How to use a factory to create objects which use S

Question

Let's assume we have a simple payment feature on an online shop. We want to manage different transactions with different processors of transactions:

• A transaction can be a payment or a refund.
• A processor of transactions can be Paypal or Payplug.

So we have the following classes:

class PaymentTransaction implements Transaction {

}

class RefundTransaction implements Transaction {

}

class PaypalProcessor implements Processor {

}

class PayplugProcessor implements Processor {

}

As suggested in this answer, we could use the following class which uses Strategy and polymorphism.

class PaymentProcessor {
     private Processor processor;
     private Transaction transaction;

     public PaymentProcessor(Processor processor, Transaction transaction) {
          this.processor = processor;
          this.transaction = transaction;
     }

     public void processPayment() {
         processor.process(transaction);
     }
}

We assume the processor and the transaction to use are given from the database. I wonder how to create the PaymentProcessor object.

It seems that an abstract factory class with only one method is still a valid Abstract Factory pattern. So, in this case I wonder if using Abstract Factory would be relevant.

• If yes, how to implement it?
• If no, should we use Factory Method pattern with a PaymentProcessorFactory class to create PaymentProcessor with his two attributes according the details given from the database?

What is a best practice to use a factory in this case?

Answer 1

Maybe you can use the builder pattern. In the builder pattern there is a class called the director, which knows the algorithm of creating a complex object. To create the components the complex object is build of the director uses a builder. Like this you can change specific components to build up the whole complex object.

In your case the PaymentProcessor (the complex object) is composed out of a Payment and a Processor, so the algorithm is to inject them into a PaymentProcessor. The builder should build the parts. To build a paypal-refund combination you should create a builder which returns a PaypalProcessor and a RefundTransaction. When you want to create a payplug-payment the builder should return a PaymentTransaction and a PayPlugProcessor.

public interface PaymentProcessorBuilder {
    public Transaction buildTransaction();
    public Processor buildProcessor();
}

public class PaypalRefundProcessorBuilder implements PaymentProcessorBuilder {
    public Transaction buildTransaction {
        return new RefundTransaction();
    }
    public Processor buildProcessor {
        return new PayPalProcessor();
    }
}

public class PayPlugPaymentProcessorBuilder implements PaymentProcessorBuilder {
    public Transaction buildTransaction {
        return PaymentTransaction();
    }
    public Processor buildProcessor {
        return new PayPlugProcessor();
    }
}

Now the Director can use the builder to compose the PaymentProcessor:

publi PaymentProcessorDirector {
    public PaymentProcessor createPaymentProcessor(PaymentProcessorBuilder builder) {
        PaymentProcessor paymentProcessor = new PaymentProcessor();

        paymentProcessor.setTransaction(builder.buildTransaction());
        paymentProcessor.setProcessor(builder.buildProcessor());

        return paymentProcessor;
    }
}

The created PaymentProcessor depends now on the passed Builder:

...
PaymentProcessorDirector director = new PaymentProcessorDirector();
PaymentProcessorBuilder builder = new PaypalRefundProcessorBuilder();

PaymentProcessor paymentProcessor = director.createPaymentProcessor(builder);
...

For each combination you can create a builder. If you pass the right builder to the director you get the wanted PaymentProcessor back.

Now the question how could you get the right builder. Therefore you can use a factory, that takes some event arguments and decides then which builder has to be made. This builder you pass in the director an get the wanted PaymentProcessor.

CAUTION: This is only one possible solution for this problem. Every solution has is advantages and disadvantages. To find the right solution you to balance the good and the bad things.

PS: Hope the syntax is correct. Im not a java developer.

EDIT: You could interprete the director of the builder pattern as a PaymentProcessorFactory with the builder itself as strategy for building the parts of the PaymentProcessor

Answer 2

We assume the processor and the transaction to use are given from the database. I wonder how to create the PaymentProcessor object.

I would define an interface that I can adapt to the database result or any other source that can provide the data needed to create a PaymentProcessor. This is also useful for unittests.

public interface PaymentProcessorFactoryArgs {
  String getProcessorType();
  String getTransactionType();
}

and then implement a factory like this.

public class PaymentProcessorFactory {

  private Map<String, Processor> processorMap = new HashMap<>();
  private Map<String, Transaction> transactionMap = new HashMap<>();

  public PaymentProcessorFactory() {
    processorMap.put("paypal", new PaypalProcessor());
    processorMap.put("payplug", new PayplugProcessor());

    transactionMap.put("refund", new RefundTransaction());
    transactionMap.put("payment", new PaymentTransaction());
  }

  public PaymentProcessor create(PaymentProcessorFactoryArgs factoryArgs) {
    String processorType = factoryArgs.getProcessorType();
    Processor processor = processorMap.get(processorType);
    if(processor == null){
      throw new IllegalArgumentException("Unknown processor type " + processorType);
    }

    String transactionType = factoryArgs.getTransactionType();
    Transaction transaction = transactionMap.get(transactionType);
    if(transaction == null){
      throw new IllegalArgumentException("Unknown transaction type " + processorType);
    }

    return new PaymentProcessor(processor, transaction);
  }
}

This is just a quick example. It would be better if you can register Processors and Transactions. E.g.

public void register(String processorType, Processor processor){
   ...
}
public void register(String transactionType, Transaction transaction){
   ...
}

You also might want to use anther type then String for the keys, maybe an enum.

In this example the Processor and Transaction objects are re-used every time a PaymentProcessor is created. If you want to create new objects for each PaymentProcessor, you can replace the Maps type

private Map<String, Factory<Processor>> processorMap = new HashMap<>();
private Map<String, Factory<Transaction>> transactionMap = new HashMap<>();

with anther factory interface. E.g.

public interface Factory<T> {
  public T newInstance();
}