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。

 1 // Defines application wide constants
 2 package Constants;
 3 import java.awt.Color;
 4 
 5 public class SketcherConstants {
 6   // Element type definitions
 7   public final static int LINE      = 101;
 8   public final static int RECTANGLE = 102;
 9   public final static int CIRCLE    = 103;
10   public final static int CURVE     = 104;
11 
12   // Initial conditions
13   public final static int DEFAULT_ELEMENT_TYPE = LINE;
14   public final static Color DEFAULT_ELEMENT_COLOR = Color.BLUE;
15 }

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。

  1 // Main window for the Sketcher application
  2 import javax.swing.*;
  3 import static java.awt.event.InputEvent.*;                             // For modifier constants
  4 import java.awt.event.*;
  5 import java.awt.Color;
  6 import static java.awt.Color.*;
  7 import static Constants.SketcherConstants.*;
  8 
  9 @SuppressWarnings("serial")
 10 public class SketcherFrame extends JFrame {
 11   // Constructor
 12   public SketcherFrame(String title) {
 13     setTitle(title);                                                   // Set the window title
 14     setJMenuBar(menuBar);                                              // Add the menu bar to the window
 15     setDefaultCloseOperation(EXIT_ON_CLOSE);
 16 
 17 
 18     JMenu fileMenu = new JMenu("File");                                // Create File menu
 19     JMenu elementMenu = new JMenu("Elements");                         // Create Elements menu
 20     fileMenu.setMnemonic('F');                                         // Create shortcut
 21     elementMenu.setMnemonic('E');                                      // Create shortcut
 22 
 23     // Construct the file drop-down menu
 24     newItem = fileMenu.add("New");                                     // Add New item
 25     openItem = fileMenu.add("Open");                                   // Add Open item
 26     closeItem = fileMenu.add("Close");                                 // Add Close item
 27     fileMenu.addSeparator();                                           // Add separator
 28     saveItem = fileMenu.add("Save");                                   // Add Save item
 29     saveAsItem = fileMenu.add("Save As...");                           // Add Save As item
 30     fileMenu.addSeparator();                                           // Add separator
 31     printItem = fileMenu.add("Print");                                 // Add Print item
 32 
 33     // Add File menu accelerators
 34     newItem.setAccelerator(KeyStroke.getKeyStroke('N',CTRL_DOWN_MASK ));
 35     openItem.setAccelerator(KeyStroke.getKeyStroke('O', CTRL_DOWN_MASK));
 36     saveItem.setAccelerator(KeyStroke.getKeyStroke('S', CTRL_DOWN_MASK));
 37     printItem.setAccelerator(KeyStroke.getKeyStroke('P', CTRL_DOWN_MASK));
 38 
 39     // Construct the Elements drop-down menu
 40     elementMenu.add(lineItem = new JRadioButtonMenuItem("Line", elementType==LINE));
 41     elementMenu.add(rectangleItem = new JRadioButtonMenuItem("Rectangle", elementType==RECTANGLE));
 42     elementMenu.add(circleItem = new JRadioButtonMenuItem("Circle", elementType==CIRCLE));
 43     elementMenu.add(curveItem = new JRadioButtonMenuItem("Curve", elementType==CURVE));
 44     ButtonGroup types = new ButtonGroup();
 45     types.add(lineItem);
 46     types.add(rectangleItem);
 47     types.add(circleItem);
 48     types.add(curveItem);
 49 
 50     // Add element type accelerators
 51     lineItem.setAccelerator(KeyStroke.getKeyStroke('L', CTRL_DOWN_MASK));
 52     rectangleItem.setAccelerator(KeyStroke.getKeyStroke('E', CTRL_DOWN_MASK));
 53     circleItem.setAccelerator(KeyStroke.getKeyStroke('I', CTRL_DOWN_MASK));
 54     curveItem.setAccelerator(KeyStroke.getKeyStroke('V', CTRL_DOWN_MASK));
 55 
 56     elementMenu.addSeparator();
 57     JMenu colorMenu = new JMenu("Color");                              // Color submenu
 58     elementMenu.add(colorMenu);                                        // Add the submenu
 59     colorMenu.add(redItem = new JCheckBoxMenuItem("Red", elementColor.equals(RED)));
 60     colorMenu.add(yellowItem = new JCheckBoxMenuItem("Yellow", elementColor.equals(YELLOW)));
 61     colorMenu.add(greenItem = new JCheckBoxMenuItem("Green", elementColor.equals(GREEN)));
 62     colorMenu.add(blueItem = new JCheckBoxMenuItem("Blue", elementColor.equals(BLUE)));
 63     ButtonGroup colors = new ButtonGroup();                            // Color menu items button group
 64     colors.add(redItem);
 65     colors.add(yellowItem);
 66     colors.add(greenItem);
 67     colors.add(blueItem);
 68 
 69     // Add element color accelerators
 70     redItem.setAccelerator(KeyStroke.getKeyStroke('R', CTRL_DOWN_MASK));
 71     yellowItem.setAccelerator(KeyStroke.getKeyStroke('Y', CTRL_DOWN_MASK));
 72     greenItem.setAccelerator(KeyStroke.getKeyStroke('G', CTRL_DOWN_MASK));
 73     blueItem.setAccelerator(KeyStroke.getKeyStroke('B', CTRL_DOWN_MASK));
 74 
 75     // Add type menu item listeners
 76     lineItem.addActionListener(new TypeListener(LINE));
 77     rectangleItem.addActionListener(new TypeListener(RECTANGLE));
 78     circleItem.addActionListener(new TypeListener(CIRCLE));
 79     curveItem.addActionListener(new TypeListener(CURVE));
 80 
 81     // Add color menu item listeners
 82     redItem.addActionListener(new ColorListener(RED));
 83     yellowItem.addActionListener(new ColorListener(YELLOW));
 84     greenItem.addActionListener(new ColorListener(GREEN));
 85     blueItem.addActionListener(new ColorListener(BLUE));
 86 
 87     menuBar.add(fileMenu);                                             // Add the file menu
 88     menuBar.add(elementMenu);                                          // Add the element menu
 89   }
 90 
 91   // Handles element type menu items
 92   class TypeListener implements ActionListener {
 93     // Constructor
 94     TypeListener(int type) {
 95       this.type = type;
 96     }
 97 
 98     // Sets the element type
 99     public void actionPerformed(ActionEvent e) {
100       elementType = type;
101     }
102 
103     private int type;                                                  // Store the type for the menu
104   }
105 
106   // Handles color menu items
107   class ColorListener implements ActionListener {
108     public ColorListener(Color color) {
109       this.color = color;
110     }
111 
112     public void actionPerformed(ActionEvent e) {
113       elementColor = color;
114     }
115 
116     private Color color;
117   }
118 
119   private JMenuBar menuBar = new JMenuBar();                           // Window menu bar
120 
121   // File menu items
122   private JMenuItem newItem,  openItem,   closeItem,
123                     saveItem, saveAsItem, printItem;
124 
125   // Element menu items
126   private JRadioButtonMenuItem lineItem,  rectangleItem, circleItem,   // Types
127                                curveItem, textItem;
128   private JCheckBoxMenuItem    redItem,   yellowItem,                  // Colors
129                                greenItem, blueItem ;
130   private Color elementColor = DEFAULT_ELEMENT_COLOR;                  // Current element color
131   private int elementType = DEFAULT_ELEMENT_TYPE;                      // Current element type
132 }

