Chapter 17 Abstract Factory Summary prepared by Kirk Scott

Design Patterns in Java Chapter 17 Abstract Factory Summary prepared by Kirk Scott

The Introduction Before the Introduction The book provides several examples of abstract factory with context This trimmed set of overheads restricts its attention to one of those examples

The Book’s Definition of the Pattern Book definition: The intent of the Abstract Factory, or Kit, is to allow creation of families of related or dependent objects.

Comment mode on: Even though the pattern name includes the word abstract, the book’s examples don’t include an abstract class Instead, the examples show a concrete superclass and a subclass of it It would be possible to do the same thing with an abstract class and two concrete subclasses

Factory Method and Abstract Factory A very large scale way of seeing what’s going on is this: With a factory method, you wanted something that implemented an interface. A number of classes satisfied this requirement, and the logic for deciding what to give the client was in the service code.

With abstract factory, there is a hierarchy of classes containing sets of overridden methods that return various objects Each of the methods is basically a factory method Each method will return an object that meets the requirement of the client.

The client may request which member of the family of classes it would like to make use of With this choice, a set of methods becomes a available Which kind of object comes back from a method call depends on the implementation of the method in the class that was chosen

Builder, Factory Method, and Abstract Factory Abstract factory falls in a spectrum with the builder and factory method patterns. With builder, a method call returned one kind of desired, specified object. With factory method, a method call returned a reference to one of a set of kinds of objects back, with no control over which one.

With abstract factory it is possible to specify which one of a set of methods for generating objects is desired. Those methods may be simple methods where the return type is a simple class reference. Those methods may also be factory methods which return objects which agree with a required supertype, but which may actually be instances of a subtype.

Developing an Abstract Factory Example from the Credit Check Factory Method Example The example is built on the factory method example of the previous unit Here is a thumbnail review of the factory method example The CreditCheckFactory has a method createCreditCheck() that returns a reference to an object that implements the CreditCheck interface Two classes, CreditCheckOnline and CreditCheckOffline implement the interface

The code of the createCreditCheck() method knows which kind of object to create based on “server” code conditions Client code will receive a reference to one of the two different kinds of objects Client code doesn’t have to to specify or care which kind of object comes back The UML diagram for this is given on the next overhead

Expanding the Example to the Abstract Factory Pattern You may realize that you would like to apply the abstract factory pattern when several related factory methods arise in an application design The idea is that there is a group of related objects that need to be created and you put the methods for creating them together in a common class

The book now adds things called billing checks and shipping checks to the credit check example It doesn’t really explain what these things are, but it’s apparent that as checks, they are classified with credit checks

An intermediate UML diagram is given on the next overhead It introduces the idea that you can start putting related things together in packages The diagram is intermediate because it doesn’t have enough information in it to show how it illustrates the abstract factory design pattern Specific comments on that will follow the diagram

In the preceding diagram there were two new classes, ShippingCheck and BillingCheck Although they are checks of some sort, they don’t seem to be new types of credit checks They don’t implement the CreditCheck interface

Not surprisingly then, the CreditCheckFactory doesn’t have a method which constructs objects of these two new classes So the question remains, what are they exactly, and what role will they play in the abstract factory design pattern?

Another UML diagram is given on the following overhead This is the book’s next step in explaining the design pattern—but it still doesn’t give the whole picture In this diagram, the checks have become interfaces Also, the CreditCheckFactory includes methods that return values that are of the types billing check, shipping check, and credit check

Review of the Contents of the Diagram The preceding overhead was a UML diagram of the com.oozinoz.credit package, which contained these elements: A CreditCheck interface A CreditCheckOffline class that implements the interface Two more interfaces, BillingCheck and ShippingCheck A CreditCheckFactory class containing methods to create instances of each kind of check

Questions about the Design so Far The diagram is clearly still not complete. Only the CreditCheck interface has an implementing class. There are no classes that implement the BillingCheck or ShippingCheck interfaces The create() methods in the CreditCheckFactory have no classes which they could construct instances of

Also, there is just the one CreditCheckFactory If there are to be families of methods, then there should be more than one factory class It also turns out that the CreditCheckFactory is now misnamed The factory class has methods for all 3 kinds of checks, not just credit checks

The issue also remains whether the CreditCheckFactory is a plain concrete class with subclasses Or would it be better to make it an interface or an abstract class

Bringing the Example to Completion—Adding Packages The book brings the example to completion in more or less the following way: There will be no direct instances of the CreditCheckFactory class in the credit package There will be other packages, one for each country, such as the U.S. or Canada

In these packages there will be classes that implement the interfaces in the credit package There will also be a factory class which extends the CreditCheckFactory class in the credit package The UML diagram on the following overhead illustrates the relationship between the credit package and the Canada package

The Canada package contains live classes BillingCheckCanada, ShippingCheckCanada, and CreditCheckCanadaOnline which implement the check interfaces The package contains the CheckFactoryCanada class which extends the CreditCheckFactory class The contents of the CheckFactoryCanada class aren’t shown in detail in this diagram However, it will either inherit or override the methods which return either a billing check, a shipping check, or a credit check

