Solution for Programmming Exercise 12.5
This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.
Exercise 12.5:
The sample program PhoneDirectoryFileDemo.java from Subsection 11.3.2 keeps data for a "phone directory" in a file in the user's home directory. Exercise 11.5 asked you to revise that program to use an XML format for the data. Both programs have a simple command-line user interface. For this exercise, you should provide a GUI interface for the phone directory data. You can base your program either on the original sample program or on the modified version from the exercise. Use a JTable to hold the data. The user should be able to edit all the entries in the table. Also, the user should be able to add and delete rows. Include either buttons or menu commands that can be used to perform these actions. The delete command should delete the selected row, if any. New rows should be added at the end of the table. For this program, you can use a standard DefaultTableModel.
Your program should load data from the file when it starts and save data to the file when it ends, just as the two previous programs do. For a GUI program, you can't simply save the data at the end of the main() routine, since main() terminates as soon as the window shows up on the screen. You want to save the data when the user closes the window and ends the program. There are several approaches. One is to use a WindowListener to detect the event that occurs when the window closes. Another is to use a "Quit" command to end the program; when the user quits, you can save the data and close the window (by calling its dispose() method), and end the program. If you use the "Quit" command approach, you don't want the user to be able to end the program simply by closing the window. To accomplish this, you should call
frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
where frame refers to the JFrame that you have created for the program's user interface. When using a WindowListener, you want the close box on the window to close the window, not end the program. For this, you need
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
When the listener is notified of a window closed event, it can save the data and end the program.
Most of the JTable and DefaultTableModel methods that you need for this exercise are discussed in Subsection 12.4.3, but there are a few more that you need to know about. To determine which row is selected in a JTable, call table.getSelectedRow(). This method returns the row number of the selected row, or returns -1 if no row is selected. To specify which cell is currently being edited, you can use:
table.setRowSelectionInterval(rowNum, rowNum); // Selects row number rowNum. table.editCellAt( rowNum, colNum ); // Edit cell at position (rowNum,colNum). phoneTable.getEditorComponent().requestFocus(); // Put input cursor in cell.
One particularly troublesome point is that the data that is in the cell that is currently being edited is not in the table model. The value in the edit cell is not put into the table model until after the editing is finished. This means that even though the user sees the data in the cell, it's not really part of the table data yet. If you lose that data, the user would be justified in complaining. To make sure that you get the right data when you save the data at the end of the program, you have to turn off editing before retrieving the data from the model. This can be done with the following method:
private void stopEditing() {
if (table.getCellEditor() != null)
table.getCellEditor().stopCellEditing();
}
This method must also be called before modifying the table by adding or deleting rows; if such modifications are made while editing is in progress, the effect can be very strange.
There are many, many ways to organize the program. I will discuss just one. The main GUI class in my program is a subclass of JPanel, named PhoneDirectoryPanel. This panel holds the table and two buttons that are used for adding and deleting rows. In the main() routine, the data from the file is read into a variable of type TreeMap<String,String>. Somehow, that data has to get into the panel. I decided to pass it as a parameter to a constructor of the form
public PhoneDirectoryPanel(TreeMap<String,String> initialPhoneBook)
When the program ends, the data has to be gotten back out of the panel object so it can be saved to the file. I defined a method in the PhoneDirectoryPanel class that can be used to get the data:
public TreeMap<String, String> getPhoneBook()
Aside from the constructor, some instance variables, and the getPhoneBook() method, the only other thing in the panel class is the stopEditing() method that was mentioned in the exercise. While the program is running, the data is managed by the table.
In the command-line version, the code for saving the data is at the end of the main() routine. I moved it into a static method
private static void saveData(TreeMap<String,String> newPhoneBook)
This method has to be called when the program ends. I decided to use a WindowListener to save the data when the window is closed. The main() routine creates the window, puts a PhoneDirectoryPanel in the window, and adds a listener to the window:
JFrame window = new JFrame("PhoneBook");
final PhoneDirectoryPanel panel = new PhoneDirectoryPanel(phoneBook);
window.setContentPane( panel );
window.pack();
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screenSize.width - window.getWidth())/2, 80 );
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
window.addWindowListener( new WindowAdapter() {
// When the user clicks the close box of the window,
// the window will be disposed (that is, closed), and the
// windowClosed method in this WindowListener will be
// called. This method saves the phone book data and
// calls System.exit() to terminate the program.
