Java基础之在窗口中绘图——利用多态性使用鼠标自由绘图（Sketcher 7 with a crosshair cursor）

控制台程序。

在Sketcher中创建形状时，并不知道应该以什么顺序创建不同类型的形状，这完全取决于使用Sketcher程序生成草图的人。因此需要绘制形状，对它们执行其他操作而不必知道图形是什么。当然，多态性在这里是有帮助的。

最简单的方法可能是为Sketcher形状类定义通用的基类，并在每个形状类中包含一个成员来存储某种java.awt.geom形状对象，接着把对形状类对象的引用存储为基类类型以获得多态行为。

1、通用基类：

 import java.awt.*;
 import java.io.Serializable;
 import static Constants.SketcherConstants.*;
 import java.awt.geom.*;

 public abstract class Element implements Serializable{

   public Element(Point position, Color color) {
     this.position = new Point(position);
     this.color = color;
   }

   protected Element(Color color) {
     this.color = color;
   }

   // Returns the color of the element
   public Color getColor() {
     return color;
   }

   // Returns the position of the element
   public Point getPosition() {
     return position;
   }

   // Returns the bounding rectangle enclosing an element boundary
   public java.awt.Rectangle getBounds() {
     return bounds;
   }

   // Create a new element
   public static Element createElement(int type, Color color, Point start, Point end) {
     switch(type) {
       case LINE:
         return new Element.Line(start, end, color);
       case RECTANGLE:
          return new Rectangle(start, end, color);
       case CIRCLE:
         return new Circle(start, end, color);
       case CURVE:
         return new Curve(start, end, color);
       default:
        assert false;                                                   // We should never get to here
     }
     return null;
   }

   // Nested class defining a line
   public static class Line extends Element {
     public Line(Point start, Point end, Color color) {
       super(start, color);
       line = new Line2D.Double(origin.x, origin.y, end.x - position.x, end.y - position.y);
       bounds = new java.awt.Rectangle(
                      Math.min(start.x ,end.x),    Math.min(start.y, end.y),
                      Math.abs(start.x - end.x)+1, Math.abs(start.y - end.y)+1);
     }

     // Change the end point for the line
     public void modify(Point start, Point last) {
       line.x2 = last.x - position.x;
       line.y2 = last.y - position.y;
       bounds = new java.awt.Rectangle(
                    Math.min(start.x ,last.x),    Math.min(start.y, last.y),
                    Math.abs(start.x - last.x)+1, Math.abs(start.y - last.y)+1);
     }

     // Display the line
     public  void draw(Graphics2D g2D) {
      g2D.setPaint(color);                                              // Set the line color
      g2D.translate(position.x, position.y);                            // Move context origin
      g2D.draw(line);                                                   // Draw the line
      g2D.translate(-position.x, -position.y);                          // Move context origin back
     }
     private Line2D.Double line;
     private final static long serialVersionUID = 1001L;
   }

   // Nested class defining a rectangle
   public static class Rectangle extends Element {
     public Rectangle(Point start, Point end, Color color) {
       super(new Point(Math.min(start.x, end.x), Math.min(start.y, end.y)), color);
       rectangle = new Rectangle2D.Double(
         origin.x, origin.y,                                            // Top-left corner
         Math.abs(start.x - end.x), Math.abs(start.y - end.y));         // Width & height
       bounds = new java.awt.Rectangle(
                     Math.min(start.x ,end.x),    Math.min(start.y, end.y),
                     Math.abs(start.x - end.x)+1, Math.abs(start.y - end.y)+1);
     }

     // Display the rectangle
     public  void draw(Graphics2D g2D) {
       g2D.setPaint(color);                                             // Set the rectangle color
       g2D.translate(position.x, position.y);                           // Move context origin
       g2D.draw(rectangle);                                             // Draw the rectangle
       g2D.translate(-position.x, -position.y);                         // Move context origin back
     }

     // Method to redefine the rectangle
     public void modify(Point start, Point last) {
       bounds.x = position.x = Math.min(start.x, last.x);
       bounds.y = position.y = Math.min(start.y, last.y);
       rectangle.width = Math.abs(start.x - last.x);
       rectangle.height = Math.abs(start.y - last.y);
       bounds.width = (int)rectangle.width +1;
       bounds.height = (int)rectangle.height + 1;
     }

     private Rectangle2D.Double rectangle;
     private final static long serialVersionUID = 1001L;
   }

