如何保存从抽象类扩展的不同类

Question

I have an application with many DisplayFields (rendered on a canvas) eg text, boolean, date etc. These DisplayFields have many variables in common such as color, shadow parameters, fill etc.

Because of these common values, I have defined BasicField as an abstract class.

From this abstract class I extend DisplayFieldText, DisplayFieldBoolean etc. Each of these is a separate class with additional variables specific to the field's function.

Now I need to save all DisplayFields to disk. So each field needs to save its core (Abstract Class variables) as well as its own parameters.

How do I go about this without lots of duplication of code?

Answer 1

Caveat: I'm not sure how you're going about saving your data to disk, so this might not apply.

I'm going to assume that you're currently saving to disk using some structure that looks something like this:

class DisplayField {
    private int data;

    public void saveData() {
        // save data here
    }
}

This is a perfectly acceptable way to save your data, but when you introduce the parent class it would get kind of messy:

abstract class BasicField {
    public String sharedData1;
    public double sharedData2;
}

class DisplayField extends BasicField {
    private int data;

    public void saveData() {
        // save parent's data every time in every subclass!! Not fun!
        // save DisplayField's private data
    }
}

A good way to solve this problem is to use polymorphism. Since all the subclasses of BasicField are going to need to save their data, you can define the saveData() method in BasicField, and call it in the subclasses. This has a lot of other side-benefits, for example if you have a BasicField object, you know that it will be able to save its data. Also if you add more data to the BasicField later, you will only have to change a couple lines of code in BasicField instead of modifying all of its subclasses.

abstract class BasicField {
    public String sharedData1;
    public double sharedData2;

    public void saveData() {
        // save sharedData1
        // save sharedData2
    }
}

class DisplayField extends BasicField {
    private int data;

    public void saveData() {
        super.saveData();
        // save DisplayField's private data
    }
}