public void windowClosed(WindowEvent evt) {
saveData(panel.getPhoneBook());
System.exit(0);
}
});
After it does this, the main() routine ends, and the window takes over. The constructor of the panel class has to create a table and a table model using the data from the TreeMap, initialPhoneBook. To do this, it copies the data into a two-dimensional array of Strings that can be used in the constructor for the DefaultTableModel:
int entryCount = initialPhoneBook.size();
String[][] entries;
if (entryCount == 0)
entries = new String[1][2]; // Represents an empty row.
else {
entries = new String[entryCount][2];
int index = 0;
for (Map.Entry<String, String> entry : initialPhoneBook.entrySet()) {
entries[index][0] = entry.getKey();
entries[index][1] = entry.getValue();
index++;
}
}
String[] columnHeads = new String[] { "Name", "Number" };
model = new DefaultTableModel(entries,columnHeads);
table = new JTable(model);
Note that if there are no entries in the phone book, I place one empty row in the table, since the user would have to do that anyway before they could do anything else with the table. (Also, I need a real array when I create the model, since the number of columns is taken from the size of the array.) I also did some customization of the table's appearance. The constructor also creates an "Add Row" button and a "Delete Row" button. It's worth looking at the ActionListener for the "Add Row" button:
JButton addRowButton = new JButton("Add Row");
addRowButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Add a row at the end of the table. Also, select
// that row and set the first cell in that row to
// be the cell that is currently being edited.
stopEditing();
model.addRow( new String[] { null, null } );
int newRow = model.getRowCount() - 1; // Number of the row that was added.
table.setRowSelectionInterval(newRow, newRow);
table.editCellAt( newRow, 0 );
Component c = table.getEditorComponent();
if (c != null) // (Should not be null.)
c.requestFocus();
}
});
The statement model.addRow( new String[] { null, null } ); adds a row at the end of the table. The data that is to go into the cells in that row is passed to the addRow() method as an array. In this case, the null values mean that both cells will initially be empty. Before adding the row, I call stopEditing(), as suggested in the exercise, and after adding the row, I select the first cell in the row for editing, again following the exercise.
The only other thing to do in the panel class is to define the getPhoneBooK() method. This method has to get the data out of the table model and add it to a TreeMap. One question was, what to do about empty cells in the table? A row with one or two empty rows represents missing or incomplete data. One possibility is just to ignore all such rows. However, I decided that having a name in the phone book with no number attached to it would be OK. On the other hand, it doesn't make any sense to have a number without a name, so my policy is to ignore any row in which the "name" cell is empty. You can see how it's done in the solution shown below.
One other issue that came up is what to do if an error occurs while trying to read or write the data file. The command-line version of the program simply prints a message to standard output. For a GUI program, however, the user probably won't even see standard output, so sending the output there does no good. A much better way to report errors in a GUI program is to use the JOptionPane class, so I converted error reports from the original program to use JOptionPane instead of System.out.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.table.DefaultTableModel;
import java.io.*;
import java.util.Map;
import java.util.TreeMap;
import javax.xml.parsers.*;
import org.w3c.dom.*;
/**
* This program lets the user keep a persistent "phone book" that
* contains names and phone numbers. The data for the phone book
* is stored in a file in the user's home directory. This is
* a GUI version of the program, with the phonebook displayed
* in a table that the user can edit. The data files for this
* version of the program are in XML format.
*
* WARNING: This program will save a file named ".phone_book_xml_demo"
* in the home directory of the user who runs it. On some computers,
* this will be a "hidden" file.
*/
public class PhoneDirectoryGUI {
/**
* The name of the file in which the phone book data is kept. The
* file is stored in the user's home directory. The "." at the
* beginning of the file name means that the file will be a
* "hidden" file on Unix-based computers, including Linux and
* Mac OS X.
*/
private static String DATA_FILE_NAME = ".phone_book_xml_demo";
/**
* A File object created from the absolute path name of the file.
*/
private static File dataFile;
/**
* Holds the phone book data as it was read from the data file
* when the program started. The data in this map is not modified
* after it has been read.
*/
private static TreeMap<String,String> phoneBook;
public static void main(String[] args) {
phoneBook = new TreeMap<String,String>();
File userHomeDirectory = new File( System.getProperty("user.home") );
dataFile = new File( userHomeDirectory, DATA_FILE_NAME );
/* If the data file already exists, then the data in the file is
* read and is used to initialize the phone directory. The user
* is informed before the file is created and is given a chance to
* exit the program immediately.
