does a factory just return an implementation of an interface?

Question

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

Answer 1

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!)