添加到ArrayList时Java NullPointerException? [英] Java NullPointerException when adding to ArrayList?

问题描述

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

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);

为什么会这样?

推荐答案

根据你提供的部分代码，看起来你还没有初始化myPolygon

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

这篇关于添加到ArrayList时Java NullPointerException?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！