Dependency Injection in Apex

PaymentInterface

public interface PaymentInterface{
   String processPayment();
}

PaypalPaymentService

public class PaypalPaymentService implements PaymentInterface{
   public String processPayment(){
        // 1. Connect to payment processor
        // 2. ???
        // 3. Profit!
        return 'Paypal successfully processed your payment!';
    }
}

ServiceFactory

public class ServiceFactory{
    public static PaymentInterface getPaymentInterface(){
        return (PaymentInterface)Type.forName(InheritenceSettings__c.getInstance().PaymentService__c).newInstance();
    }
}

StripePaymentService

public class StripePaymentService implements PaymentInterface{
    public String processPayment(){
        // 1. Connect to payment processor
        // 2. ???
        // 3. Profit!
        return 'Stripe successfully processed your payment!';
    }
}

This code is from my blog article Dependency Injection in Apex