📄 disc3-1.htm
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<HTML><HEAD> <TITLE>Discussion of Creational Patterns</TITLE>
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<A NAME="creational_discussion"></A>
<A NAME="disc3-1"></A>
<P>There are two common ways to parameterize a system by the classes of
objects it creates. One way is to subclass the class that creates the
objects; this corresponds to using the
<A HREF="pat3cfs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/pat3cfs.htm" TARGET="_mainDisplayFrame">Factory Method (107)</A> pattern. The main drawback of this
approach is that it can require creating a new subclass just to change
the class of the product. Such changes can cascade. For example,
when the product creator is itself created by a factory method, then
you have to override its creator as well.</P>
<A NAME="disc3-2"></A>
<A NAME="protocol-vs-abstrfact"></A>
<A NAME="build-vs-abstfact"></A>
<P>The other way to parameterize a system relies more on object
composition: Define an object that's responsible for knowing the class
of the product objects, and make it a parameter of the system. This
is a key aspect of the
<A HREF="pat3afs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/pat3afs.htm" TARGET="_mainDisplayFrame">Abstract Factory (87)</A>,
<A HREF="pat3bfs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/pat3bfs.htm" TARGET="_mainDisplayFrame">Builder (97)</A>, and
<A HREF="pat3dfs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/pat3dfs.htm" TARGET="_mainDisplayFrame">Prototype (117)</A>
patterns. All three involve creating a new "factory object" whose
responsibility is to create product objects. Abstract Factory has the
factory object producing objects of several classes. Builder has the
factory object building a complex product incrementally using a
correspondingly complex protocol. Prototype has the factory object
building a product by copying a prototype object. In this case, the
factory object and the prototype are the same object, because the
prototype is responsible for returning the product.</P>
<A NAME="disc3-3"></A>
<P>Consider the drawing editor framework described in the Prototype
pattern. There are several ways to parameterize a GraphicTool by the
class of product:</P>
<UL>
<A NAME="disc3-4"></A>
<LI>By applying the Factory Method pattern, a subclass of GraphicTool will be
created for each subclass of Graphic in the palette. GraphicTool will
have a NewGraphic operation that each GraphicTool subclass will
redefine.</LI>
<P></P>
<A NAME="disc3-5"></A>
<LI>By applying the Abstract Factory pattern, there will be a class hierarchy
of GraphicsFactories, one for each Graphic subclass. Each factory
creates just one product in this case: CircleFactory will create
Circles, LineFactory will create Lines, and so on. A GraphicTool will
be parameterized with a factory for creating the appropriate kind of
Graphics.</LI>
<P></P>
<A NAME="disc3-6"></A>
<LI>By applying the Prototype pattern, each subclass of Graphics will
implement the Clone operation, and a GraphicTool will be parameterized
with a prototype of the Graphic it creates.</LI>
</UL>
<A NAME="disc3-7"></A>
<P>Which pattern is best depends on many factors. In our drawing editor
framework, the Factory Method pattern is easiest to use at first.
It's easy to define a new subclass of GraphicTool, and the instances
of GraphicTool are created only when the palette is defined. The main
disadvantage here is that GraphicTool subclasses proliferate, and none
of them does very much.</P>
<A NAME="disc3-8"></A>
<P>Abstract Factory doesn't offer much of an improvement, because it
requires an equally large GraphicsFactory class hierarchy. Abstract
Factory would be preferable to Factory Method only if there were
already a GraphicsFactory class hierarchy—either because the
compiler provides it automatically (as in Smalltalk or Objective C) or
because it's needed in another part of the system.</P>
<A NAME="disc3-9"></A>
<P>Overall, the Prototype pattern is probably the best for the drawing
editor framework, because it only requires implementing a Clone
operation on each Graphics class. That reduces the number of classes,
and Clone can be used for purposes other than pure instantiation (e.g.,
a Duplicate menu operation).</P>
<A NAME="disc3-10"></A>
<P>Factory Method makes a design more customizable and only a little more
complicated. Other design patterns require new classes, whereas
Factory Method only requires a new operation. People often use
Factory Method as the standard way to create objects, but it isn't
necessary when the class that's instantiated never changes or when
instantiation takes place in an operation that subclasses can easily
override, such as an initialization operation.</P>
<A NAME="disc3-11"></A>
<P>Designs that use Abstract Factory, Prototype, or Builder are even more
flexible than those that use Factory Method, but they're also more
complex. Often, designs start out using Factory Method and evolve
toward the other creational patterns as the designer discovers where
more flexibility is needed. Knowing many design patterns gives you
more choices when trading off one design criterion against another.</P>
<A NAME="disc3-12"></A>
<A NAME="last"></A>
<P><A HREF="#top"><IMG SRC="up3-1.gif" tppabs="http://ultra/development/DesignPatterns/lowres/gifsb/up3.gif" BORDER=0></A><BR>
<A HREF="chap4fs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/chap4fs.htm" TARGET="_mainDisplayFrame"><IMG SRC="rightar3-1.gif" tppabs="http://ultra/development/DesignPatterns/lowres/gifsb/rightar3.gif"
ALIGN=TOP BORDER=0></A> <A HREF="chap4fs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/chap4fs.htm"
TARGET="_mainDisplayFrame">Structural Patterns</A><BR>
<A HREF="pat3efs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/pat3efs.htm" TARGET="_mainDisplayFrame"><IMG SRC="leftarr3-1.gif" tppabs="http://ultra/development/DesignPatterns/lowres/gifsb/leftarr3.gif"
ALIGN=TOP BORDER=0></A> <A HREF="pat3efs-1.htm" tppabs="http://ultra/development/DesignPatterns/lowres/pat3efs.htm"
TARGET="_mainDisplayFrame">Singleton</A>
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