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To implement the payment app, we can utilize the low level design principles we have learned so far. Let's start by creating a PaymentProcessor interface that defines the contract for processing payments.

1public interface PaymentProcessor {
2  void processPayment(Payment payment);
3}

The processPayment method accepts a Payment object as an argument and performs the necessary operations to ___ the payment.

Next, we can implement the PaymentProcessor interface for different payment methods. For example, let's create a CreditCardPaymentProcessor class and a PayPalPaymentProcessor class.

1class CreditCardPaymentProcessor implements PaymentProcessor {
2  public void processPayment(Payment payment) {
3    // Logic to process credit card payment
4    System.out.println("Processing credit card payment: " + payment.getAmount());
5  }
6}
7
8class PayPalPaymentProcessor implements PaymentProcessor {
9  public void processPayment(Payment payment) {
10    // Logic to process PayPal payment
11    System.out.println("Processing PayPal payment: " + payment.getAmount());
12  }
13}

Within the main method of our application, we can create an instance of the desired payment processor based on the selected payment method, and then process a payment by calling the processPayment method.

1String paymentMethod = "Credit Card";
2PaymentProcessor paymentProcessor = PaymentProcessorFactory.createPaymentProcessor(paymentMethod);
3
4Payment payment = new Payment();
5payment.setAmount(100);
6paymentProcessor.processPayment(payment);

This code snippet demonstrates how we can implement the payment app using low level design principles in Java. We create the necessary interfaces and classes to represent the payment processors, and then use the PaymentProcessorFactory to create an instance of the desired payment processor based on the selected payment method. Finally, we process a payment using the chosen payment processor.

Feel free to explore different payment methods and add more functionality to the payment app based on your requirements and low level design principles.