Overriding constructors in Java

Question

Hi,

For school I need to learn Java and since I'm used to C++ (like Cocoa/Objective-C) based languages, I get really frustrated on Java.

I've made a super-class (that can also be used as a base-class):

public class CellView {
    public CellViewHelper helper; // CellViewHelper is just an example

    public CellView() {
        this.helper = new CellViewHelper();
        this.helper.someVariable = <anything>;

        System.out.println("CellView_constructor");
    }

    public void draw() {
        System.out.println("CellView_draw");
    }

    public void needsRedraw() {
        this.draw();
    }

}

public class ImageCellView extends CellView {

    public Image someImage;

    public ImageCellView() {
        super();
        this.someImage = new Image();
        System.out.println("ImageCellView_constructor");
    }

    public void setSomeParam() {
        this.needsRedraw(); // cannot be replaced by this.draw(); since it's some more complicated.
    }

    @Override public void draw() {
        super.draw();
        System.out.println("ImageCellView_draw");
    }

}

Now, when I call it like this:

ImageCellView imageCellView = new ImageCellView();
imageCellView.setSomeParam();

I get this:

CellView_constructor

ImageCellView_constructor

CellView_draw

However, I want it to be:

CellView_constructor

ImageCellView_constructor

CellView_draw

ImageCellView_draw

How can I do this?

Thanks in advance,

Tim

EDIT:

I also implemented this method to CellView:

public void needsRedraw() {
    this.draw();
}

And this to ImageCellView:

public void setSomeParam() {
    this.needsRedraw(); // cannot be replaced by this.draw(); since it's some more complicated.
}

And I've been calling this:

ImageCellView imageCellView = new ImageCellView();
imageCellView.setSomeParam();

Does this causes the problem (when I call a function from the super it calls to the super only)? How can I solve this... (without having to redefine/override the needsRedraw()-method in every subclass?)

Answer 1

The short answer is "you can't."

Objects are constructed from the bottom up, calling base class initializers before subclass initializers and base class consrtuctors before subclass constructors.

EDIT:

The code you have looks good, based on your edit. I would go through the mundane tasks like ensuring that you have compiled your code after you've added you System.out.println calls to your subclass

Answer 2

You should get proper output.

I tried you example just commented unrelated things:

import java.awt.Image;

public class CellView {
    //public CellViewHelper helper; // CellViewHelper is just an example

    public CellView() {
        //this.helper = new CellViewHelper();
        //this.helper.someVariable = <anything>;

        System.out.println("CellView_constructor");
    }

    public void draw() {
        System.out.println("CellView_draw");
    }

    public static void main(String[] args) {
        ImageCellView imageCellView = new ImageCellView();
        imageCellView.draw();
    }
}

class ImageCellView extends CellView {

    public Image someImage;

    public ImageCellView() {
        super();
        //this.someImage = new Image();
        System.out.println("ImageCellView_constructor");
    }

    @Override public void draw() {
        super.draw();
        System.out.println("ImageCellView_draw");
    }

}

and I get following output:

CellView_constructor

ImageCellView_constructor

CellView_draw

ImageCellView_draw

This is exactly what you want, and this is what your code print's.