添加到ArrayList時Java NullPointerException？

Question

即使對象似乎正確存在，我的代碼仍然拋出一個NullPointerException異常。

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); 

這是怎麼發生的？

Answer 1

根據您所提供的代碼的部分，它看起來像你還沒有初始化myPolygon

Answer 2

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

Answer 3

確保您初始化列表：

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

另外請注意，這是最好的將myPolygon定義爲List（接口）而不是ArrayList（實現）。