java-有人可以幫我看看這個繪圖代碼的問題在哪嗎？感謝！

有人可以幫我看看這個繪圖代碼的問題在哪嗎？感謝！

import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.util.ArrayList;
import java.awt.Point;

class Mouse extends MouseAdapter
{
    protected ArrayList<Point> points=null;
    int endX=0,endY=0;
    public void mouseClicked(MouseEvent e)
    {
        if(e.getClickCount()==1)
            points.add(new Point(e.getX(),e.getY()));
        else if(e.getClickCount()==2)
        {
            endX=e.getX();
            endY=e.getY();
        }
    }
}

class MyPanel extends JPanel 
{
    private Mouse mouse=new Mouse();
    public MyPanel()
    {
        this.addMouseListener(mouse);
    }
    public void paint(Graphics g)
    {
        int k=mouse.points.size();
        for(int i=0;i<k-1;i++)
            g.drawLine(mouse.points.get(i).x, mouse.points.get(i).y, mouse.points.get(i+1).x, mouse.points.get(i+1).y);
        g.drawLine(mouse.endX,mouse.endY,mouse.points.get(1).x, mouse.points.get(1).y);
    }
}

public class Paint extends JFrame{
    private MyPanel mpl=new MyPanel();
    public Paint()
    {
        this.add(mpl);
        this.setSize(400,400);
        this.setVisible(true);
    }
    public static void main(String[] args) throws Exception
    {
        new Paint();
    }
}

最佳回答：

int k=mouse.points.size();
上面這一句有問題。mouse.points是null，你還要調用size（）方法，可不就是空指針異常了呢。