Abstraction

// ========== Abstract Class ==========
public abstract class PaymentProcessor {
    protected String merchantId;
    protected double transactionFee;

    public PaymentProcessor(String merchantId) {
        this.merchantId = merchantId;
    }

    // Abstract method - WHAT to do (no HOW)
    public abstract boolean processPayment(double amount);

    // Abstract method
    public abstract void refund(String transactionId);

    // Concrete method - shared implementation
    public double calculateFee(double amount) {
        return amount * transactionFee;
    }

    // Template method
    public final void executeTransaction(double amount) {
        System.out.println("Starting transaction...");
        validateMerchant();
        if (processPayment(amount)) {
            System.out.println("Payment successful!");
            logTransaction(amount);
        } else {
            System.out.println("Payment failed!");
        }
    }

    private void validateMerchant() {
        System.out.println("Validating merchant: " + merchantId);
    }

    private void logTransaction(double amount) {
        System.out.println("Logged transaction of $" + amount);
    }
}

// ========== Concrete Implementation 1 ==========
public class CreditCardProcessor extends PaymentProcessor {
    private String apiKey;

    public CreditCardProcessor(String merchantId, String apiKey) {
        super(merchantId);
        this.apiKey = apiKey;
        this.transactionFee = 0.029; // 2.9%
    }

    @Override
    public boolean processPayment(double amount) {
        System.out.println("Processing credit card payment of $" + amount);
        // Complex credit card logic hidden here
        // - Connect to payment gateway
        // - Encrypt card data
        // - Handle 3D Secure
        // - Process with bank
        return true;
    }

    @Override
    public void refund(String transactionId) {
        System.out.println("Refunding credit card transaction: " + transactionId);
    }
}

// ========== Concrete Implementation 2 ==========
public class UPIProcessor extends PaymentProcessor {
    public UPIProcessor(String merchantId) {
        super(merchantId);
        this.transactionFee = 0.0; // No fee for UPI
    }

    @Override
    public boolean processPayment(double amount) {
        System.out.println("Processing UPI payment of Rs." + amount);
        // Complex UPI logic hidden here
        // - Generate QR code or collect VPA
        // - Send request to NPCI
        // - Wait for user approval
        return true;
    }

    @Override
    public void refund(String transactionId) {
        System.out.println("Initiating UPI refund: " + transactionId);
    }
}

// ========== Usage ==========
public class Main {
    public static void main(String[] args) {
        // User does not need to know HOW payment works
        PaymentProcessor processor = new CreditCardProcessor("MERCH123", "api_key");
        processor.executeTransaction(100.00);

        System.out.println();

        processor = new UPIProcessor("MERCH123");
        processor.executeTransaction(500.00);
    }
}