Factory Pattern When to use factory methods

Question

When is it a good idea to use factory methods within an object instead of a Factory class

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They re also useful when you need several  constructors  with the same parameter type but with different behavior

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UML from     Product  It defines an interface of the objects the Factory method creates   ConcreteProduct  Implements Product interface  Creator  Declares the Factory method  ConcreateCreator   Implements the Factory method to return an instance of a ConcreteProduct  Problem statement  Create a Factory of Games by using Factory Methods  which defines the game interface   Code snippet   import java util HashMap       Product interface as per UML diagram    interface Game         createGame is a complex method  which executes a sequence of game steps        public void createGame          ConcreteProduct implementation as per UML diagram    class Chess implements Game      public Chess               public void createGame            System out println  ---------------------------------------            System out println  Create Chess game            System out println  Opponents 2            System out println  Define 64 blocks            System out println  Place 16 pieces for White opponent            System out println  Place 16 pieces for Black opponent            System out println  Start Chess game            System out println  ---------------------------------------            class Checkers implements Game      public Checkers               public void createGame            System out println  ---------------------------------------            System out println  Create Checkers game            System out println  Opponents 2 or 3 or 4 or 6            System out println  For each opponent  place 10 coins            System out println  Start Checkers game            System out println  ---------------------------------------            class Ludo implements Game      public Ludo               public void createGame            System out println  ---------------------------------------            System out println  Create Ludo game            System out println  Opponents 2 or 3 or 4            System out println  For each opponent  place 4 coins            System out println  Create two dices with numbers from 1-6            System out println  Start Ludo game            System out println  ---------------------------------------                Creator interface as per UML diagram    interface IGameFactory       public Game getGame String gameName         ConcreteCreator implementation as per UML diagram    class GameFactory implements IGameFactory         HashMap lt String Game gt  games   new HashMap lt String Game gt                      Since Game Creation is complex process  we don t want to create game using new operator every time          Instead we create Game only once and store it in Factory  When client request a specific game           Game object is returned from Factory instead of creating new Game on the fly  which is time consuming             public GameFactory             games put Chess class getName   new Chess             games put Checkers class getName   new Checkers             games put Ludo class getName   new Ludo                       public Game getGame String gameName           return games get gameName            public class NonStaticFactoryDemo      public static void main String args             if   args length  lt  1               System out println  Usage  java FactoryDemo gameName                return                     GameFactory factory   new GameFactory            Game game   factory getGame args 0            if   game    null                                    game createGame                System out println  Game   game getClass   getName              else              System out println args 0      Game does not exists in factory                                   output   java NonStaticFactoryDemo Chess --------------------------------------- Create Chess game Opponents 2 Define 64 blocks Place 16 pieces for White opponent Place 16 pieces for Black opponent Start Chess game --------------------------------------- Game Chess   This example shows a Factory class by implementing a FactoryMethod    Game is the interface for all type of games  It defines complex method  createGame   Chess  Ludo  Checkers are different variants of games  which provide implementation to createGame   public Game getGame String gameName  is FactoryMethod in IGameFactory class GameFactory pre-creates different type of games in constructor  It implements IGameFactory factory method   game Name is passed as command line argument to NotStaticFactoryDemo getGame in GameFactory accepts a game name and returns corresponding Game object    Factory      Creates objects without exposing the instantiation logic to the client    FactoryMethod     Define an interface for creating an object  but let the subclasses decide which class to instantiate  The Factory method lets a class defer instantiation to subclasses   Use case   When to use  Client doesn t know what concrete classes it will be required to create at runtime  but just wants to get a class that will do the job

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It is important to clearly differentiate the idea behind using factory or factory method  Both are meant to address mutually exclusive different kind of object creation problems   Let s be specific about  factory method    First thing is that  when you are developing library or APIs which in turn will be used for further application development  then factory method is one of the best selections for creation pattern  Reason behind  We know that when to create an object of required functionality s  but type of object will remain undecided or it will be decided ob dynamic parameters being passed   Now the point is  approximately same can be achieved by using factory pattern itself but one huge drawback will introduce into the system if factory pattern will be used for above highlighted problem  it is that your logic of crating different objects sub classes objects  will be specific to some business condition so in future when you need to extend your library s functionality for other platforms In more technically  you need to add more sub classes of basic interface or abstract class so factory will return those objects also in addition to existing one based on some dynamic parameters  then every time you need to change extend  the logic of factory class which will be costly operation and not good from design perspective  On the other side  if  factory method  pattern will be used to perform the same thing then you just need to create additional functionality sub classes  and get it registered dynamically by injection which doesn t require changes in your base code   interface Deliverable                       abstract class DefaultProducer         public void taskToBeDone                     Deliverable deliverable   factoryMethodPattern              protected abstract Deliverable factoryMethodPattern       class SpecificDeliverable implements Deliverable         SPECIFIC TASK CAN BE WRITTEN HERE        class SpecificProducer extends DefaultProducer        protected Deliverable factoryMethodPattern                  return new SpecificDeliverable             public class MasterApplicationProgram        public static void main String arg                   DefaultProducer defaultProducer   new SpecificProducer            defaultProducer taskToBeDone