Sketcher类和以前一样：

 1 // Sketching application
 2 import javax.swing.*;
 3 import java.awt.*;
 4 import java.awt.event.*;
 5 
 6 public class Sketcher {
 7   public static void main(String[] args) {
 8      theApp = new Sketcher();                                          // Create the application object
 9    SwingUtilities.invokeLater(new Runnable() {
10             public void run() {
11                           theApp.createGUI();                          // Call GUI creator
12             }
13         });
14   }
15 
16   // Method to create the application GUI
17   private void createGUI() {
18     window = new SketcherFrame("Sketcher");                            // Create the app window
19     Toolkit theKit = window.getToolkit();                              // Get the window toolkit
20     Dimension wndSize = theKit.getScreenSize();                        // Get screen size
21 
22     // Set the position to screen center & size to half screen size
23     window.setSize(wndSize.width/2, wndSize.height/2);                 // Set window size
24     window.setLocationRelativeTo(null);                                // Center window
25 
26     window.addWindowListener(new WindowHandler());                     // Add window listener
27     window.setVisible(true);
28   }
29 
30   // Handler class for window events
31   class WindowHandler extends WindowAdapter {
32     // Handler for window closing event
33     @Override
34     public void windowClosing(WindowEvent e) {
35       window.dispose();                                                // Release the window resources
36       System.exit(0);                                                  // End the application
37     }
38   }
39 
40   private SketcherFrame window;                                        // The application window
41   private static Sketcher theApp;                                      // The application object
42 }

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。