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }

        // Transaction 2: Insufficient Funds (Checked Exception Handled)
        System.out.println("
-------------------------------------------------");
        try {
            processor.executeTransfer("vendor@icici", 45000.0); // Only 35000 left
        } catch (InsufficientFundsException e) {
            System.out.println("❌ PAYMENT REJECTED: " + e.getMessage());
            System.out.printf("   💡 Suggested Action: Please add ₹%,.2f to complete payment.%n", e.getShortfall());
        }

        // Transaction 3: Fraud Radar Block (Unchecked Exception Handled)
        System.out.println("
-------------------------------------------------");
        try {
            processor.executeTransfer("blacklisted_hacker@fraud", 500.0);
        } catch (FraudDetectedException e) {
            System.out.println("🛑 SECURITY LOCKDOWN: " + e.getMessage());
            System.out.println("   🔒 Compliance Alert: Event dispatched to Cyber Cell Incident Desk.");
        } catch (InsufficientFundsException e) {
            System.out.println("Error: " + e.getMessage());
        }

        System.out.println("
=================================================");
        System.out.println("🎉 PART 04 (CORE OOP & EXCEPTIONS) COMPLETED 100%!");
        System.out.println("=================================================");
    }
}