Understanding Abstract Factory pattern

问题

I've read about abstract factory patter on wiki. But I don't understand really profit by using this pattern. Can you get an example in which is hard to avoid abstract factory pattern. Consider the following Java code:

public abstract class FinancialToolsFactory {
    public abstract TaxProcessor createTaxProcessor();
    public abstract ShipFeeProcessor createShipFeeProcessor();
}

public abstract class ShipFeeProcessor {
    abstract void calculateShipFee(Order order);
}

public abstract class TaxProcessor {
    abstract void calculateTaxes(Order order);
}

// Factories
public class CanadaFinancialToolsFactory extends FinancialToolsFactory {
    public TaxProcessor createTaxProcessor() {
        return new CanadaTaxProcessor();
    }
    public ShipFeeProcessor createShipFeeProcessor() {
        return new CanadaShipFeeProcessor();
    }
}

public class EuropeFinancialToolsFactory extends FinancialToolsFactory {
   public TaxProcessor createTaxProcessor() {
        return new EuropeTaxProcessor();
    }
    public ShipFeeProcessor createShipFeeProcessor() {
        return new EuropeShipFeeProcessor();
    }
}

// Products
public class EuropeShipFeeProcessor extends ShipFeeProcessor {
    public void calculateShipFee(Order order) {
        // insert here Europe specific ship fee calculation
    }
}   

public class CanadaShipFeeProcessor extends ShipFeeProcessor {
    public void calculateShipFee(Order order) {
        // insert here Canada specific ship fee calculation
    }
}

public class EuropeTaxProcessor extends TaxProcessor {
    public void calculateTaxes(Order order) {
        // insert here Europe specific tax calculation
    }
}

public class CanadaTaxProcessor extends TaxProcessor {
    public void calculateTaxes(Order order) {
        // insert here Canada specific tax calculation
    }
}

If we need to just create objects in a code below 1-2 times in a code then we can use just new operator. And why we need in abstract factory?

回答1:

You are missing half of the work :)

void processOrder(FinancialToolsFactory ftf,Order o) {

  tft.createTaxProcessor().calculateTaxes(o);
  tft.createShipFeeProcessor().calculateShipFee(o);
}

this code works as well as you pass a canadian or european implementation of FinancialToolsFactory (you can externalize the implementor class to external resource and instantiate with a Class.newInstance(), for example).

In this case one of the real benefits of pattern usage is not writing the code that implements the pattern, but who use that code!

PS: My answer is intentionally incomplete and try to answer just this specific question; a discussion about pattern and their benefits is too big!

回答2:

You'd take advantage of this pattern if you were to support different implementations transparently. By delegating the decision of which implementation to use to the factory, you have a single point in your code where that decision is made (a.k.a. single responsibility).

The abstract factory pattern takes this concept beyond by aggregating related factories, such as different financial tools factories in your sample.

Now, if you only instantiate your financial tools once or twice in your code, using factories is over-engineering. The gain comes when you need to instantiate different implementations of the same interfaces in different places many times and you want to be able to work without worrying about which implementation you are using or how that decision is made.

回答3:

There are quite some resources about this pattern on the web, and it's hard to guess what might be the best way of explaining its purpose in a way that sounds "plausible" for you.

But I think that the key point is:

With this pattern, someone who wants to create an instance of a particular implementation of an interface does not need to know what this particular implementation is. The call to the new operator is hidden inside the factory, and the user of the factory does not need to know the concrete class.

This makes it easier to switch the implementation later: You don't have to find and adjust all places where new ConcreteClass() was called and change it to new OtherConcreteClass() in order to use a different implementation. You just pass a different factory around, and everybody who uses this factory automatically creates instances of OtherConcreteClass (without even knowing that he does so...)

来源：https://stackoverflow.com/questions/21706319/understanding-abstract-factory-pattern

标签

java

design-patterns

factory-pattern