   // Nested class defining a circle
   public static class Circle extends Element {
     public Circle(Point center, Point circum, Color color) {
       super(color);

       // Radius is distance from center to circumference
       double radius = center.distance(circum);
       position = new Point(center.x - (int)radius, center.y - (int)radius);
       circle = new Ellipse2D.Double(origin.x, origin.y, 2.*radius, 2.*radius);
       bounds = new java.awt.Rectangle(position.x, position.y,
                               1 + (int)circle.width, 1+(int)circle.height);
     }

     // Display the circle
     public  void draw(Graphics2D g2D) {
       g2D.setPaint(color);                                             // Set the circle color
       g2D.translate(position.x, position.y);                           // Move context origin
       g2D.draw(circle);                                                // Draw the circle
       g2D.translate(-position.x, -position.y);                         // Move context origin back
     }

     // Recreate this circle
     public void modify(Point center, Point circum) {
       double radius = center.distance(circum);
       circle.width = circle.height = 2*radius;
       position.x = center.x - (int)radius;
       position.y = center.y - (int)radius;
       bounds = new java.awt.Rectangle(position.x, position.y,
                               1 + (int)circle.width, 1+(int)circle.height);
     }

     private Ellipse2D.Double circle;
     private final static long serialVersionUID = 1001L;
   }

   // Nested class defining a curve
   public static class Curve extends Element {
     public Curve(Point start, Point next, Color color) {
       super(start, color);
       curve = new GeneralPath();
       curve.moveTo(origin.x, origin.y);                                // Set current position as origin
       curve.lineTo(next.x - position.x, next.y - position.y);          // Add segment
       bounds = new java.awt.Rectangle(
                    Math.min(start.x ,next.x),    Math.min(start.y, next.y),
                    Math.abs(next.x - start.x)+1, Math.abs(next.y - start.y)+1);
     }

     // Add another segment
     public void modify(Point start, Point next) {
       curve.lineTo(next.x - position.x, next.y - position.y);          // Add segment
       bounds.add(new java.awt.Rectangle(next.x,next.y, 1, 1));         // Extend bounds
     }

     // Display the curve
     public  void draw(Graphics2D g2D) {
      g2D.setPaint(color);                                              // Set the curve color
      g2D.translate(position.x, position.y);                            // Move context origin
      g2D.draw(curve);                                                  // Draw the curve
      g2D.translate(-position.x, -position.y);                          // Move context origin back
     }

     private GeneralPath curve;
     private final static long serialVersionUID = 1001L;
   }

   // Abstract Element class methods
   public abstract void draw(Graphics2D g2D);
   public abstract void modify(Point start, Point last);

   // Element class fields
   protected Point position;                                            // Position of a shape
   protected Color color;                                               // Color of a shape
   protected java.awt.Rectangle bounds;                                 // Bounding rectangle
   protected static final Point origin = new Point();                   // Origin for elements
   private final static long serialVersionUID = 1001L;
 }

2、实现了链表和Iterable<Element>接口的模型：

 import java.io.Serializable;
 import java.util.*;

 public class SketcherModel extends Observable implements Serializable, Iterable<Element> {

   //Remove an element from the sketch
   public boolean remove(Element element) {
     boolean removed = elements.remove(element);
     if(removed) {
       setChanged();
       notifyObservers(element.getBounds());
     }
     return removed;
   }

   //Add an element to the sketch
   public void add(Element element) {
     elements.add(element);
     setChanged();
     notifyObservers(element.getBounds());
   }

   // Get iterator for sketch elements
   public Iterator<Element> iterator() {
     return elements.iterator();
   }

   protected LinkedList<Element> elements = new LinkedList<>();
   private final static long serialVersionUID = 1001L;
 }

3、视图：

 import javax.swing.JComponent;
 import java.util.*;
 import java.awt.*;
 import java.awt.event.MouseEvent;
 import javax.swing.event.MouseInputAdapter;

 @SuppressWarnings("serial")
 public class SketcherView extends JComponent implements Observer {
   public SketcherView(Sketcher theApp) {
     this.theApp = theApp;
     MouseHandler handler = new MouseHandler();                         // create the mouse listener
     addMouseListener(handler);                                         // Listen for button events
     addMouseMotionListener(handler);                                   // Listen for motion events
   }

