程序不知道哪里出错了？

Question

package test;import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.Iterator;public class Main {
public static void main(String[] args) {
new MyFrame("ly");
}
}class MyFrame extends Frame {
private ArrayList<Point> pointsList; public MyFrame(String s) {
super(s);
setBounds(200, 200, 500, 400);
setVisible(true);
addMouseListener(new Mouse());
} @Override
public void paint(Graphics g) {
Color oldColor = g.getColor();
Iterator<Point> i = pointsList.iterator();
try {
while (i.hasNext()) {
g.fillOval(i.next().x, i.next().y, 5, 5);
} } catch (Exception e) {
// TODO: handle exception
}
g.setColor(oldColor);
} public ArrayList<Point> getPointsList() {
return pointsList;
}
}class Mouse extends MouseAdapter {
@Override
public void mouseClicked(MouseEvent e) {
MyFrame f = (MyFrame) e.getSource();
f.getPointsList().add(e.getPoint());
f.repaint();
}
}

Answer 1

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.MyFrame.paint(Main.java:31)
at sun.awt.RepaintArea.paintComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at sun.awt.windows.WWindowPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.MyFrame.paint(Main.java:31)
at sun.awt.RepaintArea.paintComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at sun.awt.windows.WWindowPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.MyFrame.paint(Main.java:31)
at sun.awt.RepaintArea.paintComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at sun.awt.windows.WWindowPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.Mouse.mouseClicked(Main.java:53)
at java.awt.Component.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Window.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at test.Mouse.mouseClicked(Main.java:53)
at java.awt.Component.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Window.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 2

Iterator<Point> i = pointsList.iterator();这一句报空指针异常，因为pointsList=null;

Answer 3

加pointsList=new  ArrayList<Point>();可避免空指针异常
如：
                  Color oldColor = g.getColor();
pointsList=new  ArrayList<Point>();
Iterator<Point> i = pointsList.iterator();

Answer 4

class MyFrame extends Frame {
private ArrayList<Point> pointsList;public MyFrame(String s) {
super(s);
setBounds(200, 200, 500, 400);
setVisible(true);
addMouseListener(new Mouse());
}这里的pointsList没有实例化，导致“空指”异常的发生。
可以用
private ArrayList<Point> pointsList＝new ArrayList<Point>(){};
应该可以，你试试.

Answer 5

追踪了半天，发现这句有问题
while (i.hasNext()) {
g.fillOval(i.next().x, i.next().y, 5, 5);
}改成下面这个就没有问题了
while (i.hasNext()) {
Point p = i.next();
g.setColor(Color.BLUE);
g.fillOval(p.x, p.y, 10, 10);
}
已经用了泛型，问什么非要转换为Point对象才能fillOval（）不报异常？？？

Answer 6

i.next().x, i.next().yi.next() 每调用一次，返回的结果都不一样
前一个.x和后一个.y不是同一个Point对象