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我正在學習Java幾周，並且遇到了「項目」問題。由鼠標單擊問題引起的圓圈
我想寫一個簡單的（顯然不適合我）程序，通過鼠標點擊畫布上的一個圓圈，在鼠標點擊的座標上！
這裏是我的代碼：
MyGame3.java 包mygame3;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import javax.swing.JFrame;
public class MyGame3 extends JFrame{
int WIDTH, HEIGHT;
Image dbImage;
Graphics dbg;
static ArrayList<Circle> circles;
public MyGame3(){
WIDTH = 600;
HEIGHT = 600;
setSize(WIDTH, HEIGHT);
setTitle("My Game 3");
setBackground(Color.white);
setResizable(false);
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
addMouseListener(new MouseAdapter() {
public void mousePressed(MouseEvent e){
System.out.println(e.getX() + " " + e.getY());
circles.add(new Circle(e.getX(), e.getY()));
}
});
}
@Override
public void paint(Graphics g){
dbImage = createImage(getWidth(), getHeight());
dbg = dbImage.getGraphics();
draw(dbg);
g.drawImage(dbImage, 0, 0, this);
}
public void draw(Graphics g){
if (circles.size() > 0){
for (Circle c: circles){
c.draw(g);
}
repaint();
}
}
public static void main (String[] args){
MyGame3 main = new MyGame3();
if (circles.size() > 0){
for (Circle c: circles){
Thread circle = new Thread(c);
circle.start();
}
}
}
}
Circle.java：
package mygame3;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.geom.Ellipse2D;
public class Circle implements Runnable{
Ellipse2D circle;
public Circle(int x, int y){
circle = new Ellipse2D.Double(x, y, 30, 30);
}
public void draw (Graphics g){
g.setColor(Color.blue);
g.drawOval((int)circle.getX(),
(int)circle.getY(),
(int)circle.getWidth(),
(int)circle.getHeight());
}
@Override
public void run() {
try{
while(true){
Thread.sleep(2);
}
}
catch(Exception e){
System.err.println(e.getMessage());
}
}
}
這裏是我2次點擊屏幕上得到的錯誤：
run:
Exception in thread "main" java.lang.NullPointerException
at mygame3.MyGame3.main(MyGame3.java:59)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mygame3.MyGame3.draw(MyGame3.java:47)
at mygame3.MyGame3.paint(MyGame3.java:42)
at javax.swing.RepaintManager$3.run(RepaintManager.java:819)
at javax.swing.RepaintManager$3.run(RepaintManager.java:796)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:769)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:718)
at javax.swing.RepaintManager.access$1100(RepaintManager.java:62)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1677)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mygame3.MyGame3.draw(MyGame3.java:47)
at mygame3.MyGame3.paint(MyGame3.java:42)
at javax.swing.RepaintManager$3.run(RepaintManager.java:819)
at javax.swing.RepaintManager$3.run(RepaintManager.java:796)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:769)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:718)
at javax.swing.RepaintManager.access$1100(RepaintManager.java:62)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1677)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
245 386
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mygame3.MyGame3$1.mousePressed(MyGame3.java:33)
at java.awt.Component.processMouseEvent(Component.java:6502)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Window.processEvent(Window.java:2022)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
245 386
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mygame3.MyGame3$1.mousePressed(MyGame3.java:33)
at java.awt.Component.processMouseEvent(Component.java:6502)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Window.processEvent(Window.java:2022)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
BUILD SUCCESSFUL (total time: 3 seconds)
感謝任何迴應！