   // Method called by Observable object when it changes
   public void update(Observable o, Object rectangle) {
     if(rectangle != null) {
       repaint((java.awt.Rectangle)rectangle);
     } else {
       repaint();
     }
   }

   // Method to draw on the view
   @Override
   public void paint(Graphics g) {
     Graphics2D g2D = (Graphics2D)g;                                    // Get a 2D device context
     for(Element element: theApp.getModel()) {                          // For each element in the model
       element.draw(g2D);                                               // ...draw the element
     }
   }

   class MouseHandler extends MouseInputAdapter {
     @Override
     public void mousePressed(MouseEvent e)  {
       start = e.getPoint();                                            // Save the cursor position in start
       buttonState = e.getButton();                                     // Record which button was pressed
       if(buttonState == MouseEvent.BUTTON1) {
         g2D = (Graphics2D)getGraphics();                               // Get graphics context
         g2D.setXORMode(getBackground());                               // Set XOR mode
       }
     }

     @Override
     public void mouseDragged(MouseEvent e) {
       last = e.getPoint();                                             // Save cursor position

       if(buttonState == MouseEvent.BUTTON1) {
         if(tempElement == null) {                                      // Is there an element?
           tempElement = Element.createElement(                         // No, so create one
                                  theApp.getWindow().getElementType(),
                                  theApp.getWindow().getElementColor(),
                                  start, last);
         } else {
           tempElement.draw(g2D);                                       // Yes draw to erase it
           tempElement.modify(start, last);                             // Now modify it
         }
         tempElement.draw(g2D);                                         // and draw it
      }
     }

     @Override
     public void mouseReleased(MouseEvent e) {
       if(e.getButton() == MouseEvent.BUTTON1) {
         buttonState = MouseEvent.NOBUTTON;                             // Reset the button state

         if(tempElement != null) {                                      // If there is an element...
           theApp.getModel().add(tempElement);                          // ...add it to the model...
           tempElement = null;                                          // ...and reset the field
         }
         if(g2D != null) {                                              // If there's a graphics context
           g2D.dispose();                                               // ...release the resource...
           g2D = null;                                                  // ...and reset field to null
         }
         start = last = null;                                           // Remove any points
       }
     }

     @Override
     public void mouseEntered(MouseEvent e) {
       setCursor(Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR));
     }

     @Override
     public void mouseExited(MouseEvent e) {
       setCursor(Cursor.getDefaultCursor());
     }

     private Point start;                                               // Stores cursor position on press
     private Point last;                                                // Stores cursor position on drag
     private Element tempElement = null;                                // Stores a temporary element
     private int buttonState = MouseEvent.NOBUTTON;                     // Records button state
     private Graphics2D g2D = null;                                     // Temporary graphics context
    }

   private Sketcher theApp;                                             // The application object
 }

4、窗口框架：

 // Frame for the Sketcher application
 import javax.swing.*;
 import javax.swing.border.*;
 import java.awt.event.*;
 import java.awt.*;

 import static java.awt.event.InputEvent.*;
 import static java.awt.AWTEvent.*;
 import static java.awt.Color.*;
 import static Constants.SketcherConstants.*;
 import static javax.swing.Action.*;

 @SuppressWarnings("serial")
 public class SketcherFrame extends JFrame {
   // Constructor
   public SketcherFrame(String title, Sketcher theApp) {
     setTitle(title);                                                   // Set the window title
     this.theApp = theApp;                                              // Save app. object reference
     setJMenuBar(menuBar);                                              // Add the menu bar to the window
     setDefaultCloseOperation(EXIT_ON_CLOSE);                           // Default is exit the application

     createFileMenu();                                                  // Create the File menu
     createElementMenu();                                               // Create the element menu
     createColorMenu();                                                 // Create the element menu
     createToolbar();
     toolBar.setRollover(true);
     getContentPane().add(toolBar, BorderLayout.NORTH);
   }

   // Create File menu item actions
   private void createFileMenuActions() {
     newAction = new FileAction("New", 'N', CTRL_DOWN_MASK);
     openAction = new FileAction("Open", 'O', CTRL_DOWN_MASK);
     closeAction = new FileAction("Close");
     saveAction = new FileAction("Save", 'S', CTRL_DOWN_MASK);
     saveAsAction = new FileAction("Save As...");
     printAction = new FileAction("Print", 'P', CTRL_DOWN_MASK);
     exitAction = new FileAction("Exit", 'X', CTRL_DOWN_MASK);