User · Answer

Any class deferring the object creation to its sub class for the object it needs to work with can be seen as an example of Factory pattern   I have mentioned in detail in an another answer at https   stackoverflow com a 49110001 504133

User · Answer

They re also useful when you need several  constructors  with the same parameter type but with different behavior

User · Answer

Factory methods should be considered as an alternative to constructors - mostly when constructors aren t expressive enough  ie   class Foo    public Foo bool withBar       is not as expressive as   class Foo    public static Foo withBar      public static Foo withoutBar        Factory classes are useful when you need a complicated process for constructing the object  when the construction need a dependency that you do not want for the actual class  when you need to construct different objects etc

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It is good idea to use factory methods inside object when    Object s class doesn t know what exact sub-classes it have to create Object s class is designed so that objects it creates were specified by sub-classes Object s class delegates its duties to auxiliary sub-classes and doesn t know what exact class will take these duties   It is good idea to use abstract factory class when    Your object shouldn t depend on how its inner objects are created and designed Group of linked objects should be used together and you need to serve this constraint Object should be configured by one of several possible families of linked objects that will be a part of your parent object It is required to share child objects showing interfaces only but not an implementation

User · Answer

Factory classes are more heavyweight  but give you certain advantages  In cases when you need to build your objects from multiple  raw data sources they allow you to encapsulate only the building logic  and maybe the aggregation of the data  in one place  There it can be tested in abstract without being concerned with the object interface   I have found this a useful pattern  particularly where I am unable to replace and inadequate ORM and want to efficiently instantiate many objects from DB table joins or stored procedures

User · Answer

Factory classes are more heavyweight  but give you certain advantages  In cases when you need to build your objects from multiple  raw data sources they allow you to encapsulate only the building logic  and maybe the aggregation of the data  in one place  There it can be tested in abstract without being concerned with the object interface   I have found this a useful pattern  particularly where I am unable to replace and inadequate ORM and want to efficiently instantiate many objects from DB table joins or stored procedures

User · Answer

It s really a matter of taste  Factory classes can be abstracted interfaced away as necessary  whereas factory methods are lighter weight  and also tend to be testable  since they don t have a defined type  but they will require a well-known registration point  akin to a service locator but for locating factory methods

User · Answer

It is good idea to use factory methods inside object when    Object s class doesn t know what exact sub-classes it have to create Object s class is designed so that objects it creates were specified by sub-classes Object s class delegates its duties to auxiliary sub-classes and doesn t know what exact class will take these duties   It is good idea to use abstract factory class when    Your object shouldn t depend on how its inner objects are created and designed Group of linked objects should be used together and you need to serve this constraint Object should be configured by one of several possible families of linked objects that will be a part of your parent object It is required to share child objects showing interfaces only but not an implementation

User · Answer

Factory classes are more heavyweight  but give you certain advantages  In cases when you need to build your objects from multiple  raw data sources they allow you to encapsulate only the building logic  and maybe the aggregation of the data  in one place  There it can be tested in abstract without being concerned with the object interface   I have found this a useful pattern  particularly where I am unable to replace and inadequate ORM and want to efficiently instantiate many objects from DB table joins or stored procedures

User · Answer

Factory classes are useful for when the object type that they return has a private constructor  when different factory classes set different properties on the returning object  or when a specific factory type is coupled with its returning concrete type    WCF uses ServiceHostFactory classes to retrieve ServiceHost objects in different situations  The standard ServiceHostFactory is used by IIS to retrieve ServiceHost instances for  svc files  but a WebScriptServiceHostFactory is used for services that return serializations to JavaScript clients  ADO NET Data Services has its own special  DataServiceHostFactory and ASP NET has its  ApplicationServicesHostFactory since its services have private constructors   If you only have one class that s consuming the factory  then you can just use a factory method within that class

