Design issue on nested structure

问题:

I´m working on a project that deals with a huge amount of different kinds of objects retrieved from a database. For further considerations let´s suppose we have a much simpler structure as follows (for simplicity access-modifiers and such are omited):

class Outer {
    Mid mid;
    string someString;

    void createMid(string str) {
        // some work
        this.mid = new Mid(str);
    }
}

class Mid {
    Inner inner;
    string someOtherString;
}

class Inner {
    string innerString;
}

class Inner2 : Inner {
    // some custom behaviour for innerString
}

Now we create an instance of Outer that also instantiates its nested objects. Now we want to change some behaviour within the Inner-class by subclassing it (let´s say we want to set its innerString-property to some different value than in its base-implementation). My current approach for this is to subclass Inner. To create an instance of this derived class we also have to subclass Mid, whereby we also subclass Outer. Whilst creating this nested structure consists of many operations in the Outer-object we thus should also override most of our code only to instantiate Inner2-instance and thus also another instance of Mid.

I know that sounds like a huge design-flaw but I don´t come to any other solution for this kind of design than subclassing the whole structure and overriding the whole createMid-method just for this relative small sub-object. Is there any other approach for subclassing Inner without also subclassing all the others?

回答1:

Make use of the factory pattern: Instead of instantiating Inner inside of Mid directly, call:

this.mid = InnerFactory.Create(/*parameters*/);

InnerFactory can then create Inner or Inner2, depending on the arguments.

What may be a little tricky is to provide the parameters so that the factory can decided which class to instantiate.