Javanotes 6.0, Section 6.8 -- Menus and Dialogs

Section 6.8

Menus and Dialogs

We have already encountered many of the basic aspects of GUI programming, but professional programs use many additional features. We will cover some of the advanced features of Java GUI programming in Chapter 13, but in this section we look briefly at a few more basic features that are essential for writing GUI programs. I will discuss these features in the context of this "MosaicDraw" program:

Try clicking and dragging on the large black drawing area. As you move the mouse, you will leave a trail of little colored squares. There is some random variation in the color of the squares. (This is meant to make the picture look a little more like a real mosaic, which is a picture made out of small colored stones in which there would be some natural color variation.) There is a menu bar above the drawing area. The "Control" menu contains commands for filling and clearing the drawing area, along with a few options that affect the appearance of the picture. The "Color" menu lets the user select the color that will be used when the user draws. The "Tools" menu affects the behavior of the mouse. Using the default "Draw" tool, the mouse leaves a trail of single squares. Using the "Draw 3x3" tool, the mouse leaves a swath of colored squares that is three squares wide. There are also "Erase" tools, which let the user set squares back to their default black color.

The drawing area of the program is a panel that belongs to the MosaicPanel class, a subclass of JPanel that is defined in MosaicPanel.java. MosaicPanel is a highly reusable class for representing mosaics of colored rectangles. It does not directly support drawing on the mosaic, but it does support setting the color of each individual square. The MosaicDraw program installs a mouse listener on the panel; the mouse listener responds to mousePressed and mouseDragged events on the panel by setting the color of the square that contains the mouse. This is a nice example of applying a listener to an object to do something that was not programmed into the object itself.

Most of the programming for MosaicDraw can be found in MosaicDrawController.java. (It could have gone into the MosaicPanel class, if I had not decided to use that pre-existing class in unmodified form.) It is the MosaicDrawController class that creates a MosaicPanel object and adds a mouse listener to it. It also creates the menu bar that is shown at the top of the program and implements all the commands in the menu bar. It has an instance method getMosaicPanel() that returns a reference to the mosaic panel that it has created, and it has another instance method getMenuBar() that returns a menu bar for the program. These methods are used to obtain the panel and menu bar so that they can be added to an applet or a frame.

To get a working program, an object of type JApplet or JFrame is needed. The files MosaicDrawApplet.java and MosaicDrawFrame.java define the applet and frame versions of the program. These are rather simple classes; they simply create a MosaicDrawController object and use its mosaic panel and menu bar. I urge you to study these files, along with MosaicDrawController.java. I will not be discussing all aspects of the code here, but you should be able to understand it all after reading this section. As for MosaicPanel.java, it uses some techniques that you would not understand at this point, but I encourage you to at least read the comments in this file to learn about the API for mosaic panels.

6.8.1 Menus and Menubars

MosaicDraw is the first example that we have seen that uses a menu bar. Fortunately, menus are very easy to use in Java. The items in a menu are represented by the class JMenuItem (this class and other menu-related classes are in package javax.swing). Menu items are used in almost exactly the same way as buttons. In fact, JMenuItem and JButton are both subclasses of a class, AbstractButton, that defines their common behavior. In particular, a JMenuItem is created using a constructor that specifies the text of the menu item, such as:

JMenuItem fillCommand = new JMenuItem( "Fill" );

You can add an ActionListener to a JMenuItem by calling the menu item's addActionListener() method. The actionPerformed() method of the action listener is called when the user selects the item from the menu. You can change the text of the item by calling its setText(String) method, and you can enable it and disable it using the setEnabled(boolean) method. All this works in exactly the same way as for a JButton.

The main difference between a menu item and a button, of course, is that a menu item is meant to appear in a menu rather than in a panel. A menu in Java is represented by the class JMenu. A JMenu has a name, which is specified in the constructor, and it has an add(JMenuItem) method that can be used to add a JMenuItem to the menu. So, the "Tools" menu in the MosaicDraw program could be created as follows, where listener is a variable of type ActionListener:

JMenu toolsMenu = new JMenu( "Tools" );  // Create a menu with name "Tools"

JMenuItem drawCommand = new JMenuItem( "Draw" );   // Create a menu item.
drawCommand.addActionListener( listener );         // Add listener to menu item.
toolsMenu.add( drawCommand );                      // Add menu item to menu.

