Form processing with inheritance

Question

I have a model that can be described as follows:

http://aoeu.se/so/animal-dog-cat.png

(Extremely simplified. My actual classes have about ten-twenty more fields each.)

I have the persistence all worked out and I'm now trying to create a form in which users should be able to create objects of these classes (dogs and cats).

I would like it to look roughly as follows:

http://aoeu.se/so/new-animal-form.png

(I already have the HTML and the enabling / disabling of input fields implemented.)

To my question: How do I best implement the form processing? I don't see how I can directly use PropertyModels and such since the type of model object depends on the first Dog/Cat choice.

If Wicket guarantees that the order in which the fields are processed is the same as the order in which they appear in the web page, then I guess I could create the model object once the animal type input is processed (since it's the first form component), and let the remaining fields use a PropertyModel.

Answer 1

At first you should think about separating the animal type selection from the rest of the form as you'll need to discard some of the entries if the user decides to change the type after inputing some data. This can be archieved by ajaxifying this part of the form and using the onChange event to clean the data and switch the models.

When you do so, you might want to think about switching from inheritance to composition to avoid copying data around. I don't know if this is still compatible with your persistance strategy but you could allways copy the finished data to your persistance layer.

For example when your CatModel doesn't inherit from AnimalModel but instead contains an AnimalModel like this:

public class CatModel {

    private final AnimalModel parent;
    private String meowingSound;

    public CatModel(AnimalModel parent) {
        this.parent = parent;
    }

    public int getNumLegs() {
        return parent.getNumLegs();
    }

    public void setNumLegs(int numLegs) {
        parent.setNumLegs(numLegs);
    }

    public String getMeowingSound() {
        return meowingSound;
    }

    public void setMeowingSound(String meowingSound) {
        this.meowingSound = meowingSound;
    }

    public AnimalModel getParent() {
        return parent;
    }

}

(interfaces skipped)

You could take the AnimalModel from a (likewise constructed) DogModel to initialize your CatModels 'inherited' data.

public class AnimalModel {

    private int numLegs;

    public int getNumLegs() {
        return numLegs;
    }

    public void setNumLegs(int numLegs) {
        this.numLegs = numLegs;
    }
}

AnimalModel for completeness...