The Java Lesson 33: Menus and submenus

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Java Lesson 33 by Jon Huhtala: Menus and submenus

Overview

A menu bar having multiple menus (each with multiple menu items, pop-up submenus, checkbox menu items, and separators) can be applied to any container that extends the Frame class. Note that other Window subclasses (such as Dialog) and Panel subclasses (such as Applet) cannot contain a menu bar.

To work with menus and submenus, you need to know the following classes:

MenuComponent

MenuBar

Menu

MenuItem

CheckboxMenuItem

The MenuComponent class

• Is part of the java.awt package

• Is an extension of the Object class

Object
	MenuComponent

• Is the abstract superclass of all menu-related components. In this respect, the class MenuComponent is analogous to the abstract superclass Component for AWT components.

• Has a few useful methods, but most menu and submenu programming involves classes that extend MenuComponent

The MenuBar class

• Is part of the java.awt package

• Is an extension of the MenuComponent class

Object
	MenuComponent
		MenuBar

• Encapsulates the platform's concept of a menu bar bound to a frame. In order to associate the menu bar with a Frame object, call the frame's setMenuBar() method. For example, if appFrame is a Frame object, a menu bar may be created and added to appFrame by coding

MenuBar mainBar = new MenuBar();
appFrame.setMenuBar(mainBar);

When initially constructed, mainbar has no menus. Its add() method may be called to add them. Other methods exist, but are of little general use.

The Menu class

• Is part of the java.awt package

• Is an extension of the MenuItem class (to be covered shortly)

Object
	MenuComponent
		MenuItem
			Menu

• Is used to construct Menu objects, which are a pull-down menu components deployed from a menu bar. Each Menu object can contain a number of menu items which can be a submenu (an instance of Menu), a simple item (an instance of MenuItem), or a check box item (an instance of CheckboxMenuItem).

• Has several constructors but the most commonly used receives a string parameter that specifies the label of the menu (the text that will appear within the menu bar). For example, if mainBar is a MenuBar object, a menu with the label "File" can be instantiated and added to mainBar by coding

Menu fileMenu = new Menu("File");
mainBar.add(fileMenu);

When initially constructed, fileMenu has no submenus, menu items, or checkbox menu items. Its add() method may be called to add them. The order in which menu items appear will be the order in which they are added.

• Has several methods but the two most frequently used are:

Method	Usage
add()	Adds the specified MenuItem to the menu
addSeparator()	Adds a separator line to the menu at the current position

Consult the help facility of your Java development environment or the Java API for more details.

The MenuItem class

• Is part of the java.awt package

• Is an extension of the MenuComponent class

Object
	MenuComponent
		MenuItem

• Is the root class for all items to be added to a menu. All items in a menu must belong to the class MenuItem, or one of its subclasses. The default MenuItem object embodies a simple labeled menu item.

• Fires an ActionEvent when a simple menu item is selected. If the menu item has a registered ActionListener with an actionPerformed() method, it will automatically be called to process the event. Note that the subclass Menu does not fire any event until one of its simple menu items is selected.

• Has several constructors but the most commonly used receives a string parameter that specifies the label of the menu item (the text that will appear within the menu). For example, if fileMenu is a Menu object, the following statements will construct a MenuItem object with the label "Open", register its ActionListener, and add it to fileMenu:

MenuItem open = new MenuItem("Open");
open.addActionListener(this);
fileMenu.add(open);

• Has many useful methods. The most commonly used are:

Method	Usage
addActionListener()	Adds the specified ActionListener to receive ActionEvent objects from this menu item
getActionCommand()	Returns the command name of the action event fired by this menu item
getLabel()	Gets the label of this menu item
isEnabled()	Determines whether the menu item is enabled
removeActionListener()	Removes the specified action listener so it no longer receives ActionEvent objects from this menu item
setActionCommand()	Sets the command name for the ActionEvent objects fired by this menu item
setEnabled()	Enables or disables the menu item
setLabel()	Sets the label of this menu item

Consult the help facility of your Java development environment or the Java API for more details.

The CheckboxMenuItem class

• Is part of the java.awt package

• Is an extension of the MenuItem class

Object
	MenuComponent
		MenuItem
			CheckboxMenuItem

• Represents a checkbox that can be included in a menu. Clicking on the checkbox in the menu changes its state from "on" to "off" or from "off" to "on."

• Fires an ItemEvent when a checkbox menu item is selected. If the item has a registered ItemListener with an itemStateChanged() method, it will automatically be called to process the event.

• Has several constructors but the most commonly used receives a string parameter that specifies the label of the checkbox menu item (the text that will appear within the menu). For example, if optionMenu is a Menu object, the following statements will construct a CheckboxMenuItem object with the label "Auto Save", register its ItemListener, and add it to optionMenu:

CheckboxMenuItem autoSave = new CheckboxMenuItem("Auto Save");
autoSave.addItemListener(this);
fileMenu.add(autoSave);

By default, the initial state of a CheckboxMenuItem is "off". An overloaded constructor may be used to initialize the object to be "on".

• Has several useful methods including:

Method	Usage
addItemListener()	Adds the specified ItemListener to receive ItemEvent objects from this checkbox menu item
getState()	Determines whether the state of this checkbox menu item is "on" or "off"
removeItemListener()	Removes the specified item listener so it no longer receives ItemEvent objects from this checkbox menu item
setState()	Sets the check box menu item to the specified state

Consult the help facility of your Java development environment or the Java API for more details.