JMenuItem eraseCommand = new JMenuItem( "Erase" ); // Create a menu item.
eraseCommand.addActionListener( listener );        // Add listener to menu item.
toolsMenu.add( eraseCommand );                     // Add menu item to menu.
   .
   .  // Create and add other menu items.
   .

Once a menu has been created, it must be added to a menu bar. A menu bar is represented by the class JMenuBar. A menu bar is just a container for menus. It does not have a name, and its constructor does not have any parameters. It has an add(JMenu) method that can be used to add menus to the menu bar. The name of the menu then appears in the menu bar. For example, the MosaicDraw program uses three menus, controlMenu, colorMenu, and toolsMenu. We could create a menu bar and add the menus to it with the statements:

JMenuBar menuBar = new JMenuBar();
menuBar.add( controlMenu );
menuBar.add( colorMenu );
menuBar.add( toolsMenu );

The final step in using menus is to use the menu bar in a JApplet or JFrame. We have already seen that an applet or frame has a "content pane." The menu bar is another component of the applet or frame, not contained inside the content pane. Both the JApplet and the JFrame classes include an instance method setMenuBar(JMenuBar) that can be used to set the menu bar. (There can only be one, so this is a "set" method rather than an "add" method.) In the MosaicDraw program, the menu bar is created by a MosaicDrawController object and can be obtained by calling that object's getMenuBar() method. Here is the basic code that is used (in somewhat modified form) to set up the interface both in the applet and in the frame version of the program:

MosaicDrawController controller = new MosaicDrawController();

MosaicPanel content = controller.getMosaicPanel();
setContentPane( content );  // Use panel from controller as content pane.
   
JMenuBar menuBar = controller.getMenuBar();
setJMenuBar( menuBar );    // Use the menu bar from the controller.

Using menus always follows the same general pattern: Create a menu bar. Create menus and add them to the menu bar. Create menu items and add them to the menus (and set up listening to handle action events from the menu items). Use the menu bar in a JApplet or JFrame by calling the setJMenuBar() method of the applet or frame.

There are other kinds of menu items, defined by subclasses of JMenuItem, that can be added to menus. One of these is JCheckBoxMenuItem, which represents menu items that can be in one of two states, selected or not selected. A JCheckBoxMenuItem has the same functionality and is used in the same way as a JCheckBox (see Subsection 6.6.3). Three JCheckBoxMenuItems are used in the "Control" menu of the MosaicDraw program. One can be used to turn the random color variation of the squares on and off. Another turns a symmetry feature on and off; when symmetry is turned on, the user's drawing is reflected horizontally and vertically to produce a symmetric pattern. And the third checkbox menu item shows and hides the "grouting" in the mosaic; the grouting is the gray lines that are drawn around each of the little squares in the mosaic. The menu item that corresponds to the "Use Randomness" option in the "Control" menu could be set up with the statements:

JMenuItem useRandomnessToggle = new JCheckBoxMenuItem( "Use Randomness" );
useRandomnessToggle.addActionListener( listener );  // Set up a listener.
useRandomnessToggle.setSelected(true);  // Randomness is initially turned on.
controlMenu.add( useRandomnessToggle );   // Add the menu item to the menu.

The "Use Randomness" JCheckBoxMenuItem corresponds to a boolean-valued instance variable named useRandomness in the MosaicDrawController class. This variable is part of the state of the controller object. Its value is tested whenever the user draws one of the squares, to decide whether or not to add a random variation to the color of the square. When the user selects the "Use Randomness" command from the menu, the state of the JCheckBoxMenuItem is reversed, from selected to not-selected or from not-selected to selected. The ActionListener for the menu item checks whether the menu item is selected or not, and it changes the value of useRandomness to match. Note that selecting the menu command does not have any immediate effect on the picture that is shown in the window. It just changes the state of the program so that future drawing operations on the part of the user will have a different effect. The "Use Symmetry" option in the "Control" menu works in much the same way. The "Show Grouting" option is a little different. Selecting the "Show Grouting" option does have an immediate effect: The picture is redrawn with or without the grouting, depending on the state of the menu item.