     // Initialize the array
     FileAction[] actions = {openAction, closeAction, saveAction, saveAsAction, printAction, exitAction};
     fileActions = actions;

     // Add toolbar icons
     newAction.putValue(LARGE_ICON_KEY, NEW24);
     openAction.putValue(LARGE_ICON_KEY, OPEN24);
     saveAction.putValue(LARGE_ICON_KEY, SAVE24);
     saveAsAction.putValue(LARGE_ICON_KEY, SAVEAS24);
     printAction.putValue(LARGE_ICON_KEY, PRINT24);

     // Add menu item icons
     newAction.putValue(SMALL_ICON, NEW16);
     openAction.putValue(SMALL_ICON, OPEN16);
     saveAction.putValue(SMALL_ICON, SAVE16);
     saveAsAction.putValue(SMALL_ICON,SAVEAS16);
     printAction.putValue(SMALL_ICON, PRINT16);

     // Add tooltip text
     newAction.putValue(SHORT_DESCRIPTION, "Create a new sketch");
     openAction.putValue(SHORT_DESCRIPTION, "Read a sketch from a file");
     closeAction.putValue(SHORT_DESCRIPTION, "Close the current sketch");
     saveAction.putValue(SHORT_DESCRIPTION, "Save the current sketch to file");
     saveAsAction.putValue(SHORT_DESCRIPTION, "Save the current sketch to a new file");
     printAction.putValue(SHORT_DESCRIPTION, "Print the current sketch");
     exitAction.putValue(SHORT_DESCRIPTION, "Exit Sketcher");
   }

   // Create the File menu
   private void createFileMenu() {
     JMenu fileMenu = new JMenu("File");                                // Create File menu
     fileMenu.setMnemonic('F');                                         // Create shortcut
     createFileMenuActions();                                           // Create Actions for File menu item

     // Construct the file drop-down menu
     fileMenu.add(newAction);                                           // New Sketch menu item
     fileMenu.add(openAction);                                          // Open sketch menu item
     fileMenu.add(closeAction);                                         // Close sketch menu item
     fileMenu.addSeparator();                                           // Add separator
     fileMenu.add(saveAction);                                          // Save sketch to file
     fileMenu.add(saveAsAction);                                        // Save As menu item
     fileMenu.addSeparator();                                           // Add separator
     fileMenu.add(printAction);                                         // Print sketch menu item
     fileMenu.addSeparator();                                           // Add separator
     fileMenu.add(exitAction);                                          // Print sketch menu item
     menuBar.add(fileMenu);                                             // Add the file menu
   }

   // Create Element  menu actions
   private void createElementTypeActions() {
     lineAction = new TypeAction("Line", LINE, 'L', CTRL_DOWN_MASK);
     rectangleAction = new TypeAction("Rectangle", RECTANGLE, 'R', CTRL_DOWN_MASK);
     circleAction =  new TypeAction("Circle", CIRCLE,'C', CTRL_DOWN_MASK);
     curveAction = new TypeAction("Curve", CURVE,'U', CTRL_DOWN_MASK);

     // Initialize the array
     TypeAction[] actions = {lineAction, rectangleAction, circleAction, curveAction};
     typeActions = actions;

     // Add toolbar icons
     lineAction.putValue(LARGE_ICON_KEY, LINE24);
     rectangleAction.putValue(LARGE_ICON_KEY, RECTANGLE24);
     circleAction.putValue(LARGE_ICON_KEY, CIRCLE24);
     curveAction.putValue(LARGE_ICON_KEY, CURVE24);

     // Add menu item icons
     lineAction.putValue(SMALL_ICON, LINE16);
     rectangleAction.putValue(SMALL_ICON, RECTANGLE16);
     circleAction.putValue(SMALL_ICON, CIRCLE16);
     curveAction.putValue(SMALL_ICON, CURVE16);

     // Add tooltip text
     lineAction.putValue(SHORT_DESCRIPTION, "Draw lines");
     rectangleAction.putValue(SHORT_DESCRIPTION, "Draw rectangles");
     circleAction.putValue(SHORT_DESCRIPTION, "Draw circles");
     curveAction.putValue(SHORT_DESCRIPTION, "Draw curves");
   }

