Java基础之处理事件——应用程序中的语义事件监听器（Sketcher 5 with element color listeners）

控制台程序。

为了标识元素的类型，可以为菜单已提供的4中元素定义常量，用作ID。这有助于执行菜单项监听器的操作，还提供了一种标识颜色类型的方式。我们会累积许多应用程序范围的常量，所以把它们定义为可以静态导入的类中的静态域会比较方便。为了导入静态域，类必须在指定的包中，所以先建立包。

为了把类放在Constants包中，需要在硬盘的合适位置建立以Constants命名的目录，然后在编译Constants包中的类时，使用-classpath选项标识Constants目录的路径。

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

 public class SketcherConstants {
   // 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;
 }

将上述代码保存为Constants目录下的SketcherConstants.java。类中的每个元素类型ID都是整数常量，并且值都是唯一的。

 // Main window for the Sketcher application
 import javax.swing.*;
 import static java.awt.event.InputEvent.*;                             // For modifier constants
 import java.awt.event.*;
 import java.awt.Color;
 import static java.awt.Color.*;
 import static Constants.SketcherConstants.*;

 @SuppressWarnings("serial")
 public class SketcherFrame extends JFrame {
   // Constructor
   public SketcherFrame(String title) {
     setTitle(title);                                                   // Set the window title
     setJMenuBar(menuBar);                                              // Add the menu bar to the window
     setDefaultCloseOperation(EXIT_ON_CLOSE);

     JMenu fileMenu = new JMenu("File");                                // Create File menu
     JMenu elementMenu = new JMenu("Elements");                         // Create Elements menu
     fileMenu.setMnemonic('F');                                         // Create shortcut
     elementMenu.setMnemonic('E');                                      // Create shortcut

     // Construct the file drop-down menu
     newItem = fileMenu.add("New");                                     // Add New item
     openItem = fileMenu.add("Open");                                   // Add Open item
     closeItem = fileMenu.add("Close");                                 // Add Close item
     fileMenu.addSeparator();                                           // Add separator
     saveItem = fileMenu.add("Save");                                   // Add Save item
     saveAsItem = fileMenu.add("Save As...");                           // Add Save As item
     fileMenu.addSeparator();                                           // Add separator
     printItem = fileMenu.add("Print");                                 // Add Print item

     // Add File menu accelerators
     newItem.setAccelerator(KeyStroke.getKeyStroke('N',CTRL_DOWN_MASK ));
     openItem.setAccelerator(KeyStroke.getKeyStroke('O', CTRL_DOWN_MASK));
     saveItem.setAccelerator(KeyStroke.getKeyStroke('S', CTRL_DOWN_MASK));
     printItem.setAccelerator(KeyStroke.getKeyStroke('P', CTRL_DOWN_MASK));

     // Construct the Elements drop-down menu
     elementMenu.add(lineItem = new JRadioButtonMenuItem("Line", elementType==LINE));
     elementMenu.add(rectangleItem = new JRadioButtonMenuItem("Rectangle", elementType==RECTANGLE));
     elementMenu.add(circleItem = new JRadioButtonMenuItem("Circle", elementType==CIRCLE));
     elementMenu.add(curveItem = new JRadioButtonMenuItem("Curve", elementType==CURVE));
     ButtonGroup types = new ButtonGroup();
     types.add(lineItem);
     types.add(rectangleItem);
     types.add(circleItem);
     types.add(curveItem);

     // Add element type accelerators
     lineItem.setAccelerator(KeyStroke.getKeyStroke('L', CTRL_DOWN_MASK));
     rectangleItem.setAccelerator(KeyStroke.getKeyStroke('E', CTRL_DOWN_MASK));
     circleItem.setAccelerator(KeyStroke.getKeyStroke('I', CTRL_DOWN_MASK));
     curveItem.setAccelerator(KeyStroke.getKeyStroke('V', CTRL_DOWN_MASK));

     elementMenu.addSeparator();
     JMenu colorMenu = new JMenu("Color");                              // Color submenu
     elementMenu.add(colorMenu);                                        // Add the submenu
     colorMenu.add(redItem = new JCheckBoxMenuItem("Red", elementColor.equals(RED)));
     colorMenu.add(yellowItem = new JCheckBoxMenuItem("Yellow", elementColor.equals(YELLOW)));
     colorMenu.add(greenItem = new JCheckBoxMenuItem("Green", elementColor.equals(GREEN)));
     colorMenu.add(blueItem = new JCheckBoxMenuItem("Blue", elementColor.equals(BLUE)));
     ButtonGroup colors = new ButtonGroup();                            // Color menu items button group
     colors.add(redItem);
     colors.add(yellowItem);
     colors.add(greenItem);
     colors.add(blueItem);

