Object-Oriented Programming: The 4 Pillars & Architecture

/**
 * DevAscent Academy — Unit 6 Capstone Project
 * Demonstrates Encapsulation, Inheritance, Polymorphism, Abstraction, and Interfaces.
 */

// 1. Interface: Behavioral Contract
interface Auditable {
    void logAuditTrail(String auditId);
}

// 2. Abstract Superclass: Domain Foundation
abstract class PaymentTransaction implements Auditable {
    private final String transactionId; // Encapsulated immutable state
    private double amount;

    // Superclass Constructor
    public PaymentTransaction(String transactionId, double amount) {
        this.transactionId = transactionId;
        this.setAmount(amount);
    }

    // Encapsulated Getter & Setter with business validation
    public double getAmount() { return this.amount; }
    public void setAmount(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("Amount must be positive.");
        this.amount = amount;
    }

    public String getTransactionId() { return this.transactionId; }

    // Abstract Method: Enforces dynamic polymorphic implementation
    public abstract void processPayment();

    // Concrete Implementation of Interface Method
    @Override
    public void logAuditTrail(String auditId) {
        System.out.printf("  [AUDIT-%s] Txn %s verified on compliance ledger.%n", 
                auditId, this.transactionId);
    }
}

// 3. Concrete Subclass A: Credit Card Payment
class CreditCardPayment extends PaymentTransaction {
    private final String cardLast4;

    public CreditCardPayment(String txnId, double amount, String cardLast4) {
        super(txnId, amount); // Calling Superclass Constructor
        this.cardLast4 = cardLast4;
    }

    @Override
    public void processPayment() {
        System.out.printf("Charging Credit Card (**** **** **** %s) | Amount: $%.2f [SUCCESS]%n",
                this.cardLast4, this.getAmount());
    }
}

// 4. Concrete Subclass B: Crypto Wallet Payment
class CryptoPayment extends PaymentTransaction {
    private final String walletAddress;

    public CryptoPayment(String txnId, double amount, String walletAddress) {
        super(txnId, amount);
        this.walletAddress = walletAddress;
    }

    @Override
    public void processPayment() {
        System.out.printf("Broadcasting to Blockchain Wallet (%s) | Amount: $%.2f [CONFIRMED]%n",
                this.walletAddress, this.getAmount());
    }
}

// 5. Execution Controller
public class PaymentArchitectureDemo {
    public static void main(String[] args) {
        System.out.println("==================================================");
        System.out.println("   DEVASCENT ENTERPRISE PAYMENT DISPATCH ENGINE   ");
        System.out.println("==================================================");

        // Polymorphic Array: Superclass reference array holding derived objects
        PaymentTransaction[] paymentQueue = {
            new CreditCardPayment("TXN-801", 249.99, "4482"),
            new CryptoPayment("TXN-802", 1200.00, "0x7A1F...99B"),
            new CreditCardPayment("TXN-803", 34.50, "1109")
        };

        // Dynamic Method Dispatch: JVM resolves correct subclass implementation at runtime!
        for (PaymentTransaction txn : paymentQueue) {
            System.out.println("Initiating Txn ID: " + txn.getTransactionId());
            txn.processPayment();  // Polymorphic dispatch
            txn.logAuditTrail("SEC99");
            System.out.println("--------------------------------------------------");
        }
    }
}