Why do I get NPE in this case and how to work around this?

Question

I have an assignment to make a class RGBImage based on another class called RBGColor which I don't have the source code (and don't need to) for but I am told it represents the color of a pixel like (0,255,0) and I was given the methods of that class.

I have written the class but I have a null point exception on one of my methods.

Some background code:

public class RGBImage{
private RGBColor[][] _image;

final RGBColor DEFAULT_COLOR_VALUE = new RGBColor(0,0,0);

public RGBImage(int rows, int cols) {
    _image = new RGBColor[rows][cols];
}

The problematic method:

public RGBColor[][] toRGBColorArray(){
    RGBColor[][] output = new RGBColor[_image.length][_image[0].length];

    for (int i = 0; i < _image.length; i++){
        for (int j = 0; j < _image[0].length; j++){
            output[i][j].setRed(_image[i][j].getRed());// NPE here.
            output[i][j].setGreen(_image[i][j].getGreen());
            output[i][j].setBlue(_image[i][j].getBlue());
        }
    }

    return output;
}

I have used setRed and getRed in other methods without problem so far so I don't understand why it's problematic now.

You just created a blank array with nothing in it and called methods on the nonexistent objects. — chrylis -cautiouslyoptimistic-, May 01 '21 at 14:23
What I didn't add is that I was given those methods from RGBColor class. — Paul Sirota, May 01 '21 at 14:25
So? You don't have any `RGBColor` _objects_ to call them on. — chrylis -cautiouslyoptimistic-, May 01 '21 at 14:30
I do, otherwise, I would have got a compiling error and not a run time error. — Paul Sirota, May 01 '21 at 14:36
Your array object exists, but the elements inside the array do not. You have an array of null objects and you are calling methods on those null objects. — takendarkk, May 01 '21 at 14:47
Ok, I see the problem. Please tell me how to do it properly. — Paul Sirota, May 01 '21 at 16:08

Why do I get NPE in this case and how to work around this?

0 Answers0