Comment mode on: I think the example would be better if the CreditCheckFactory class were abstract I also think it would be better if that class were renamed simply CheckFactory Billing and shipping checks don’t seem to be kinds of credit checks, but methods to create them are included in the factory

CreditCheckFactory only has the name it does because it was named before the example was expanded Notice that the subclass is named CheckFactoryCanada, not CreditCheckFactoryCanada In other words, implicitly the authors recognize that at the implementation level this is a check factory, not a credit check factory

Also, if you read the text closely, you’ll discover that at one point the authors refer to the CheckFactoryCanada class as a concrete factory, not an abstract factory This suggests that they recognize that the plain CreditCheckFactory doesn’t have to be a concrete class

Continuing to Outline the Example No separate package is given for the U.S., but conceptually the example is set up as if there would also be a concrete package for the U.S., analogous to the Canada package There could also be separate packages for other countries, like Mexico, etc.

The original diagram is shown again on the next overhead. Take a moment to consider what it would look like if a package for the U.S. were added to it

If set up as I would set it up, the plain credit package would contain an abstract CheckFactory class and the packages for specific countries would each contain a concrete check factory class that extended it

There is a reason that the CreditCheckOffline class in the credit package is already a concrete class that implements the CreditCheck interface The CreditCheckOffline is going to be the same for both Canada and the U.S. and can be shared by the code for both That can be taken care of with one implementing class in the parent package The original factory method pattern is still at work because there are two kinds of credit check, online and offline

The abstract factory pattern is like a collection of factory methods It is after the U.S. package is added to the picture that this becomes apparent createBillingCheck() illustrates the idea There are two kinds of billing check, Canadian and American There are different classes for each, but they both implement the common BillingCheck interface

createBillingCheck() in the Canada package implementation will return something of type BillingCheck There would also be a createBillingCheck() method in the U.S. package It would also return an object of type BillingCheck This creation method along with the others together form a family of methods for each country

There could be an additional layer of code on the server side of the example That layer would contain if/else logic to determine whether a customer that a billing check was being run on was Canadian or American

In other words, there might be a method whatKindOfCustomer() in this example That would be analogous to isAgencyUp() in the factory method example of the previous chapter The actual kind of billing check object returned to a client would depend on the outcome of a call to whatKindOfCustomer()

The ultimate client code would simply be handed a reference to an object that implemented the BillingCheck interface The client wouldn’t have to be aware or specifically request a billing check for one country or the other Such a solution would make these abstract factory methods quite analogous to plain old factory methods, with the decision about what to return embedded on the server side.

However, the application overall could also be structured more like the GUI example On the client side would be handed a reference to a customer of a certain nationality, or would itself create one Then what kind of billing check came back would simply be determined by polymorphism when createBillingCheck() was called The server side wouldn’t explicitly be in charge of determining exactly what was returned

The Implementation of the CheckFactoryCanada Class I’m skipping this and everything else in the book up until its comments on packages. Enough is enough…

Packages and Abstract Factories The book now returns to the question of packages What more they actually have to say about packages is contained in the next challenge

Challenge 17.5 Write down an argument supporting the decision to place each factory and its related classes in a separate package. Or, argue that another approach is superior.

Solution 17.5 An example justification is: Placing country-specific classes in separate packages helps our Oozinoz developers to organize our software and our development efforts. By placing the classes for each country in a separate package, we keep country-specific packages independent of one another.

We can be confident, for example, that U. S We can be confident, for example, that U.S.-specific classes have no impact on Canada-specific classes. We can also easily add support for new countries. For example, when we start doing business with Mexico, we can create a new package that provides the check services we need in a way that makes sense in that country.

This has the further advantage of letting us assign the credit This has the further advantage of letting us assign the credit.mx package to a developer who has expertise in working with services and data from Mexico.

An argument against: Although this separation is nice in theory, it’s overwrought in practice. I’d rather have only one package with all the classes in it, at least until we expand to nine or ten countries. Spreading these classes over multiple packages winds up causing me three or more times the configuration-management work when I need to implement a change that cuts across all these packages.

Comment mode on: Eventually, as software grows, you may start to use packages If so, you’ll have to decide how to divide them up. When the time comes, you’ll have to make your own decision about the right approach

The book’s argument “against” summarizes my feelings about packages at this stage Up through CSCE 222 and CSCE 302 there are no programming projects so complex that using packages is justified Trying to use them just introduces an artificial level of complexity Why, for example, Eclipse generates packages for a simple program consisting of a couple of classes is beyond me…

UML for the Pattern Although not really complete, the book’s last diagram is repeated on the following overhead. What’s missing is some indication that the factory makes use of the check classes What’s also missing is another set of classes representing another family, for example, a set of classes for the United States, Mexico, etc.

The End