$2.$ Open$/$Closed Principle $($OCP$)$:

Make the PaymentProcessor open for extension $($for handling different payment methods$)$ but closed for modification.

@startuml

interface PaymentValidator $\{$

$+$validate$($payment: Payment$)$: boolean

$\}$

interface PaymentPersister $\{$

$+$persist$($payment: Payment$)$: void

$\}$

interface PaymentMethodProcessor $\{$

$+$processPayment$($payment: Payment$)$: void

$\}$

class PaymentProcessor $\{$

$-$validator: PaymentValidator

$-$persister: PaymentPersister

$-$methodProcessor: PaymentMethodProcessor

$+$processPayment$($payment: Payment$)$: void

$\}$

class CreditCardProcessor implements PaymentMethodProcessor $\{+$processPayment$($payment: Payment$)$: void

$\}$

class PayPalProcessor implements PaymentMethodProcessor $\{+$processPayment$($payment: Payment$)$: void

$\}$

class Payment $\{$

$-$amount: double

$-$customerId: String

$-$paymentMethod: String

$\}$

PaymentProcessor $*-->$ PaymentValidator

PaymentProcessor $*-->$ PaymentPersister

PaymentProcessor $*-->$ PaymentMethodProcessor

@enduml

$$