User · Answer

They re also useful when you need several  constructors  with the same parameter type but with different behavior

User · Answer

I like thinking about design pattens in terms of my classes being  people   and the patterns are the ways that the people talk to each other   So  to me the factory pattern is like a hiring agency   You ve got someone that will need a variable number of workers   This person may know some info they need in the people they hire  but that s it   So  when they need a new employee  they call the hiring agency and tell them what they need   Now  to actually hire someone  you need to know a lot of stuff - benefits  eligibility verification  etc   But the person  hiring doesn t need to know any of this - the hiring agency handles all of that   In the same way  using a Factory allows the consumer to create new objects without having to know the details of how they re created  or what their dependencies are - they only have to give the information they actually want   public interface IThingFactory       Thing GetThing string theString      public class ThingFactory   IThingFactory       public Thing GetThing string theString                return new Thing theString  firstDependency  secondDependency             So  now the consumer of the ThingFactory can get a Thing  without having to know about the dependencies of the Thing  except for the string data that comes from the consumer

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I liken factories to the concept of libraries  For example you can have a library for working with numbers and another for working with shapes  You can store the functions of these libraries in logically named directories as Numbers or Shapes  These are generic types that could include integers  floats  dobules  longs or rectangles  circles  triangles  pentagons in the case of shapes   The factory petter uses polymorphism  dependency injection and Inversion of control   The stated purpose of the Factory Patterns is  Define an interface for creating an object  but let subclasses decide which class to instantiate  Factory Method lets a class defer instantiation to subclasses   So let s say that you are building an Operating System or Framework and you are building all the discrete components   Here is a simple example of the concept of the Factory Pattern in PHP  I may not be 100  on all of it but it s intended to serve as a simple example  I am not an expert   class NumbersFactory       public static function makeNumber   type   number              numObject   null           number   null           switch   type                 case  float                    numObject   new Float   number                    break              case  integer                    numObject   new Integer   number                    break              case  short                    numObject   new Short   number                    break              case  double                    numObject   new Double   number                    break              case  long                    numObject   new Long   number                    break              default                   numObject   new Integer   number                    break                     return  numObject              Numbers interface    abstract class Number       protected  number       public function   construct   number              this- gt number    number             abstract public function add        abstract public function subtract        abstract public function multiply        abstract public function divide         Float Implementation    class Float extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Integer Implementation    class Integer extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Short Implementation    class Short extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Double Implementation    class Double extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Long Implementation    class Long extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here           number   NumbersFactory  makeNumber   float   12 5

User · Answer

They re also useful when you need several  constructors  with the same parameter type but with different behavior

User · Answer

It is important to clearly differentiate the idea behind using factory or factory method  Both are meant to address mutually exclusive different kind of object creation problems   Let s be specific about  factory method    First thing is that  when you are developing library or APIs which in turn will be used for further application development  then factory method is one of the best selections for creation pattern  Reason behind  We know that when to create an object of required functionality s  but type of object will remain undecided or it will be decided ob dynamic parameters being passed   Now the point is  approximately same can be achieved by using factory pattern itself but one huge drawback will introduce into the system if factory pattern will be used for above highlighted problem  it is that your logic of crating different objects sub classes objects  will be specific to some business condition so in future when you need to extend your library s functionality for other platforms In more technically  you need to add more sub classes of basic interface or abstract class so factory will return those objects also in addition to existing one based on some dynamic parameters  then every time you need to change extend  the logic of factory class which will be costly operation and not good from design perspective  On the other side  if  factory method  pattern will be used to perform the same thing then you just need to create additional functionality sub classes  and get it registered dynamically by injection which doesn t require changes in your base code   interface Deliverable                       abstract class DefaultProducer         public void taskToBeDone                     Deliverable deliverable   factoryMethodPattern              protected abstract Deliverable factoryMethodPattern       class SpecificDeliverable implements Deliverable         SPECIFIC TASK CAN BE WRITTEN HERE        class SpecificProducer extends DefaultProducer        protected Deliverable factoryMethodPattern                  return new SpecificDeliverable             public class MasterApplicationProgram        public static void main String arg                   DefaultProducer defaultProducer   new SpecificProducer            defaultProducer taskToBeDone