     // Add element color accelerators
     redItem.setAccelerator(KeyStroke.getKeyStroke('R', CTRL_DOWN_MASK));
     yellowItem.setAccelerator(KeyStroke.getKeyStroke('Y', CTRL_DOWN_MASK));
     greenItem.setAccelerator(KeyStroke.getKeyStroke('G', CTRL_DOWN_MASK));
     blueItem.setAccelerator(KeyStroke.getKeyStroke('B', CTRL_DOWN_MASK));

     // Add type menu item listeners
     lineItem.addActionListener(new TypeListener(LINE));
     rectangleItem.addActionListener(new TypeListener(RECTANGLE));
     circleItem.addActionListener(new TypeListener(CIRCLE));
     curveItem.addActionListener(new TypeListener(CURVE));

     // Add color menu item listeners
     redItem.addActionListener(new ColorListener(RED));
     yellowItem.addActionListener(new ColorListener(YELLOW));
     greenItem.addActionListener(new ColorListener(GREEN));
     blueItem.addActionListener(new ColorListener(BLUE));

     menuBar.add(fileMenu);                                             // Add the file menu
     menuBar.add(elementMenu);                                          // Add the element menu
   }

   // Handles element type menu items
   class TypeListener implements ActionListener {
     // Constructor
     TypeListener(int type) {
       this.type = type;
     }

     // Sets the element type
     public void actionPerformed(ActionEvent e) {
       elementType = type;
     }

     private int type;                                                  // Store the type for the menu
   }

   // Handles color menu items
   class ColorListener implements ActionListener {
     public ColorListener(Color color) {
       this.color = color;
     }

     public void actionPerformed(ActionEvent e) {
       elementColor = color;
     }

     private Color color;
   }

   private JMenuBar menuBar = new JMenuBar();                           // Window menu bar

   // File menu items
   private JMenuItem newItem,  openItem,   closeItem,
                     saveItem, saveAsItem, printItem;

   // Element menu items
   private JRadioButtonMenuItem lineItem,  rectangleItem, circleItem,   // Types
                                curveItem, textItem;
   private JCheckBoxMenuItem    redItem,   yellowItem,                  // Colors
                                greenItem, blueItem ;
   private Color elementColor = DEFAULT_ELEMENT_COLOR;                  // Current element color
   private int elementType = DEFAULT_ELEMENT_TYPE;                      // Current element type
 }

Sketcher类和以前一样：

 // 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");                            // 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
     window.setVisible(true);
   }

   // Handler class for window events
   class WindowHandler extends WindowAdapter {
     // Handler for window closing event
     @Override
     public void windowClosing(WindowEvent e) {
       window.dispose();                                                // Release the window resources
       System.exit(0);                                                  // End the application
     }
   }

   private SketcherFrame window;                                        // The application window
   private static Sketcher theApp;                                      // The application object
 }

构建元素对象时，使用elementType和elementColor成员来设置每个菜单项的状态。只选中了对应于elementType中设置的默认类型的元素类型菜单项，因为只有这个比较生成了true，作为JRadioButtonMenuItem构造函数的参数。
因为现在有自己的包来包含SketcherConstants类定义，所以必须使用-classpath选项，告诉编译器在哪里能找到这个类。假定Constants目录是C:/Packages目录的子目录，当前目录中包含Sketcher.java和SketcherFrame.java文件，可以使用下列命令编译Sketcher:

javac -classpath ".;c:/Packages" Sketcher.java

-classpath选项定义了两条路径：当前路径，由句点指定；C:/Packages，这是包含源文件Sketcher.java的Constants目录的路径。这个命令应编译所有的代码，包括自定义的包。