Tutorial :does a factory just return an implementation of an interface?


does a factory just return an implementation of an interface? Is that the job?


Sometimes that's all a factory does, but they can also:

  • Choose a concrete implementation based on data that's only available at run-time:

    // Beverage Factory  public IBeverage CreateBeverage(DateTime orderDate) {      return orderDate.Hour > 12 ? new Beer() : new Milk();  }  
  • Perform post-construction initialization (often expensive initialization or initialization of data that's not appropriate to encapsulate within the object itself):

    // Weather report factory  public IWeatherReport CreateWeatherReport() {      WeatherReport report = new WeatherReport();      report.data = WeatherWebService.GetData();      return report;  }  
  • Initialize the new instance based on an existing instance:

    // Fittest Algorithm Factory  public Algorithm CreateNewAlgorithm() {      return this.fittestAlgorithm.Clone();  }  
  • Draw an instance from a pool instead of creating one from scratch:

    public IDbConnection CreateConnection(string connectionString) {      return this.ConnectionStacks[connectionString].Pop();  }  
  • Return a singleton instance (though yuck, and you'd better be sure it's thread safe!)


See wikipedia. Depending on what exactly you mean by "return an implementation of an interface", yes. But this definition may not be very accurate / comprehensive (especially since the Factory pattern does not necessarily require the concept of interface).


From Wiki:

The essence of the Factory Pattern is to "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.


In essence, yes.

However, depending on your language, "interface" may have a specific meaning. A factory typically returns a concrete implementation of a general contract - which can be an interface, a base class, or any other means of specialization.

Also, there are times when a factory returns the exact type you're specifying - but you use a factory for another purpose, such as managing lifetime, tracking, or some other rationale other than construction of a specialized type.


Couple more great resources on this:

The more specific answer is that a Factory is to provide pre-initialization and pre-construction work for types of objects which are common. The simplest example (I have seen) of this is a "Hammer factory", where the hammer is constructed of two objects (handle, and head) and is given a single name "wood handled claw hammer". Our factory can therefore have a single method:

(Hammer|IHammer|...) GetHammer(string hammername);  

Which can return one of several objects (an actual Hammer Object, an Interface describing the Hammer, a Base class for Hammer, etc). Possibly the most useful of these is to return an Interface which describes the hammer and allows us to implement a number of useful design patterns from there.

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