Java NullPointerException when adding to ArrayList?

Question

My code is throwing a NullPointerException, even though the object seems to properly exist.

public class IrregularPolygon {

    private ArrayList<Point2D.Double> myPolygon;

    public void add(Point2D.Double aPoint) {
        System.out.println(aPoint); // Outputs Point2D.Double[20.0, 10.0]
        myPolygon.add(aPoint); // NullPointerException gets thrown here
    }
}

// Everything below this line is called by main()

    IrregularPolygon poly = new IrregularPolygon();
    Point2D.Double a = new Point2D.Double(20,10);
    poly.add(a);

Why is this happening?

Brad Mace · Accepted Answer · 2010-10-16 17:24:00Z

55

based on the parts of the code you provided, it looks like you haven't initialized myPolygon

answered Oct 16, 2010 at 17:24

Brad Mace

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Cristian · Accepted Answer · 2010-10-16 17:24:35Z

20

private ArrayList<Point2D.Double> myPolygon = new ArrayList<Point2D.Double>();

answered Oct 16, 2010 at 17:24

Cristian

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cherouvim · Accepted Answer · 2010-10-16 17:25:23Z

12

Make sure you initialize the List:

private List<Point2D.Double> myPolygon = new ArrayList<Point2D.Double>();

Also note that it's best to define myPolygon as a List (interface) and not ArrayList (implementation).

answered Oct 16, 2010 at 17:25

cherouvim

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2 Comments

Ajay Over a year ago

Is this still true? I don't think so, because if you initialise it as an ArrayList then you do not need to specify a type on the other side of the argument anymore.

FullStackDeveloper Over a year ago

May I know why? What is the advantage?: "Also note that it's best to define myPolygon as a List (interface) and not ArrayList (implementation)"

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