My program uses a single ActionListener to respond to all of the menu items in all the menus. This is not a particularly good design, but it is easy to implement for a small program like this one. The actionPerformed() method of the listener object uses the statement

String command = evt.getActionCommand();

to get the action command of the source of the event; this will be the text of the menu item. The listener tests the value of command to determine which menu item was selected by the user. If the menu item is a JCheckBoxMenuItem, the listener must check the state of the menu item. The menu item is the source of the event that is being processed. The listener can get its hands on the menu item object by calling evt.getSource(). Since the return value of getSource() is of type Object, the return value must be type-cast to the correct type. Here, for example, is the code that handles the "Use Randomness" command:

if (command.equals("Use Randomness")) 
{
        // Set the value of useRandomness depending on the menu item's state.
   JCheckBoxMenuItem toggle = (JCheckBoxMenuItem)evt.getSource();
   useRandomness = toggle.isSelected();
}

(The actionPerformed() method uses a rather long if..then..else statement to check all the possible action commands. This would be a natural place to use a switch statement with command as the selector and all the possible action commands as cases. However, this can only be done if you are sure that the program will be run using Java 7 or later, since Strings were not allowed in switch statements in earlier versions of Java.)

In addition to menu items, a menu can contain lines that separate the menu items into groups. In the MosaicDraw program, the "Control" menu contains such a separator. A JMenu has an instance method addSeparator() that can be used to add a separator to the menu. For example, the separator in the "Control" menu was created with the statement:

controlMenu.addSeparator();

A menu can also contain a submenu. The name of the submenu appears as an item in the main menu. When the user moves the mouse over the submenu name, the submenu pops up. (There is no example of this in the MosaicDraw program.) It is very easy to do this in Java: You can add one JMenu to another JMenu using a statement such as mainMenu.add(submenu).

6.8.2 Dialogs

One of the commands in the "Color" menu of the MosaicDraw program is "Custom Color...". When the user selects this command, a new window appears where the user can select a color. This window is an example of a dialog or dialog box. A dialog is a type of window that is generally used for short, single purpose interactions with the user. For example, a dialog box can be used to display a message to the user, to ask the user a question, to let the user select a file to be opened, or to let the user select a color. In Swing, a dialog box is represented by an object belonging to the class JDialog or to a subclass.

The JDialog class is very similar to JFrame and is used in much the same way. Like a frame, a dialog box is a separate window. Unlike a frame, however, a dialog is not completely independent. Every dialog is associated with a frame (or another dialog), which is called its parent window. The dialog box is dependent on its parent. For example, if the parent is closed, the dialog box will also be closed. It is possible to create a dialog box without specifying a parent, but in that case an invisible frame is created by the system to serve as the parent.

Dialog boxes can be either modal or modeless. When a modal dialog is created, its parent frame is blocked. That is, the user will not be able to interact with the parent until the dialog box is closed. Modeless dialog boxes do not block their parents in the same way, so they seem a lot more like independent windows. In practice, modal dialog boxes are easier to use and are much more common than modeless dialogs. All the examples we will look at are modal.

Aside from having a parent, a JDialog can be created and used in the same way as a JFrame. However, I will not give any examples here of using JDialog directly. Swing has many convenient methods for creating common types of dialog boxes. For example, the color choice dialog that appears when the user selects the "Custom Color" command in the MosaicDraw program belongs to the class JColorChooser, which is a subclass of JDialog. The JColorChooser class has a static method that makes color choice dialogs very easy to use:

Color JColorChooser.showDialog( Component parentComp, 
                                        String title, Color initialColor )