User · Answer

if you want to create a different object in terms of using  It is useful   public class factoryMethodPattern         static String planName    COMMERCIALPLAN         static int units   3        public static void main String args                GetPlanFactory planFactory   new GetPlanFactory              Plan p   planFactory getPlan planName             System out print  Bill amount for     planName     of      units                             units is                p getRate              p calculateBill units              abstract class Plan         protected double rate         abstract void getRate           public void calculateBill int units                System out println units   rate              class DomesticPlan extends Plan             override       public void getRate                 rate   3 50             class CommercialPlan extends Plan             override       public void getRate                 rate   7 50             class InstitutionalPlan extends Plan             override       public void getRate                 rate   5 50             class GetPlanFactory             use getPlan method to get object of type Plan       public Plan getPlan String planType                if  planType    null                      return null                            if  planType equalsIgnoreCase  DOMESTICPLAN                        return new DomesticPlan                  else if  planType equalsIgnoreCase  COMMERCIALPLAN                        return new CommercialPlan                  else if  planType equalsIgnoreCase  INSTITUTIONALPLAN                        return new InstitutionalPlan                              return null

User · Answer

I like thinking about design pattens in terms of my classes being  people   and the patterns are the ways that the people talk to each other   So  to me the factory pattern is like a hiring agency   You ve got someone that will need a variable number of workers   This person may know some info they need in the people they hire  but that s it   So  when they need a new employee  they call the hiring agency and tell them what they need   Now  to actually hire someone  you need to know a lot of stuff - benefits  eligibility verification  etc   But the person  hiring doesn t need to know any of this - the hiring agency handles all of that   In the same way  using a Factory allows the consumer to create new objects without having to know the details of how they re created  or what their dependencies are - they only have to give the information they actually want   public interface IThingFactory       Thing GetThing string theString      public class ThingFactory   IThingFactory       public Thing GetThing string theString                return new Thing theString  firstDependency  secondDependency             So  now the consumer of the ThingFactory can get a Thing  without having to know about the dependencies of the Thing  except for the string data that comes from the consumer

User · Answer

One situation where I personally find separate Factory classes to make sense is when the final object you are trying to create relies on several other objects  E g  in PHP  Suppose you have a House object  which in turn has a Kitchen and a LivingRoom object  and the LivingRoom object has a TV object inside as well    The simplest method to achieve this is having each object create their children on their construct method  but if the properties are relatively nested  when your House fails creating you will probably spend some time trying to isolate exactly what is failing   The alternative is to do the following  dependency injection  if you like the fancy term     TVObj   new TV  param1   param2   param3    LivingroomObj   new LivingRoom  TVObj   param1   param2    KitchenroomObj   new Kitchen  param1   param2    HouseObj   new House  LivingroomObj   KitchenroomObj     Here if the process of creating a House fails there is only one place to look  but having to use this chunk every time one wants a new House is far from convenient  Enter the Factories   class HouseFactory       public function create              TVObj   new TV  param1   param2   param3            LivingroomObj   new LivingRoom  TVObj   param1   param2            KitchenroomObj   new Kitchen  param1   param2            HouseObj   new House  LivingroomObj   KitchenroomObj            return  HouseObj            houseFactory   new HouseFactory     HouseObj    houseFactory- gt create      Thanks to the factory here the process of creating a House is abstracted  in that you don t need to create and set up every single dependency when you just want to create a House  and at the same time centralized which makes it easier to maintain  There are other reasons why using separate Factories can be beneficial  e g  testability  but I find this specific use case to illustrate best how Factory classes can be useful

User · Answer

It s really a matter of taste  Factory classes can be abstracted interfaced away as necessary  whereas factory methods are lighter weight  and also tend to be testable  since they don t have a defined type  but they will require a well-known registration point  akin to a service locator but for locating factory methods

User · Answer

Factory classes are more heavyweight  but give you certain advantages  In cases when you need to build your objects from multiple  raw data sources they allow you to encapsulate only the building logic  and maybe the aggregation of the data  in one place  There it can be tested in abstract without being concerned with the object interface   I have found this a useful pattern  particularly where I am unable to replace and inadequate ORM and want to efficiently instantiate many objects from DB table joins or stored procedures

