Question

I have 2 already defined classes, we’ll call them DogActivityType and HorseActivityType.
They have the same fields, same methods, but they write to 2 different tables in the database, and of course, are named differently.

I have a function with all the business rules already working for one class, and the other class uses the same business rules.

Restrictions:

1. I have to use these 2 classes since they are used throughout other parts of the project
2. I can’t create one class and add another column (field) to distinguish between both types of classes.
3. I can’t edit the source for these 2 classes.

.
Here is my simplified version of the code:

public doAllCalculations(){
  // retrieve collection
  foreach (DogActivityType activity in allActivities){
    // a lot of code here

    // more code... 

    createMoreOf(activity);  // this is overloaded since it needs to know which DB to write to
  }
}

// using overload for same function name
private createMoreOf(DogActivityType doggyActivity){
    /// do some specific when it is a dog
}

private createMoreOf(HorseActivityType horse){
    /// do some specific when it is a horse
}

Now, the problem is: doAllCalculations() is very extensive and complicated and may change during the course of development. I don’t want to have 2 different functions (doAllCalculationsDOG() and doAllCalculationsHORSE() ) in order to do the same analysis, just because I need one for the Dog and another for the Horse class. One day someone in the project may forget to update both functions or any other bad scenario…

So, I want to use the same function for both classes. So if I edit a rule in the big calculation function, I will know it works for both classes.
I’m thinking I’ll end up with something like this:

public class AnimalActityType {

}

public doAllCalculations(){
  // retrieve collection
  foreach (AnimalActivityType activity in allActivities){
    // a lot of code here
    // more code... 
    createMoreOf(activity); 
  }
}

AnimalActityType will simulate an abstract parent, I’ll call it reverse polymorphism…
But how do DogActityType and HorseActityType know about this parent? Can I force the parent?
Is it possible? Any ideas?

Answer 1

you can try to use decorator pattern, but in very unusual way.

    class Decorator
{
    private object instance;

    public Decprator(object instance)
    {
         this.instance = instance;
    }

    public <type> SomeCommonProp
    {
      get{
        if(instance is DogActivityType)
        {
          return (instance as DogActivityType).SomeValueOrPropertyOrCall;
        }
        else
        {
          return (instance as HorseActivityType).SomeValueOrPropertyOrCall; 
        }
      }
    }
}


class MyCalculations
{
  private Decorator instance;

  public MyCalculations(Decorator inst)
  {
      instance = inst;
  }

  public <type> SomeCalculationMethod()
  {
    // here you will use instance.SomeCommonProp for your calculations
  }
}