When you call this method, a dialog box appears that allows the user to select a color. The first parameter specifies the parent of the dialog; the parent window of the dialog will be the window (if any) that contains parentComp; this parameter can be null and it can itself be a frame or dialog object. The second parameter is a string that appears in the title bar of the dialog box. And the third parameter, initialColor, specifies the color that is selected when the color choice dialog first appears. The dialog has a sophisticated interface that allows the user to change the selected color. When the user presses an "OK" button, the dialog box closes and the selected color is returned as the value of the method. The user can also click a "Cancel" button or close the dialog box in some other way; in that case, null is returned as the value of the method. This is a modal dialog, and the showDialog() does not return until the user dismisses the dialog box in some way. By using this predefined color chooser dialog, you can write one line of code that will let the user select an arbitrary color. Swing also has a JFileChooser class that makes it almost as easy to show a dialog box that lets the user select a file to be opened or saved.

The following applet demonstrates a JColorChooser dialog and three other, simpler standard dialog boxes. When you click one of the buttons, a dialog box appears. The label at the top of the applet gives you some feedback about what is happening:

The three simple dialogs in this applet are created by static methods in the class JOptionPane. This class includes many methods for making dialog boxes, but they are all variations on the three basic types shown here: a "message" dialog, a "confirm" dialog, and an "input" dialog. (The variations allow you to provide a title for the dialog box, to specify the icon that appears in the dialog, and to add other components to the dialog box. I will only cover the most basic forms here.)

A message dialog simply displays a message string to the user. The user (hopefully) reads the message and dismisses the dialog by clicking the "OK" button. A message dialog can be shown by calling the static method:

void JOptionPane.showMessageDialog( Component parentComp, String message )

The message can be more than one line long. Lines in the message should be separated by newline characters, \n. New lines will not be inserted automatically, even if the message is very long.

An input dialog displays a question or request and lets the user type in a string as a response. You can show an input dialog by calling:

String JOptionPane.showInputDialog( Component parentComp, String question )

Again, the question can include newline characters. The dialog box will contain an input box, an "OK" button, and a "Cancel" button. If the user clicks "Cancel", or closes the dialog box in some other way, then the return value of the method is null. If the user clicks "OK", then the return value is the string that was entered by the user. Note that the return value can be an empty string (which is not the same as a null value), if the user clicks "OK" without typing anything in the input box. If you want to use an input dialog to get a numerical value from the user, you will have to convert the return value into a number; see Subsection 3.7.2.

Finally, a confirm dialog presents a question and three response buttons: "Yes", "No", and "Cancel". A confirm dialog can be shown by calling:

int JOptionPane.showConfirmDialog( Component parentComp, String question )

The return value tells you the user's response. It is one of the following constants:

JOptionPane.YES_OPTION -- the user clicked the "Yes" button
JOptionPane.NO_OPTION -- the user clicked the "No" button
JOptionPane.CANCEL_OPTION -- the user clicked the "Cancel" button
JOptionPane.CLOSE_OPTION -- the dialog was closed in some other way.

By the way, it is possible to omit the Cancel button from a confirm dialog by calling one of the other methods in the JOptionPane class. Just call:

JOptionPane.showConfirmDialog(
                parent, question, title, JOptionPane.YES_NO_OPTION )

The final parameter is a constant which specifies that only a "Yes" button and a "No" button should be used. The third parameter is a string that will be displayed as the title of the dialog box window.

If you would like to see how dialogs are created and used in the sample applet, you can find the source code in the file SimpleDialogDemo.java.

6.8.3 Fine Points of Frames

In previous sections, whenever I used a frame, I created a JFrame object in a main() routine and installed a panel as the content pane of that frame. This works fine, but a more object-oriented approach is to define a subclass of JFrame and to set up the contents of the frame in the constructor of that class. This is what I did in the case of the MosaicDraw program. MosaicDrawFrame is defined as a subclass of JFrame. The definition of this class is very short, but it illustrates several new features of frames that I want to discuss:

public class MosaicDrawFrame extends JFrame 
{
   
   public static void main(String[] args) 
   {
      JFrame window = new MosaicDrawFrame();
      window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      window.setVisible(true);
   }
   
   public MosaicDrawFrame() 
   {
      super("Mosaic Draw");
      MosaicDrawController controller = new MosaicDrawController();
      setContentPane( controller.getMosaicPanel() );
      setJMenuBar( controller.getMenuBar() );
      pack();
      Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();
      setLocation( (screensize.width - getWidth())/2, 
                                 (screensize.height - getHeight())/2 );
   }
   