User · Answer

Consider a scenario when you have to design an Order and Customer class  For simplicity and initial requirements you do not feel need of factory for Order class and fill your application with many  new Order    statements  Things are working well   Now a new requirement comes into picture that Order object cannot be instantiated without Customer association  new dependency   Now You have following considerations   1- You create constructor overload which will work only for new implementations   Not acceptable   2- You change Order   signatures and change each and every invokation   Not a good practice and real pain    Instead If you have created a factory for Order Class you only have to change one line of code and you are good to go  I suggest Factory class for almost every aggregate association  Hope that helps

User · Answer

One situation where I personally find separate Factory classes to make sense is when the final object you are trying to create relies on several other objects  E g  in PHP  Suppose you have a House object  which in turn has a Kitchen and a LivingRoom object  and the LivingRoom object has a TV object inside as well    The simplest method to achieve this is having each object create their children on their construct method  but if the properties are relatively nested  when your House fails creating you will probably spend some time trying to isolate exactly what is failing   The alternative is to do the following  dependency injection  if you like the fancy term     TVObj   new TV  param1   param2   param3    LivingroomObj   new LivingRoom  TVObj   param1   param2    KitchenroomObj   new Kitchen  param1   param2    HouseObj   new House  LivingroomObj   KitchenroomObj     Here if the process of creating a House fails there is only one place to look  but having to use this chunk every time one wants a new House is far from convenient  Enter the Factories   class HouseFactory       public function create              TVObj   new TV  param1   param2   param3            LivingroomObj   new LivingRoom  TVObj   param1   param2            KitchenroomObj   new Kitchen  param1   param2            HouseObj   new House  LivingroomObj   KitchenroomObj            return  HouseObj            houseFactory   new HouseFactory     HouseObj    houseFactory- gt create      Thanks to the factory here the process of creating a House is abstracted  in that you don t need to create and set up every single dependency when you just want to create a House  and at the same time centralized which makes it easier to maintain  There are other reasons why using separate Factories can be beneficial  e g  testability  but I find this specific use case to illustrate best how Factory classes can be useful

User · Answer

I liken factories to the concept of libraries  For example you can have a library for working with numbers and another for working with shapes  You can store the functions of these libraries in logically named directories as Numbers or Shapes  These are generic types that could include integers  floats  dobules  longs or rectangles  circles  triangles  pentagons in the case of shapes   The factory petter uses polymorphism  dependency injection and Inversion of control   The stated purpose of the Factory Patterns is  Define an interface for creating an object  but let subclasses decide which class to instantiate  Factory Method lets a class defer instantiation to subclasses   So let s say that you are building an Operating System or Framework and you are building all the discrete components   Here is a simple example of the concept of the Factory Pattern in PHP  I may not be 100  on all of it but it s intended to serve as a simple example  I am not an expert   class NumbersFactory       public static function makeNumber   type   number              numObject   null           number   null           switch   type                 case  float                    numObject   new Float   number                    break              case  integer                    numObject   new Integer   number                    break              case  short                    numObject   new Short   number                    break              case  double                    numObject   new Double   number                    break              case  long                    numObject   new Long   number                    break              default                   numObject   new Integer   number                    break                     return  numObject              Numbers interface    abstract class Number       protected  number       public function   construct   number              this- gt number    number             abstract public function add        abstract public function subtract        abstract public function multiply        abstract public function divide         Float Implementation    class Float extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Integer Implementation    class Integer extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Short Implementation    class Short extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Double Implementation    class Double extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here            Long Implementation    class Long extends Number       public function add                implementation goes here            public function subtract                implementation goes here            public function multiply                implementation goes here            public function divide                implementation goes here           number   NumbersFactory  makeNumber   float   12 5

User · Answer

Factory classes are useful for when the object type that they return has a private constructor  when different factory classes set different properties on the returning object  or when a specific factory type is coupled with its returning concrete type    WCF uses ServiceHostFactory classes to retrieve ServiceHost objects in different situations  The standard ServiceHostFactory is used by IIS to retrieve ServiceHost instances for  svc files  but a WebScriptServiceHostFactory is used for services that return serializations to JavaScript clients  ADO NET Data Services has its own special  DataServiceHostFactory and ASP NET has its  ApplicationServicesHostFactory since its services have private constructors   If you only have one class that s consuming the factory  then you can just use a factory method within that class