   // Create the Elements menu
   private void createElementMenu() {
     createElementTypeActions();
     elementMenu = new JMenu("Elements");                               // Create Elements menu
     elementMenu.setMnemonic('E');                                      // Create shortcut
     createRadioButtonDropDown(elementMenu, typeActions, lineAction);
     menuBar.add(elementMenu);                                          // Add the element menu
   }

   // Create Color menu actions
   private void createElementColorActions() {
     redAction = new ColorAction("Red", RED, 'R', CTRL_DOWN_MASK|ALT_DOWN_MASK);
     yellowAction = new ColorAction("Yellow", YELLOW, 'Y', CTRL_DOWN_MASK|ALT_DOWN_MASK);
     greenAction = new ColorAction("Green", GREEN, 'G', CTRL_DOWN_MASK|ALT_DOWN_MASK);
     blueAction = new ColorAction("Blue", BLUE, 'B', CTRL_DOWN_MASK|ALT_DOWN_MASK);

     // Initialize the array
     ColorAction[] actions = {redAction, greenAction, blueAction, yellowAction};
     colorActions = actions;

     // Add toolbar icons
     redAction.putValue(LARGE_ICON_KEY, RED24);
     greenAction.putValue(LARGE_ICON_KEY, GREEN24);
     blueAction.putValue(LARGE_ICON_KEY, BLUE24);
     yellowAction.putValue(LARGE_ICON_KEY, YELLOW24);

     // Add menu item icons
     redAction.putValue(SMALL_ICON, RED16);
     blueAction.putValue(SMALL_ICON, BLUE16);
     greenAction.putValue(SMALL_ICON, GREEN16);
     yellowAction.putValue(SMALL_ICON, YELLOW16);

     // Add tooltip text
     redAction.putValue(SHORT_DESCRIPTION, "Draw in red");
     blueAction.putValue(SHORT_DESCRIPTION, "Draw in blue");
     greenAction.putValue(SHORT_DESCRIPTION, "Draw in green");
     yellowAction.putValue(SHORT_DESCRIPTION, "Draw in yellow");
   }

   // Create the Color menu
   private void createColorMenu() {
     createElementColorActions();
     colorMenu = new JMenu("Color");                                    // Create Elements menu
     colorMenu.setMnemonic('C');                                        // Create shortcut
     createRadioButtonDropDown(colorMenu, colorActions, blueAction);
     menuBar.add(colorMenu);                                            // Add the color menu
   }

   // Menu creation helper
   private void createRadioButtonDropDown(JMenu menu, Action[] actions, Action selected) {
     ButtonGroup group = new ButtonGroup();
     JRadioButtonMenuItem item = null;
     for(Action action : actions) {
       group.add(menu.add(item = new JRadioButtonMenuItem(action)));
       if(action == selected) {
         item.setSelected(true);                                        // This is default selected
       }
     }
   }

   // Create toolbar buttons on the toolbar
   private void createToolbar() {
     for(FileAction action: fileActions){
       if(action != exitAction && action != closeAction)
         addToolbarButton(action);                                      // Add the toolbar button
     }
     toolBar.addSeparator();

     // Create Color menu buttons
     for(ColorAction action:colorActions){
         addToolbarButton(action);                                      // Add the toolbar button
     }

     toolBar.addSeparator();

     // Create Elements menu buttons
     for(TypeAction action:typeActions){
         addToolbarButton(action);                                      // Add the toolbar button
     }
  }

   // Create and add a toolbar button
   private void addToolbarButton(Action action) {
     JButton button = new JButton(action);                              // Create from Action
     button.setBorder(BorderFactory.createCompoundBorder(               // Add button border
            new EmptyBorder(2,5,5,2),                                   // Outside border
            BorderFactory.createRaisedBevelBorder()));                  // Inside border
     button.setHideActionText(true);                                    // No label on the button
     toolBar.add(button);                                               // Add the toolbar button
   }

   // Return the current drawing color
   public Color getElementColor() {
     return elementColor;
   }

   // Return the current element type
   public int getElementType() {
     return elementType;
   }