}

The constructor in this class begins with the statement super("Mosaic Draw"), which calls the constructor in the superclass, JFrame. The parameter specifies a title that will appear in the title bar of the window. The next three lines of the constructor set up the contents of the window; a MosaicDrawController is created, and the content pane and menu bar of the window are obtained from the controller. The next line is something new. If window is a variable of type JFrame (or JDialog), then the statement window.pack() will resize the window so that its size matches the preferred size of its contents. (In this case, of course, "pack()" is equivalent to "this.pack()"; that is, it refers to the window that is being created by the constructor.) The pack() method is usually the best way to set the size of a window. Note that it will only work correctly if every component in the window has a correct preferred size. This is only a problem in two cases: when a panel is used as a drawing surface and when a panel is used as a container with a null layout manager. In both these cases there is no way for the system to determine the correct preferred size automatically, and you should set a preferred size by hand. For example:

panel.setPreferredSize( new Dimension(400, 250) );

The last two lines in the constructor position the window so that it is exactly centered on the screen. The line

Dimension screensize = Toolkit.getDefaultToolkit().getScreenSize();

determines the size of the screen. The size of the screen is screensize.width pixels in the horizontal direction and screensize.height pixels in the vertical direction. The setLocation() method of the frame sets the position of the upper left corner of the frame on the screen. The expression "screensize.width - getWidth()" is the amount of horizontal space left on the screen after subtracting the width of the window. This is divided by 2 so that half of the empty space will be to the left of the window, leaving the other half of the space to the right of the window. Similarly, half of the extra vertical space is above the window, and half is below.

Note that the constructor has created the window and set its size and position, but that at the end of the constructor, the window is not yet visible on the screen. (More exactly, the constructor has created the window object, but the visual representation of that object on the screen has not yet been created.) To show the window on the screen, it will be necessary to call its instance method, window.setVisible(true).

In addition to the constructor, the MosaicDrawFrame class includes a main() routine. This makes it possible to run MosaicDrawFrame as a stand-alone application. (The main() routine, as a static method, has nothing to do with the function of a MosaicDrawFrame object, and it could (and perhaps should) be in a separate class.) The main() routine creates a MosaicDrawFrame and makes it visible on the screen. It also calls

window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

which means that the program will end when the user closes the window. Note that this is not done in the constructor because doing it there would make MosaicDrawFrame less flexible. It is possible, for example, to write a program that lets the user open multiple MosaicDraw windows. In that case, we don't want to end the program just because the user has closed one of the windows. Furthermore, it is possible for an applet to create a frame, which will open as a separate window on the screen. An applet is not allowed to "terminate the program" (and it's not even clear what that should mean in the case of an applet), and attempting to do so will produce an exception. There are other possible values for the default close operation of a window:

JFrame.DO_NOTHING_ON_CLOSE -- the user's attempts to close the window by clicking its close box will be ignored.
JFrame.HIDE_ON_CLOSE -- when the user clicks its close box, the window will be hidden just as if window.setVisible(false) were called. The window can be made visible again by calling window.setVisible(true). This is the value that is used if you do not specify another value by calling setDefaultCloseOperation.
JFrame.DISPOSE_ON_CLOSE -- the window is closed and any operating system resources used by the window are released. It is not possible to make the window visible again. (This is the proper way to permanently get rid of a window without ending the program. You can accomplish the same thing by calling the instance method window.dispose().)