User · Answer

if you want to create a different object in terms of using  It is useful   public class factoryMethodPattern         static String planName    COMMERCIALPLAN         static int units   3        public static void main String args                GetPlanFactory planFactory   new GetPlanFactory              Plan p   planFactory getPlan planName             System out print  Bill amount for     planName     of      units                             units is                p getRate              p calculateBill units              abstract class Plan         protected double rate         abstract void getRate           public void calculateBill int units                System out println units   rate              class DomesticPlan extends Plan             override       public void getRate                 rate   3 50             class CommercialPlan extends Plan             override       public void getRate                 rate   7 50             class InstitutionalPlan extends Plan             override       public void getRate                 rate   5 50             class GetPlanFactory             use getPlan method to get object of type Plan       public Plan getPlan String planType                if  planType    null                      return null                            if  planType equalsIgnoreCase  DOMESTICPLAN                        return new DomesticPlan                  else if  planType equalsIgnoreCase  COMMERCIALPLAN                        return new CommercialPlan                  else if  planType equalsIgnoreCase  INSTITUTIONALPLAN                        return new InstitutionalPlan                              return null

User · Answer

It s really a matter of taste  Factory classes can be abstracted interfaced away as necessary  whereas factory methods are lighter weight  and also tend to be testable  since they don t have a defined type  but they will require a well-known registration point  akin to a service locator but for locating factory methods

User · Answer

Consider a scenario when you have to design an Order and Customer class  For simplicity and initial requirements you do not feel need of factory for Order class and fill your application with many  new Order    statements  Things are working well   Now a new requirement comes into picture that Order object cannot be instantiated without Customer association  new dependency   Now You have following considerations   1- You create constructor overload which will work only for new implementations   Not acceptable   2- You change Order   signatures and change each and every invokation   Not a good practice and real pain    Instead If you have created a factory for Order Class you only have to change one line of code and you are good to go  I suggest Factory class for almost every aggregate association  Hope that helps

User · Answer

Factory methods should be considered as an alternative to constructors - mostly when constructors aren t expressive enough  ie   class Foo    public Foo bool withBar       is not as expressive as   class Foo    public static Foo withBar      public static Foo withoutBar        Factory classes are useful when you need a complicated process for constructing the object  when the construction need a dependency that you do not want for the actual class  when you need to construct different objects etc

User · Answer

Any class deferring the object creation to its sub class for the object it needs to work with can be seen as an example of Factory pattern   I have mentioned in detail in an another answer at https   stackoverflow com a 49110001 504133

User · Answer

I think it will depend of loose coupling degree that you want to bring to your code   Factory method decouples things very well but factory class no   In other words  it s easier to change things if you use factory method than if you use a simple factory  known as factory class    Look into this example  https   connected2know com programming java-factory-pattern    Now  imagine that you want to bring a new Animal  In Factory class you need to change the Factory but in the factory method  no  you only need to add a new subclass

User · Answer

Factory classes are useful for when the object type that they return has a private constructor  when different factory classes set different properties on the returning object  or when a specific factory type is coupled with its returning concrete type    WCF uses ServiceHostFactory classes to retrieve ServiceHost objects in different situations  The standard ServiceHostFactory is used by IIS to retrieve ServiceHost instances for  svc files  but a WebScriptServiceHostFactory is used for services that return serializations to JavaScript clients  ADO NET Data Services has its own special  DataServiceHostFactory and ASP NET has its  ApplicationServicesHostFactory since its services have private constructors   If you only have one class that s consuming the factory  then you can just use a factory method within that class

User · Answer

Factory methods should be considered as an alternative to constructors - mostly when constructors aren t expressive enough  ie   class Foo    public Foo bool withBar       is not as expressive as   class Foo    public static Foo withBar      public static Foo withoutBar        Factory classes are useful when you need a complicated process for constructing the object  when the construction need a dependency that you do not want for the actual class  when you need to construct different objects etc

User · Answer

Factory classes are useful for when the object type that they return has a private constructor  when different factory classes set different properties on the returning object  or when a specific factory type is coupled with its returning concrete type    WCF uses ServiceHostFactory classes to retrieve ServiceHost objects in different situations  The standard ServiceHostFactory is used by IIS to retrieve ServiceHost instances for  svc files  but a WebScriptServiceHostFactory is used for services that return serializations to JavaScript clients  ADO NET Data Services has its own special  DataServiceHostFactory and ASP NET has its  ApplicationServicesHostFactory since its services have private constructors   If you only have one class that s consuming the factory  then you can just use a factory method within that class