   // Set radio button menu checks
   private void setChecks(JMenu menu, Object eventSource) {
     if(eventSource instanceof JButton){
       JButton button = (JButton)eventSource;
       Action action = button.getAction();
       for(int i = 0 ; i<menu.getItemCount() ; ++i) {
         JMenuItem item = menu.getItem(i);
         item.setSelected(item.getAction() == action);
       }
     }
   }

   // Inner class defining Action objects for File menu items
   class FileAction extends AbstractAction {
     // Create action with a name
     FileAction(String name) {
       super(name);
     }

     // Create action with a name and accelerator
     FileAction(String name, char ch, int modifiers) {
       super(name);
       putValue(ACCELERATOR_KEY, KeyStroke.getKeyStroke(ch, modifiers));

       // Now find the character to underline
       int index = name.toUpperCase().indexOf(ch);
       if(index != -1) {
         putValue(DISPLAYED_MNEMONIC_INDEX_KEY, index);
       }
     }

     // Event handler
     public void actionPerformed(ActionEvent e) {
       // You will add action code here eventually...
     }
   }

   // Inner class defining Action objects for Element type menu items
   class TypeAction extends AbstractAction {
     // Create action with just a name property
     TypeAction(String name, int typeID) {
       super(name);
       this.typeID = typeID;
     }

     // Create action with a name and an accelerator
     private TypeAction(String name,int typeID, char ch, int modifiers) {
       this(name, typeID);
       putValue(ACCELERATOR_KEY, KeyStroke.getKeyStroke(ch, modifiers));

       // Now find the character to underline
       int index = name.toUpperCase().indexOf(ch);
       if(index != -1) {
         putValue(DISPLAYED_MNEMONIC_INDEX_KEY, index);
       }
     }

     public void actionPerformed(ActionEvent e) {
       elementType = typeID;
       setChecks(elementMenu, e.getSource());
     }

     private int typeID;
   }

   // Handles color menu items
   class ColorAction  extends AbstractAction {
     // Create an action with a name and a color
     public ColorAction(String name, Color color) {
       super(name);
       this.color = color;
     }

     // Create an action with a name, a color, and an accelerator
     public ColorAction(String name, Color color, char ch, int modifiers) {
       this(name, color);
       putValue(ACCELERATOR_KEY, KeyStroke.getKeyStroke(ch, modifiers));

       // Now find the character to underline
       int index = name.toUpperCase().indexOf(ch);
       if(index != -1) {
         putValue(DISPLAYED_MNEMONIC_INDEX_KEY, index);
       }
     }

     public void actionPerformed(ActionEvent e) {
       elementColor = color;
       setChecks(colorMenu, e.getSource());
     }

     private Color color;
   }

   // File actions
   private FileAction newAction, openAction, closeAction, saveAction, saveAsAction, printAction, exitAction;
   private FileAction[] fileActions;                                    // File actions as an array

   // Element type actions
   private TypeAction lineAction, rectangleAction, circleAction, curveAction;
   private TypeAction[] typeActions;                                    // Type actions as an array

 // Element color actions
   private ColorAction redAction, yellowAction,greenAction, blueAction;
   private ColorAction[] colorActions;                                  // Color actions as an array

   private JMenuBar menuBar = new JMenuBar();                           // Window menu bar
   private JMenu elementMenu;                                           // Elements menu
   private JMenu colorMenu;                                             // Color menu

   private Color elementColor = DEFAULT_ELEMENT_COLOR;                  // Current element color
   private int elementType = DEFAULT_ELEMENT_TYPE;                      // Current element type
   private JToolBar toolBar = new JToolBar();                           // Window toolbar
   private Sketcher theApp;                                             // The application object
 }

5、自定义常量包：

 // Defines application wide constants
 package Constants;
 import java.awt.Color;
 import javax.swing.*;

 public class SketcherConstants {
   // Path for images
   public final static String imagePath = "E:/JavaProject/BeginningJava/Images/";