I've written an applet version of the MosaicDraw program that appears on a Web page as a single button. When the user clicks the button, the applet opens a MosaicDrawFrame. In this case, the applet sets the default close operation of the window to JFrame.DISPOSE_ON_CLOSE. You can try the applet here:

The file MosaicDrawLauncherApplet.java contains the source code for the applet. One interesting point in the applet is that the text of the button changes depending on whether a window is open or not. If there is no window, the text reads "Launch MosaicDraw". When the window is open, it changes to "Close MosaicDraw", and clicking the button will close the window. The change is implemented by attaching a WindowListener to the window. The listener responds to WindowEvents that are generated when the window opens and closes. Although I will not discuss window events further here, you can look at the source code for an example of how they can be used.

6.8.4 Creating Jar Files

As the final topic for this chapter, we look again at jar files. Recall that a jar file is a "java archive" that can contain a number of class files. When creating a program that uses more than one class, it's usually a good idea to place all the classes that are required by the program into a jar file. If that is done, then a user will only need that one file to run the program. Subsection 6.2.4 discusses how a jar file can be used for an applet. Jar files can also be used for stand-alone applications. In fact, it is possible to make a so-called executable jar file. A user can run an executable jar file in much the same way as any other application, usually by double-clicking the icon of the jar file. (The user's computer must have a correct version of Java installed, and the computer must be configured correctly for this to work. The configuration is usually done automatically when Java is installed, at least on Windows and Mac OS.)

The question, then, is how to create a jar file. The answer depends on what programming environment you are using. The two basic types of programming environment -- command line and IDE -- were discussed in Section 2.6. Any IDE (Integrated Programming Environment) for Java should have a command for creating jar files. In the Eclipse IDE, for example, it can be done as follows: In the Package Explorer pane, select the programming project (or just all the individual source code files that you need). Right-click on the selection, and choose "Export" from the menu that pops up. In the window that appears, select "JAR file" and click "Next". In the window that appears next, enter a name for the jar file in the box labeled "JAR file". (Click the "Browse" button next to this box to select the file name using a file dialog box.) The name of the file should end with ".jar". If you are creating a regular jar file, not an executable one, you can hit "Finish" at this point, and the jar file will be created. You could do this, for example, if the jar file contains an applet but no main program. To create an executable file, hit the "Next" button twice to get to the "Jar Manifest Specification" screen. At the bottom of this screen is an input box labeled "Main class". You have to enter the name of the class that contains the main() routine that will be run when the jar file is executed. If you hit the "Browse" button next to the "Main class" box, you can select the class from a list of classes that contain main() routines. Once you've selected the main class, you can click the "Finish" button to create the executable jar file. (Note that newer versions of Eclipse also have an option for exporting an executable Jar file in fewer steps.)

It is also possible to create jar files on the command line. The Java Development Kit includes a command-line program named jar that can be used to create jar files. If all your classes are in the default package (like most of the examples in this book), then the jar command is easy to use. To create a non-executable jar file on the command line, change to the directory that contains the class files that you want to include in the jar. Then give the command

jar  cf  JarFileName.jar  *.class

where JarFileName can be any name that you want to use for the jar file. The "*" in "*.class" is a wildcard that makes *.class match every class file in the current directory. This means that all the class files in the directory will be included in the jar file. If you want to include only certain class files, you can name them individually, separated by spaces. (Things get more complicated if your classes are not in the default package. In that case, the class files must be in subdirectories of the directory in which you issue the jar command. See Subsection 2.6.4.)

Making an executable jar file on the command line is more complicated. There has to be some way of specifying which class contains the main() routine. This is done by creating a manifest file. The manifest file can be a plain text file containing a single line of the form

Main-Class: ClassName

where ClassName should be replaced by the name of the class that contains the main() routine. For example, if the main() routine is in the class MosaicDrawFrame, then the manifest file should read "Main-Class: MosaicDrawFrame". You can give the manifest file any name you like. Put it in the same directory where you will issue the jar command, and use a command of the form

jar  cmf  ManifestFileName  JarFileName.jar  *.class

to create the jar file. (The jar command is capable of performing a variety of different operations. The first parameter to the command, such as "cf" or "cmf", tells it which operation to perform.)

By the way, if you have successfully created an executable jar file, you can run it on the command line using the command "java -jar". For example:

java  -jar  JarFileName.jar

End of Chapter 6