Java的NullPointerException异常时增加ArrayList的？ [英] Java NullPointerException when adding to ArrayList?

问题描述

我的code抛出NullPointerException异常，即使对象似乎正常存在。

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);

为什么会出现这种情况？

Why is this happening?

推荐答案

根据您所提供的code的部分，它看起来像你还没有初始化 myPolygon

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

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