救命啊！怎么总是出错？？！大家帮我看下那里出错误！

Question

package frame;import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import javax.swing.*;public class MyApplet extends Applet {
  private boolean isStandalone = false;
  BorderLayout borderLayout1 = new BorderLayout();
  JPanel command = new JPanel();
  JPanel workArea = new JPanel();
  JPanel parameter = new JPanel();
  JButton jButton1 = new JButton();
  JButton line = new JButton();  Operation operation = null;  Canvas image;
  Point startPoint, endPoint;
  int mouse;  //Get a parameter value
  public String getParameter(String key, String def) {
    return isStandalone ? System.getProperty(key, def) :
      (getParameter(key) != null ? getParameter(key) : def);
  }  //Construct the applet
  public MyApplet() {
  }  //Initialize the applet
  public void init() {
    startPoint = new Point(-1,-1);
    try {
      jbInit();
    }
    catch(Exception e) {
      e.printStackTrace();
    }
  }  //Component initialization
  private void jbInit() throws Exception {
    this.setLayout(borderLayout1);
    workArea.setBackground(Color.white);
    workArea.addMouseMotionListener(new MyApplet_workArea_mouseMotionAdapter(this));
    workArea.addMouseListener(new MyApplet_workArea_mouseAdapter(this));
    //workArea.addMouseMotionListener(new MyApplet_workArea_mouseMotionAdapter(this));
    jButton1.setText("jButton1");
    line.setText("墙壁");
    line.addActionListener(new MyApplet_line_actionAdapter(this));
    this.add(command,  BorderLayout.NORTH);
    command.add(line, null);
    this.add(workArea, BorderLayout.CENTER);
    this.add(parameter,  BorderLayout.EAST);
    parameter.add(jButton1, null);
  }  //Get Applet information
  public String getAppletInfo() {
    return "Applet Information";
  }  //Get parameter info
  public String[][] getParameterInfo() {
    return null;
  }  void line_actionPerformed(ActionEvent e) {
    mouse = 1;
    operation = new Operation(mouse);
  }  void workArea_mousePressed(MouseEvent e) {
    startPoint = new Point(e.getX(), e.getY());
    
  }  void workArea_mouseDragged(MouseEvent e){
    if(startPoint.x!=-1&&startPoint.y!=-1 ){
      image = operation.choose();
    }
  }
  void workArea_mouseReleased(MouseEvent e) {
    endPoint = new Point(e.getX(), e.getY());
    operation = new Operation(mouse);
    operation.date(startPoint, endPoint);
    workArea.add(image);
  }
}class MyApplet_line_actionAdapter implements java.awt.event.ActionListener {
  MyApplet adaptee;  MyApplet_line_actionAdapter(MyApplet adaptee) {
    this.adaptee = adaptee;
  }
  public void actionPerformed(ActionEvent e) {
    adaptee.line_actionPerformed(e);
  }
}class MyApplet_workArea_mouseAdapter extends java.awt.event.MouseAdapter {
  MyApplet adaptee;  MyApplet_workArea_mouseAdapter(MyApplet adaptee) {
    this.adaptee = adaptee;
  }
  public void mousePressed(MouseEvent e) {
    adaptee.workArea_mousePressed(e);
  }
  public void mouseReleased(MouseEvent e) {
    adaptee.workArea_mouseReleased(e);
  }
}class MyApplet_workArea_mouseMotionAdapter extends java.awt.event.MouseMotionAdapter {
  MyApplet adaptee;  MyApplet_workArea_mouseMotionAdapter(MyApplet adaptee) {
    this.adaptee = adaptee;
  }
  public void mouseDragged(MouseEvent e) {
    adaptee.workArea_mouseDragged(e);
  }
}
辅助类：
package frame;import java.awt.*;public class Operation {
  Process process = null;  Canvas image;
  Point startPoint, endPoint;
  int mouse;  public Operation(int mouse) {
    this.mouse = mouse;
  }  void date(Point startPoint, Point endPoint) {
    process = new Process(startPoint, endPoint);
    this.startPoint = process.startPoint;
    this.endPoint = process.endPoint;
  }  Canvas choose() {
    switch (mouse) {
      case 1:
        image = new Line(startPoint, endPoint);
        return image;
        //break;
      default:
        return image;
    }
  }
}
直线类：
package frame;import java.awt.*;public class Line extends Canvas{
  Point startPoint, endPoint;
  public Line(Point startPoint, Point endPoint) {
    this.startPoint = startPoint;
    this.endPoint = endPoint;
  }  void upDate(Graphics g) {
    g.setColor(Color.blue);
    g.drawLine(startPoint.x, startPoint.y, endPoint.x, endPoint.y);
  }
}
辅助类：
package frame;import java.awt.*;public class Process {
  Point startPoint, endPoint;  public Process(Point startPoint, Point endPoint) {
    this.startPoint = startPoint;
    this.endPoint = endPoint;
  }
}

Answer 1

错误提示：
java.lang.NullPointerException at frame.MyApplet.workArea_mouseDragged(MyApplet.java:83) at frame.MyApplet_workArea_mouseMotionAdapter.mouseDragged(MyApplet.java:128) at java.awt.Component.processMouseMotionEvent(Component.java:5148) at javax.swing.JComponent.processMouseMotionEvent(JComponent.java:2779) at java.awt.Component.processEvent(Component.java:4901) at java.awt.Container.processEvent(Container.java:1569) at java.awt.Component.dispatchEventImpl(Component.java:3615) at java.awt.Container.dispatchEventImpl(Container.java:1627) at java.awt.Component.dispatchEvent(Component.java:3477) at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:3483) at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3215) at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3128) at java.awt.Container.dispatchEventImpl(Container.java:1613) at java.awt.Component.dispatchEvent(Component.java:3477) at java.awt.EventQueue.dispatchEvent(EventQueue.java:456) at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:201) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:145) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:137) at java.awt.EventDispatchThread.run(EventDispatchThread.java:100)

Answer 2

应该是这个方法抛了空指针异常
void workArea_mouseDragged(MouseEvent e){
    if(startPoint.x!=-1&&startPoint.y!=-1 ){
      image = operation.choose();
    }
  }
楼主DEBUG下e有没有值？如果有，看看函数中其它的变量的值，应该就是这里。

Answer 3

我要开发一个有绘图功能的Applet，要求将绘制出来的图形作为一个组件对象来对待，以便能够在用户界面监听到用户对图形的操作，请问一下如何写这个图形对象的类