从画图程序开始，走进GUI编程世界

;
  else{
   File file=showDialog("保存图片","保存","保存图片",'s',
    new File(files.getCurrentDirectory(),filename));
   if(file==null)
    return;
   else
    if(file.exists())
     if(JOptionPane.NO_OPTION==JOptionPane.showConfirmDialog(DrawFrame.this,file.getName()+"已存在，覆盖？"
     ,"确认保存",JOptionPane.YES_NO_OPTION,JOptionPane.WARNING_MESSAGE))
      return;
   saveDraw(file);
  }
 }
 private void saveDraw(File outFile){
  try{
   ObjectOutputStream out=new ObjectOutputStream(new
         BufferedOutputStream(
         new FileOutputStream(outFile)));
   out.writeObject(theApp.getModel());
   out.flush();
   out.close();
  }
  catch(IOException e){
   System.out.println(e);
   JOptionPane.showMessageDialog(DrawFrame.this,"错误","保存失败",JOptionPane.ERROR_MESSAGE);
   return;
  }
  if(outFile!=modelFile){
   modelFile=outFile;
   filename=modelFile.getName();
   setTitle(frameTitle+modelFile.getPath());
  }
  changed=false;
 }
 public void openDraw(File inFile){
  try{
   ObjectInputStream in=new ObjectInputStream(new
         BufferedInputStream(
         new FileInputStream(inFile)));
   theApp.insertModel((DrawModel)in.readObject());
   in.close();
   modelFile=inFile;
   filename=modelFile.getName();
   setTitle(frameTitle+modelFile.getPath());
  }
  catch(Exception e){
   System.out.println(e);
   JOptionPane.showMessageDialog(DrawFrame.this,"错误","打开失败",JOptionPane.ERROR_MESSAGE);
   return;
  }
 }
 public void checkForSave(){
  if(changed)
   if(JOptionPane.YES_OPTION==JOptionPane.showConfirmDialog(DrawFrame.this,"文件已修改，保存？"
     ,"确认保存",JOptionPane.YES_NO_OPTION,JOptionPane.WARNING_MESSAGE))
    saveIt();
 }
}

package boya;
import java.io.*;
import java.util.*;
class DrawModel extends Observable implements Serializable{
 protected LinkedList elements=new LinkedList();
 public void add(Elements element){
  elements.add(element);
  setChanged();
  notifyObservers(element.getBounds());
 }
 public boolean remove(Elements element){
  boolean removed=elements.remove(element);
  if(removed){
   setChanged();
   notifyObservers(element.getBounds());
  }
  return removed;
 }
 public Iterator getIterator(){
  return elements.listIterator();
 }
}

package boya;
import java.awt.*;
import java.util.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
class DrawView extends JComponent implements Observer,ID{
 private Draw theApp;
 public DrawView(Draw theApp){
  this.theApp=theApp;
  MouseHandler handler=new MouseHandler();
  addMouseListener(handler);
  addMouseMotionListener(handler);
 }
 class MouseHandler extends MouseInputAdapter{
  private Point start,end;
  private Elements temp;
  private Graphics2D g2D;
  private Elements createElement(Point start,Point end){
   switch(theApp.getWindow().getElementType()){
    case LINE:
     return new Elements.Line(start,end,theApp.getWindow().getElementColor());
    case RECTANGLE:
     return new Elements.Rectangle(start,end,theApp.getWindow().getElementColor());
    case CIRCLE:
     return new Elements.Circle(start,end,theApp.getWindow().getElementColor());
    case CURVE:
     return new Elements.Curve(start,end,theApp.getWindow().getElementColor());
   }
   return null;
  }
  public void mousePressed(MouseEvent e){
   start=e.getPoint();
   int modifier=e.getModifiers();
   if((modifier&e.BUTTON1_MASK)!=0){
    g2D=(Graphics2D)getGraphics();
    g2D.setXORMode(getBackground());
    g2D.setPaint(theApp.getWindow().getElementColor());
   }
  }
  public void mouseDragged(MouseEvent e){
   end=e.getPoint();
   int modifier=e.getModifiers();
   if((modifier&e.BUTTON1_MASK)!=0&&(theApp.getWindow()
    .getElementType()!=TEXT)){
    if(temp==null)
     temp=createElement(start,end);
    else{
     temp.draw(g2D);
     temp.moving(start,end);
    }
    temp.draw(g2D);
   }
  }
  public void mouseReleased(MouseEvent e){
   int modifier=e.getModifiers();
   if(e.isPopupTrigger()){
    start=e.getPoint();
    theApp.getWindow().getPopup().show((Component)e.getSource(),
         start.x,start.y);
    start=null;
   }
   else if((modifier&e.BUTTON1_DOWN_MASK)==0&&(theApp.getWindow()
    .getElementType()!=TEXT)){
    if(temp!=null){
     theApp.getModel().add(temp);
     temp=null;
    }
    if(g2D!=null){
     g2D.dispose();
     g2D=null;
    }
    start=end=null;
   }
  }
  public void mouseClicked(MouseEvent e){
   int modifier=e.getModifiers();
   if((modifier&e.BUTTON1_MASK)!=0&&(theApp.getWindow()
    .getElementType()==TEXT)){
    start=e.getPoint();
    String text=JOptionPane.showInputDialog(
       (Component)e.getSource(),"输入文字"
       ,"文字",JOptionPane.PLAIN_MESSAGE);
    if(text!=null){
     g2D=(Graphics2D)getGraphics();
     Font font=theApp.getWindow().getFont();
     temp=new Elements.Text(font,text,start,
       theApp.getWindow().getElementColor(),
       font.getStringBounds(text,g2D.getFontRenderContext()).getBounds());
     temp.draw(g2D);
     if(temp!=null)
      theApp.getModel().add(temp);
     temp=null;
     g2D.dispose();
     g2D=null;
     start=null;
    }
   }
  }
   
 }
 public void update(Observable o,Object rectangle){
  if(rectangle==null)
   repaint();
  else repaint((Rectangle)rectangle);
 }
 public void paint(Graphics g){
  Graphics2D g2D=(Graphics2D)g;
  Iterator elements=theApp.getModel().getIterator();
  Elements element;
  while(elements.hasNext()){
   element=(Elements)elements.next();
   element.draw(g2D);
  }
 }
}

package boya;
import java.io.*;
import java.awt.*;
import java.util.*;
import java.awt.geom.*;
abstract class Elements implements Serializable{
 protected Color color;
 public Elements(Color color){
  this.color=color;
 }
 public Color getColor(){
  return color;
 }
 public abstract java.awt.Rectangle getBounds(