What Is a Factory in Java

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What Is a Factory
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"Factory" is one of the most overused and imprecise words in our industry Some use the term "Factory pattern" to refer to a Factory Method [DP], some use the term to refer to an Abstract Factory DP], some use the term to refer to both patterns, and some use the [ term to refer to any code that creates objects Our lack of a commonly understood definition of "Factory" limits our ability to know when a design could benefit from a Factory So I'll offer my definition, which is both broad and bounded: A class that implements one or more Creation Methods is a Factory This is true if the Creation Methods are static or nonstatic; if the return type of the Creation Methods is an interface, abstract class, or concrete class; or if the class that implements the Creation Methods also implements noncreational responsibilities A Factory Method [DP] is a nonstatic method that returns a base class or interface type and that is implemented in a hierarchy to enable polymorphic creation (see Introduce Polymorphic Creation with Factory Method, 88) A Factory Method must be defined/implemented by a class and one or more subclasses of the class The class and subclasses each act as Factories However, we don't say that a Factory Method is a Factory An Abstract Factory is "an interface for creating families of related or dependent objects without specifying their concrete classes" [DP, 87] Abstract Factories are designed to be substitutable at runtime, so a system may be configured to use a specific, concrete implementor of an Abstract Factory Every Abstract Factory is a Factory, though not every Factory is an Abstract Factory Classes that are Factories, not Abstract Factories, sometimes evolve into Abstract Factories when a need arises to support the creation of several families of related or dependent objects The next diagram, which uses bold lines to designate methods that create objects, illustrates common differences between sample Factory Method, Factory, and Abstract Factory structures
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I've seen numerous systems in which the Factory pattern was overused For example, if every object in a system is created by using a Factory, instead of direct instantiation (eg, new StringNode( )),the system probably has an overabundance of Factories Overusing this pattern often occurs when people always decouple client code from code that chooses between which classes to instantiate or how to instantiate them For example, the following createQuery() method makes a choice about which of two query classes to instantiate: public class Query public void createQuery() throws QueryException if (usingSDVersion52()) {
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query = new QuerySD52();
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} else {
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query = new QuerySD51();
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To eliminate the conditional logic in the above code, some would refactor it to use a QueryFactory: public class Query public void createQuery() throws QueryException
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query = queryFactorycreateQuery();
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QueryFactory now encapsulates the choice of what concrete query class to instantiate Yet doesQueryFactory improve the design of this code It certainly doesn't consolidate creation sprawl, and if it only decouples the Query class from the code that instantiates one of the two concrete queries, it most definitely does not add enough value to merit its existence This illustrates the point that it's best not to implement a Factory unless it really improves the design of your code or enables you to create/configure objects in a way that wasn't possible with direct instantiation
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Benefits and Liabilities
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+ + Consolidates creation logic and instantiation/configuration preferences Decouples a client from creation logic Complicates a design when direct instantiation would do
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Mechanics
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These mechanics assume your Factory will be implemented as a class, rather than as an interface implemented by a class If you need a Factory interface that is implemented by a class, you must make minor modifications to these mechanics
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1 An instantiator is a class that collaborates withother classes to instantiate a product (ie, an instance of some class) If the instantiator doesn't instantiate theproduct using a Creation Method, modify it and, if necessary, also modify the product's class, so the instantiation occurs through a Creation Method
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Compile and test 2 Create a new class that will become your factory Name your factory based on what it creates (eg,NodeFactory, LoanFactory)
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Compile 3 Apply Move Method [F] to move the Creation Method to the factory If the Creation Method is static, you can make it nonstatic after moving it to the factory
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Compile 4 Update the instantiator to instantiate the factory and call the factory to obtain an instance of the class
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Compile and test that the instantiator still functions correctly Repeat this step for any instantiators that could no longer compile because of changes made during step 3 5 Data and methods from the other classes are still being used in the instantiation Move whatever makes sense into the factory, so it handles as much of the creation work as possible This may involve moving where the factory gets instantiated and who instantiates it
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