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Android

Posted by Andrew Zhang
Andrew Zhang
4,364 Points

List of Custom Classes

I'm trying to put what I've learnt into real examples.

The declaration:

    protected List<ColourBoxes> mBoxes;

The snippet of the code is:

if(i == correctNumber){
                ColourBoxes correctBox = new ColourBoxes(levelColour()+3);
                mBoxes.add(correctBox);
            }

and the colourboxes class is:

public class ColourBoxes {
    int mColour;

    public ColourBoxes(int colour) {
        this.mColour = colour;
    }

}

However, when I tried to run it, it keeps erroring out on the mBoxes.add line because as it's returning a null pointer.

anyone can explain to me why this is the case?

Cheers :)

1 Answer

Ratik Sharma
Ratik Sharma
32,885 Points

Simple fix. You generally get a null pointer error when you don't initialise a object. Do the following:

// Replace this
protected List<ColourBoxes> mBoxes;

// With this
protected List<ColourBoxes> mBoxes = new ArrayList<ColourBoxes>();

Let me know if that helps!

Andrew Zhang
Andrew Zhang
4,364 Points

That has worked, thanks very much Ratik!