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UML from     Product  It defines an interface of the objects the Factory method creates   ConcreteProduct  Implements Product interface  Creator  Declares the Factory method  ConcreateCreator   Implements the Factory method to return an instance of a ConcreteProduct  Problem statement  Create a Factory of Games by using Factory Methods  which defines the game interface   Code snippet   import java util HashMap       Product interface as per UML diagram    interface Game         createGame is a complex method  which executes a sequence of game steps        public void createGame          ConcreteProduct implementation as per UML diagram    class Chess implements Game      public Chess               public void createGame            System out println  ---------------------------------------            System out println  Create Chess game            System out println  Opponents 2            System out println  Define 64 blocks            System out println  Place 16 pieces for White opponent            System out println  Place 16 pieces for Black opponent            System out println  Start Chess game            System out println  ---------------------------------------            class Checkers implements Game      public Checkers               public void createGame            System out println  ---------------------------------------            System out println  Create Checkers game            System out println  Opponents 2 or 3 or 4 or 6            System out println  For each opponent  place 10 coins            System out println  Start Checkers game            System out println  ---------------------------------------            class Ludo implements Game      public Ludo               public void createGame            System out println  ---------------------------------------            System out println  Create Ludo game            System out println  Opponents 2 or 3 or 4            System out println  For each opponent  place 4 coins            System out println  Create two dices with numbers from 1-6            System out println  Start Ludo game            System out println  ---------------------------------------                Creator interface as per UML diagram    interface IGameFactory       public Game getGame String gameName         ConcreteCreator implementation as per UML diagram    class GameFactory implements IGameFactory         HashMap lt String Game gt  games   new HashMap lt String Game gt                      Since Game Creation is complex process  we don t want to create game using new operator every time          Instead we create Game only once and store it in Factory  When client request a specific game           Game object is returned from Factory instead of creating new Game on the fly  which is time consuming             public GameFactory             games put Chess class getName   new Chess             games put Checkers class getName   new Checkers             games put Ludo class getName   new Ludo                       public Game getGame String gameName           return games get gameName            public class NonStaticFactoryDemo      public static void main String args             if   args length  lt  1               System out println  Usage  java FactoryDemo gameName                return                     GameFactory factory   new GameFactory            Game game   factory getGame args 0            if   game    null                                    game createGame                System out println  Game   game getClass   getName              else              System out println args 0      Game does not exists in factory                                   output   java NonStaticFactoryDemo Chess --------------------------------------- Create Chess game Opponents 2 Define 64 blocks Place 16 pieces for White opponent Place 16 pieces for Black opponent Start Chess game --------------------------------------- Game Chess   This example shows a Factory class by implementing a FactoryMethod    Game is the interface for all type of games  It defines complex method  createGame   Chess  Ludo  Checkers are different variants of games  which provide implementation to createGame   public Game getGame String gameName  is FactoryMethod in IGameFactory class GameFactory pre-creates different type of games in constructor  It implements IGameFactory factory method   game Name is passed as command line argument to NotStaticFactoryDemo getGame in GameFactory accepts a game name and returns corresponding Game object    Factory      Creates objects without exposing the instantiation logic to the client    FactoryMethod     Define an interface for creating an object  but let the subclasses decide which class to instantiate  The Factory method lets a class defer instantiation to subclasses   Use case   When to use  Client doesn t know what concrete classes it will be required to create at runtime  but just wants to get a class that will do the job

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Factory methods should be considered as an alternative to constructors - mostly when constructors aren t expressive enough  ie   class Foo    public Foo bool withBar       is not as expressive as   class Foo    public static Foo withBar      public static Foo withoutBar        Factory classes are useful when you need a complicated process for constructing the object  when the construction need a dependency that you do not want for the actual class  when you need to construct different objects etc

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I think it will depend of loose coupling degree that you want to bring to your code   Factory method decouples things very well but factory class no   In other words  it s easier to change things if you use factory method than if you use a simple factory  known as factory class    Look into this example  https   connected2know com programming java-factory-pattern    Now  imagine that you want to bring a new Animal  In Factory class you need to change the Factory but in the factory method  no  you only need to add a new subclass

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It s really a matter of taste  Factory classes can be abstracted interfaced away as necessary  whereas factory methods are lighter weight  and also tend to be testable  since they don t have a defined type  but they will require a well-known registration point  akin to a service locator but for locating factory methods

[design-patterns] Factory Pattern. When to use factory methods?

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