*/
if ( ! dataFile.exists() ) {
int response = JOptionPane.showConfirmDialog(null,
"No phone book data file found. To create a new one,\n"
+ "click OK. To exit the program now, click CANCEL.\n\n"
+ "(The name of the file will be:\n "
+ dataFile.getAbsolutePath() + ")",
"Create phonebook?", JOptionPane.OK_CANCEL_OPTION);
if (response == JOptionPane.CANCEL_OPTION)
System.exit(1);
}
else {
System.out.println("Reading phone book data...");
try {
DocumentBuilder docReader =
DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document xmldoc = docReader.parse(dataFile);
Element root = xmldoc.getDocumentElement();
if (! root.getTagName().equals("phone_directory"))
throw new Exception();
NodeList nodes = root.getChildNodes();
for (int i = 0; i < nodes.getLength(); i++) {
if ( nodes.item(i) instanceof Element ) {
Element entry = (Element)nodes.item(i);
if (! entry.getTagName().equals("entry"))
throw new Exception();
String entryName = entry.getAttribute("name");
String entryNumber = entry.getAttribute("number");
if (entryName.length() == 0)
throw new Exception();
phoneBook.put(entryName,entryNumber);
}
}
}
catch (Exception e) {
JOptionPane.showMessageDialog(null,
"An error occurred while trying to read\n"
+ "the phone directory from the file:\n "
+ dataFile.getAbsolutePath()
+"\n\nThis program cannot continue.");
System.exit(1);
}
}
/* The phone book has been read successfully (if it existed). Open
* a window where the user can view and edit the phone directory.
*/
JFrame window = new JFrame("PhoneBook");
final PhoneDirectoryPanel panel = new PhoneDirectoryPanel(phoneBook);
window.setContentPane( panel );
window.pack();
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
window.setLocation( (screenSize.width - window.getWidth())/2, 80 );
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
window.addWindowListener( new WindowAdapter() {
// When the user clicks the close box of the window,
// the window will be disposed (that is, closed), and the
// windowClosed method in this WindowListener will be
// called. This method saves the phone book data and
// calls System.exit() to terminate the program.
public void windowClosed(WindowEvent evt) {
saveData(panel.getPhoneBook());
System.exit(0);
}
});
} // end main()
/**
* Before ending the program, write the current contents of the
* phone directory, but only if some changes have been made to
* the directory. This is called by a window listener when the
* window is closed. If an error occurs while the data is being
* saved, a pop-up box will inform the user (but this is unlikely).
* @param newPhoneBook the phone book data that has possibly been
* edited by the user. If this is the same as the data that
* was read from the file originally, this method does not
* re-save the unchanged data. If the data has changed, the
* new data is written to the file.
*/
private static void saveData(TreeMap<String,String> newPhoneBook) {
if (phoneBook.equals(newPhoneBook) )
System.out.println("No changes to phone book.");
else{
System.out.println("Saving phone directory changes to file " +
dataFile.getAbsolutePath() + " ...");
PrintWriter out;
try {
out = new PrintWriter( new FileWriter(dataFile) );
}
catch (IOException e) {
JOptionPane.showMessageDialog(null,"Whoops! Some error occurred while\n"
+ "trying to save your phone book. Sorry.");
return;
}
out.println("<?xml version=\"1.0\"?>");
out.println("<phone_directory>");
for ( Map.Entry<String,String> entry : newPhoneBook.entrySet() ) {
out.print(" <entry name='");
out.print(entry.getKey());
out.print("' number='");
out.print(entry.getValue());
out.println("'/>");
}
out.println("</phone_directory>");
out.close();
if (out.checkError()) {
JOptionPane.showMessageDialog(null,"Whoops! Some error occurred while\n"
+ "trying to save your phone book. Sorry.");
}
}
}
/**
* This class defines the GUI for viewing and editing the phone
* book. The main program adds an object of this type to a frame.
*/
private static class PhoneDirectoryPanel extends JPanel {
private JTable phoneTable; // For showing/editing the phone book.
private DefaultTableModel model; // The model for the table. (This is
// where the data is actually stored.)
