添加到ArrayList时是否为Java NullPointerException？

Question

我的代码抛出一个NullPointerException,即使该对象似乎正确存在.

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);

为什么会这样？

Answer 1

根据您提供的代码部分,看起来您尚未初始化 myPolygon

Answer 2

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

Answer 3

确保初始化List:

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

另请注意,最好将myPolygon定义为List(接口)而不是ArrayList(实现).