   // Toolbar icons
   public final static Icon NEW24 = new ImageIcon(imagePath + "New24.gif");
   public final static Icon OPEN24 = new ImageIcon(imagePath + "Open24.gif");
   public final static Icon SAVE24 = new ImageIcon(imagePath + "Save24.gif");
   public final static Icon SAVEAS24 = new ImageIcon(imagePath + "SaveAs24.gif");
   public final static Icon PRINT24 = new ImageIcon(imagePath + "Print24.gif");

   public final static Icon LINE24 = new ImageIcon(imagePath + "Line24.gif");
   public final static Icon RECTANGLE24 = new ImageIcon(imagePath + "Rectangle24.gif");
   public final static Icon CIRCLE24 = new ImageIcon(imagePath + "Circle24.gif");
   public final static Icon CURVE24 = new ImageIcon(imagePath + "Curve24.gif");

   public final static Icon RED24 = new ImageIcon(imagePath + "Red24.gif");
   public final static Icon GREEN24 = new ImageIcon(imagePath + "Green24.gif");
   public final static Icon BLUE24 = new ImageIcon(imagePath + "Blue24.gif");
   public final static Icon YELLOW24 = new ImageIcon(imagePath + "Yellow24.gif");

   // Menu item icons
   public final static Icon NEW16 = new ImageIcon(imagePath + "new16.gif");
   public final static Icon OPEN16 = new ImageIcon(imagePath + "Open16.gif");
   public final static Icon SAVE16 = new ImageIcon(imagePath + "Save16.gif");
   public final static Icon SAVEAS16 = new ImageIcon(imagePath + "SaveAs16.gif");
   public final static Icon PRINT16 = new ImageIcon(imagePath + "print16.gif");

   public final static Icon LINE16 = new ImageIcon(imagePath + "Line16.gif");
   public final static Icon RECTANGLE16 = new ImageIcon(imagePath + "Rectangle16.gif");
   public final static Icon CIRCLE16 = new ImageIcon(imagePath + "Circle16.gif");
   public final static Icon CURVE16 = new ImageIcon(imagePath + "Curve16.gif");

   public final static Icon RED16 = new ImageIcon(imagePath + "Red16.gif");
   public final static Icon GREEN16 = new ImageIcon(imagePath + "Green16.gif");
   public final static Icon BLUE16 = new ImageIcon(imagePath + "Blue16.gif");
   public final static Icon YELLOW16 = new ImageIcon(imagePath + "Yellow16.gif");

   // Element type definitions
   public final static int LINE      = 101;
   public final static int RECTANGLE = 102;
   public final static int CIRCLE    = 103;
   public final static int CURVE     = 104;

   // Initial conditions
   public final static int DEFAULT_ELEMENT_TYPE = LINE;
   public final static Color DEFAULT_ELEMENT_COLOR = Color.BLUE;
 }

6、应用程序主程序：

 // Sketching application
 import javax.swing.*;
 import java.awt.*;
 import java.awt.event.*;

 public class Sketcher {
   public static void main(String[] args) {
      theApp = new Sketcher();                                          // Create the application object
    SwingUtilities.invokeLater(new Runnable() {
             public void run() {
                           theApp.createGUI();                          // Call GUI creator
             }
         });
   }

   // Method to create the application GUI
   private void createGUI() {
     window = new SketcherFrame("Sketcher", this);                      // Create the app window
     Toolkit theKit = window.getToolkit();                              // Get the window toolkit
     Dimension wndSize = theKit.getScreenSize();                        // Get screen size

     // Set the position to screen center & size to half screen size
     window.setSize(wndSize.width/2, wndSize.height/2);                 // Set window size
     window.setLocationRelativeTo(null);                                // Center window

     window.addWindowListener(new WindowHandler());                     // Add window listener

     sketch = new SketcherModel();                                      // Create the model
     view = new SketcherView(this);                                     // Create the view
     sketch.addObserver(view);                                          // Register view with the model
     window.getContentPane().add(view, BorderLayout.CENTER);
     window.setVisible(true);
   }

   // Return a reference to the application window
   public SketcherFrame getWindow() {
      return window;
   }

   // Return a reference to the model
   public SketcherModel getModel() {
      return sketch;
   }

   // Return a reference to the view
   public SketcherView getView() {
      return view;
   }

   // Handler class for window events
   class WindowHandler extends WindowAdapter {
     // Handler for window closing event
     @Override
     public void windowClosing(WindowEvent e) {
       // Code to be added here...
     }
   }

   private SketcherModel sketch;                                        // The data model for the sketch
   private SketcherView view;                                           // The view of the sketch
   private SketcherFrame window;                                        // The application window
   private static Sketcher theApp;                                      // The application object
 }