/**
* Constructor creates the table and shows it with an "Add row"
* and a "Delete Row" button beneath it.
* @param initialPhoneBook contains the phone book data that is initially
* added to the table. (This comes from the main program and
* contains the data that was read from the file.)
*/
public PhoneDirectoryPanel(TreeMap<String,String> initialPhoneBook) {
/* First create the arrays that hold the data for the table
* and the names of its columns. These arrays are used to
* create the table model.
*/
int entryCount = initialPhoneBook.size();
String[][] entries;
if (entryCount == 0)
entries = new String[1][2];
else {
entries = new String[entryCount][2];
int index = 0;
for (Map.Entry<String, String> entry : initialPhoneBook.entrySet()) {
entries[index][0] = entry.getKey();
entries[index][1] = entry.getValue();
index++;
}
}
String[] columnHeads = new String[] { "Name", "Number" };
/* Create the table model from the data and column name arrays,
* and use it to create the JTable. After this, the official
* phone book data is what's in the table, and the arrays and
* initialPhoneBook are no longer used.
*/
model = new DefaultTableModel(entries,columnHeads);
phoneTable = new JTable(model);
/* Customize the appearance of the table.
*/
phoneTable.setRowHeight(27);
phoneTable.setShowGrid(true);
phoneTable.setGridColor(Color.BLACK);
phoneTable.getTableHeader().setReorderingAllowed(false);
/* Create the two buttons, and set up listeners to respond
* when the user clicks them.
*/
JButton addRowButton = new JButton("Add Row");
addRowButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Add a row at the end of the table. Also, select
// that row and set the first cell in that row to
// be the cell that is currently being edited.
stopEditing();
model.addRow( new String[] { null, null } );
int newRow = model.getRowCount() - 1;
phoneTable.setRowSelectionInterval(newRow, newRow);
phoneTable.editCellAt( newRow, 0 );
Component c = phoneTable.getEditorComponent();
if (c != null) // (Should not be null.)
c.requestFocus();
}
});
JButton deleteRowButton = new JButton("Delete Row");
deleteRowButton.addActionListener( new ActionListener() {
public void actionPerformed(ActionEvent evt) {
// Delete the selected row, if there is one. If more than
// one cell is selected, the first selected row is deleted.
stopEditing();
int selectedRow = phoneTable.getSelectedRow();
if (selectedRow >= 0)
model.removeRow(selectedRow);
}
});
/* Build the layout for the panel.
*/
setLayout(new BorderLayout(2,2));
setBackground(Color.GRAY);
setBorder(BorderFactory.createLineBorder(Color.GRAY,2));
add( new JScrollPane(phoneTable), BorderLayout.CENTER );
JPanel buttonBar = new JPanel();
buttonBar.add(addRowButton);
buttonBar.add(deleteRowButton);
add( buttonBar, BorderLayout.SOUTH );
} // end constructor
/**
* This method is called before modifying the table or getting
* the data out of the table. If a cell is currently being
* edited, it turns off editing of that cell and changes the
* data model to match the current content of the editor. (Note
* that the table model does not change while the cell is being
* edited.)
*/
private void stopEditing() {
if (phoneTable.getCellEditor() != null)
phoneTable.getCellEditor().stopCellEditing();
}
/**
* This method is called when the program ends to get the phone
* book data, which might have been edited by the user. The
* data is in the table model. This method gets the data from
* the table model and puts it into a TreeMap, which is then
* returned as the value of the method. Note that if a row in
* the table contains an empty name, it is ignored. However,
* if there is an empty number for a non-empty name, the number
* is changed to "(unknown)" and the row is added to the TreeMap.
* Note that by using a TreeMap, we allow only one number for
* a given name. If the user has used the same name in more than
* one row, the first row with that name will be lost;
* it would probably be better to warn the user about this or to
* take some other, more reasonable answer (like adding a number
* to the end of the duplicate name).
*/
public TreeMap<String, String> getPhoneBook() {
stopEditing();
TreeMap<String,String> phoneBook = new TreeMap<String,String>();
for (int row = 0; row < model.getRowCount(); row++) {
String name = (String)model.getValueAt(row, 0);
String number = (String)model.getValueAt(row, 1);
if (number == null || number.trim().length() == 0)
number = "(unknown)";
if (name != null && name.trim().length() > 0)
phoneBook.put(name,number);
}
return phoneBook;
}
}
}