Example:

import java.awt.*;
import java.awt.event.*;

public class App extends Frame implements WindowListener,
ActionListener, ItemListener {

// References for menu components

MenuBar mainBar;
Menu fileMenu;
MenuItem exit;
Menu viewMenu;
Menu sizeSubmenu;
MenuItem large;
MenuItem medium;
MenuItem small;
CheckboxMenuItem bold;
CheckboxMenuItem italics;

// Message to be displayed and its initial font

Label message = new Label("Hello World!");
Font myFont = new Font("SanSerif", Font.PLAIN, 24);

public static void main(String[] args) {
App myWindow = new App("A stylish menu");
myWindow.setSize(300,200);
myWindow.setVisible(true);
}

public App(String title) {
super(title);
setLayout(new BorderLayout());
addWindowListener(this);

// Create and set the frame's menu bar (unpopulated)

mainBar = new MenuBar();
setMenuBar(mainBar);

// Create the "File" menu and add it to the frame's menu bar

fileMenu = new Menu("File");
fileMenu.addSeparator();
exit = new MenuItem("Exit");
exit.addActionListener(this);
fileMenu.add(exit);
mainBar.add(fileMenu);

// Build the "Text Size" submenu in order to reference it shortly

sizeSubmenu = new Menu("Text Size");
large = new MenuItem("Large");
large.addActionListener(this);
sizeSubmenu.add(large);
medium = new MenuItem("Medium");
medium.addActionListener(this);
sizeSubmenu.add(medium);
small = new MenuItem("Small");
small.addActionListener(this);
sizeSubmenu.add(small);

// Create the "View" menu and add it to the frame's menu bar

viewMenu = new Menu("View");
viewMenu.add(sizeSubmenu);
viewMenu.addSeparator();
bold = new CheckboxMenuItem("Bold");
bold.addItemListener(this);
viewMenu.add(bold);
italics = new CheckboxMenuItem("Italics");
italics.addItemListener(this);
viewMenu.add(italics);
mainBar.add(viewMenu);

// Set the message in the frame

message.setFont(myFont);
message.setBackground(Color.black);
message.setForeground(Color.white);
message.setAlignment(Label.CENTER);
add(message);
}

public void actionPerformed(ActionEvent e) {
if (e.getSource().equals(exit)) {
dispose();
System.exit(0);
}
else {
if (e.getSource().equals(large)) {
myFont = new Font(myFont.getFontName(), myFont.getStyle(), 36);
}
if (e.getSource().equals(medium)) {
myFont = new Font(myFont.getFontName(), myFont.getStyle(), 24);
}
if (e.getSource().equals(small)) {
myFont = new Font(myFont.getFontName(), myFont.getStyle(), 12);
}
message.setFont(myFont);
}
}

public void itemStateChanged(ItemEvent e) {
if (e.getSource().equals(bold)) {
if (bold.getState() == true) {
myFont = new Font(myFont.getFontName(),
myFont.getStyle() + Font.BOLD, myFont.getSize());
}
else {
myFont = new Font(myFont.getFontName(),
myFont.getStyle() - Font.BOLD, myFont.getSize());
}
}
if (e.getSource().equals(italics)) {
if (italics.getState() == true) {
myFont = new Font(myFont.getFontName(),
myFont.getStyle() + Font.ITALIC, myFont.getSize());
}
else {
myFont = new Font(myFont.getFontName(),
myFont.getStyle() - Font.ITALIC, myFont.getSize());
}
}
message.setFont(myFont);
}

public void windowClosing(WindowEvent e) {
dispose();
System.exit(0);
}

public void windowOpened(WindowEvent e) {}
public void windowActivated(WindowEvent e) {}
public void windowIconified(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowDeactivated(WindowEvent e) {}
public void windowClosed(WindowEvent e) {}
}

Notes:

1. The class implements the ActionListener interface in order to handle the selection of menu items and the ItemListener interface in order to handle the selection of checkbox menu items

2. Within the application's constructor, the order of instantiation is critical. The main menu bar (mainBar) must exist before a menu (such as fileMenu) may be added to it. Similarly, the "Text Size" submenu (sizeSubmenu) is built prior to adding it to the "View" menu (viewMenu). Also notice that each MenuItem object has a registered ActionListener and each CheckboxMenuItem object has a registered ItemListener.

3. The actionPerformed() method begins by testing for the "Exit" menu selection. If it was chosen, the application is immediately terminated. Otherwise, a determination is made as to which text size the user selected, a new font is created to meet their request, and the new font is applied to the message being displayed.

4. The itemStateChanged() method determines which CheckboxMenuItem was chosen (bold or italics), creates a new font to meet their request, and applies it to the message being displayed.

Lab exercise for Ferris students

E-mail your answers to this assignment no later than the due date listed in the class schedule.

Review questions

1. Which of the following may a menu contain? (choose two)

1. a List object

2. a separator

3. a Menu object

4. a Checkbox object

5. a Panel object

2. Which of the following may contain a menu bar?

1. Applet

2. Canvas

3. Panel

4. Dialog

5. none of the above

3. Code a single statement to create a checkbox menu item named gridLines with a label of "Grid Lines" and initially set to the "on" state.

4. If someItem is a MenuItem object, which one of the following can legally replace the XXXXX in the following statement?

someItem.addXXXXXListener(this);

1. Action

2. Item

3. MenuAction

4. MenuItem

5. both A and B

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