添加到ArrayList时Java NullPointerException？
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Java NullPointerException when adding to ArrayList?
提问by waiwai933
My code is throwing a NullPointerException, even though the object seems to properly exist.
我的代码抛出 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);
Why is this happening?
为什么会这样？
采纳答案by Brad Mace
based on the parts of the code you provided, it looks like you haven't initialized myPolygon
根据您提供的代码部分，您似乎尚未初始化 myPolygon
回答by Cristian
private ArrayList<Point2D.Double> myPolygon = new ArrayList<Point2D.Double>();
回答by cherouvim
Make sure you initialize the List:
确保初始化列表：
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).
另请注意，最好将 myPolygon 定义为 List（接口）而不是 ArrayList（实现）。
