17.2. Exception-Handling Overview

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15.12. Creating Multiple Windows

Occasionally, you may want to create multiple windows in an application. The application opens a new window to perform the specified task. The new windows are called subwindows, and the main frame is called the main window .

To create a subwindow from an application, you need to create a subclass of JFrame that defines the task and tells the new window what to do. You can then create an instance of this subclass in the application and launch the new window by setting the frame instance to be visible.

Listing 15.11 gives a program that creates a main window with a text area in the scroll pane and a button named "Show Histogram." When the user clicks the button, a new window appears that displays a histogram to show the occurrences of the letters in the text area. Figure 15.31 contains a sample run of the program.

Figure 15.31. The histogram is displayed in a separate frame.

Here are the major steps in the program:

1.	Create a main class for the frame named MultipleWindowsDemo in Listing 15.11. Add a text area inside a scroll pane, and place the scroll pane in the center of the frame. Create a button "Show Histogram" and place it in the south of the frame.
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2.	Create a subclass of JPanel named Histogram in Listing 15.12. The class contains a data field named count of the int[] type, which counts the occurrences of twenty-six letters. The values in count are displayed in the histogram.
3.	Implement the actionPerformed handler in MultipleWindowsDemo , as follows : Create an instance of Histogram . Count the letters in the text area and pass the count to the Histogram object. Create a new frame and place the Histogram object in the center of frame. Display the frame.

Listing 15.11. MultipleWindowsDemo.java

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 1   import   java.awt.*; 2   import   java.awt.event.*; 3   import   javax.swing.*; 4 5   public class   MultipleWindowsDemo   extends   JFrame { 6   private   JTextArea jta; 7   private   JButton jbtShowHistogram =   new   JButton(   "Show Histogram"   ); 8    private   Histogram histogram =   new   Histogram();  9 10  // Create a new frame to hold the histogram panel  11    private   JFrame histogramFrame =   new   JFrame();  12 13   public   MultipleWindowsDemo() { 14  // Store text area in a scroll pane  15 JScrollPane scrollPane =   new   JScrollPane(jta =   new   JTextArea()); 16 scrollPane.setPreferredSize(new Dimension(   300   ,   200   )); 17 jta.setWrapStyleWord(   true   ); 18 jta.setLineWrap(   true   ); 19 20  // Place scroll pane and button in the frame  21 add(scrollPane, BorderLayout.CENTER); 22 add(jbtShowHistogram, BorderLayout.SOUTH); 23 24  // Register listener  25  jbtShowHistogram.addActionListener(   new   ActionListener() {  26  /** Handle the button action */  27   public void   actionPerformed(ActionEvent e) { 28  // Count the letters in the text area  29   int   [] count = countLetters(); 30 31  // Set the letter count to histogram for display  32 histogram.showHistogram(count); 33 34  // Show the frame  35 histogramFrame.setVisible(   true   ); 36 } 37 }); 38 39  // Create a new frame to hold the histogram panel  40  histogramFrame.add(histogram);  41  histogramFrame.pack();  42 histogramFrame.setTitle(   "Histogram"   ); 43 } 44 

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 45  /** Count the letters in the text area */  46   private int   [] countLetters() { 47  // Count for 26 letters  48   int   [] count =   new int   [   26   ]; 49 50  // Get contents from the text area  51 String text = jta.getText(); 52 53  // Count occurrence of each letter (case insensitive)  54   for   (   int   i =     ; i < text.length(); i++) { 55   char   character = text.charAt(i); 56 57   if   ((character >=   'A'   ) && (character <=   'Z'   )) { 58 count[(   int   )character -   65   ]++;  // The ASCII for 'A' is 65  59 } 60   else if   ((character >=   'a'   ) && (character <=   'z'   )) { 61 count[(   int   )character -   97   ]++;  // The ASCII for 'a' is 97  62 } 63 } 64 65   return   count;  // Return the count array  66 } 67 68   public static void   main(String[] args) { 69 MultipleWindowsDemo frame =   new   MultipleWindowsDemo(); 70 frame.setLocationRelativeTo(   null   );  // Center the frame  71 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 72 frame.setTitle(   "MultipleWindowsDemo"   ); 73 frame.pack(); 74 frame.setVisible(   true   ); 75 } 76 } 

Listing 15.12. Histogram.java

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 1   import   javax.swing.*; 2   import   java.awt.*; 3 4   public class    Histogram   extends   JPanel  { 5  // Count the occurrence of 26 letters  6   private int   [] count; 7 8  /** Set the count and display histogram */  9   public void   showHistogram(   int   [] count) { 10   this   .count = count; 11 repaint(); 12 } 13 14  /** Paint the histogram */  15   protected void   paintComponent(Graphics g) { 16   if   (count ==   null   )   return   ;  // No display if count is null  17 18   super   .paintComponent(g); 19 20  // Find the panel size and bar width and interval dynamically  21   int   width = getWidth(); 22   int   height = getHeight(); 

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 23   int   interval = (width -   40   ) / count.length; 24   int   individualWidth = (   int   )(((width -   40   ) /   24   ) *     .   60   ); 25 26  // Find the maximum count. The maximum count has the highest bar  27   int   maxCount =     ; 28   for   (   int   i =     ; i < count.length; i++) { 29   if   (maxCount < count[i]) 30 maxCount = count[i]; 31 } 32 33  // x is the start position for the first bar in the histogram  34   int   x =   30   ; 35 36  // Draw a horizontal base line  37 g.drawLine(   10   , height -   45   , width -   10   , height -   45   ); 38   for   (   int   i =     ; i < count.length; i++) { 39  // Find the bar height  40   int   barHeight = 41 (   int   )(((   double   )count[i] / (   double   )maxCount) * (height -   55   )); 42 43  // Display a bar (i.e., rectangle)  44 g.drawRect(x, height -   45   - barHeight, individualWidth, 45 barHeight); 46 47  // Display a letter under the base line  48 g.drawString((   char   )(   65   + i) +   " "   , x, height -   30   ); 49 50  // Move x for displaying the next character  51 x += interval; 52 } 53 } 54 55  /** Override getPreferredSize */  56   public   Dimension getPreferredSize() { 57   return new   Dimension(   300   ,   300   ); 58 } 59 } 

The program contains two classes: MultipleWindowsDemo and Histogram . Their relationship is shown in Figure 15.32.

Figure 15.32. MultipleWindowsDemo uses Histogram to display a histogram of the occurrences of the letters in a text area in the frame.

MultipleWindowsDemo is a frame that holds a text area in a scroll pane and a button. Histogram is a subclass of JPanel that displays a histogram for the occurrences of letters in the text area.

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When the user clicks the "Show Histogram" button, the handler counts the occurrences of letters in the text area. Letters are counted regardless of their case. Nonletter characters are not counted. The count is stored in an int array of twenty-six elements. The first element stores the count for letter ' a' or ' A ,' and the last element in the array stores the count for letter ' z' or ' Z .' The count array is passed to the histogram for display.

The MultipleWindowsDemo class contains a main method. The main method creates an instance of MultipleWindowsDemo and displays the frame. The MultipleWindowsDemo class also contains an instance of JFrame , named histogramFrame , which holds an instance of Histogram . When the user clicks the "Show Histogram" button, histogramFrame is set visible to display the histogram.

The height and width of the bars in the histogram are determined dynamically according to the window size of the histogram.

You cannot add an instance of JFrame to a container. For example, adding histogramFrame to the main frame would cause a runtime exception. However, you can create a frame instance and set it visible to launch a new window.