We have learned how to handle events and use layout managers.These two topics form the basis for the design and implementation of a Graphical User Interface.
Now we are learning about the various components that we can combine with layout and event handling to produce an effective Graphical User Interface.
The available components are defined by classes in the package java.awt. Our approach is to group those classes into categories and study the material on a category basis. As of this writing, it looks as if the remaining categories are:
java.lang.Object | +----MenuShortcut | +----java.awt.MenuComponent | +----java.awt.MenuBar | +----java.awt.MenuItem | +----java.awt.Menu | +----java.awt.CheckboxMenuItem | +----java.awt.PopupMenu |
Previous lessons discussed and provided sample programs for the following classes.
This lesson will make use of the same set of classes, except this time we will substitute CheckboxMenuItem in place of MenuItem.
In addition, whereas the previous lesson used an ActionListener class to process the events generated by selecting menu items, this lesson will use an ItemListener class for processing events generated by selecting menu items.
We will defer detailed consideration of the PopupMenu class to a subsequent lesson.
A previous lesson discussed the MenuComponent, Menu, MenuItem, MenuBar, and MenuShortcut classes in detail. We won't repeat that here. We will begin our detailed discussion her with the CheckboxMenuItem class.
Unlike the ordinary menu items discussed in a previous lesson, these menu items behave much like checkbox components behave. An operational description of the behavior of menu items of this type is contained in the discussion of the sample program that follows.
CheckboxMenuItem has no fields and the following public constructors:
This class provides about nine methods, with some of the more interesting ones listed below.
In the case of our sample program, the desired action will simply be to display the identification and state of the menu item that was selected.
This application places one menu in a Frame object. The menu contains three items of type CheckboxMenuItem. An item of this type in a menu looks pretty much like an ordinary menu item until you select it. When you select it, a checkmark or other visual indication that it has been selected appears on the menu. Selecting the item again causes the checkmark to be removed. These operations cause the "state" of the item to change. A checked item has a state of true and an unchecked item has a state of false.
Selection of an item generates an ItemEvent that can be trapped using an object of type ItemListener which overrides the method itemStateChanged(ItemEvent e).
When such an event is trapped, several pieces of important information including the new state, the item label, and the item name are contained in the ItemEvent object passed in as a parameter. This information can be used to identify which item has changed state and to implement the required action.
In this program, the action of the overridden itemStateChanged() method is to display a line of text on the screen showing the source information contained in the ItemEvent object.
Typical screen outputs for menu items being selected and deselected
are shown below. Line breaks were manually inserted in this material to
make it fit on the screen.
java.awt.CheckboxMenuItem[ chkmenuitem0,label=First Item,state=true] java.awt.CheckboxMenuItem[ chkmenuitem1,label=Second Item,state=true] java.awt.CheckboxMenuItem[ chkmenuitem2,label=Third Item,state=true] java.awt.CheckboxMenuItem[ chkmenuitem0,label=First Item,state=false] java.awt.CheckboxMenuItem[ chkmenuitem1,label=Second Item,state=false] java.awt.CheckboxMenuItem[ chkmenuitem2,label=Third Item,state=false] |
The program was tested using JDK 1.1.3 running under Win95.
The order that items are instantiated and associated is important. The first interesting code fragment instantiates several CheckboxMenuItem objects which will be added to a Menu object to produce a menu with several choices.
The Menu object will be added to a MenuBar object to produce a menu bar assembly with one menu.
The MenuBar object will then be associated with the Frame object.
That's the structural overview. Now let's go back to the CheckboxMeunItem
objects. The following code is typical of that required to instantiate
those objects.
CheckboxMenuItem firstMenuItem =
new CheckboxMenuItem("First Item"); |
CheckBoxMenuProcessor eventProcessor = new CheckBoxMenuProcessor(); firstMenuItem.addItemListener(eventProcessor); |
The next interesting code fragment is class that implements the ItemListener
interface. Again, it is only interesting because it implements an interface
that we haven't used before and overrides a method that we haven't used
before. Otherwise, the code in the class definition is straightforward.
class CheckBoxMenuProcessor implements ItemListener{
public void itemStateChanged(ItemEvent e){
//Display the menu item that generated the ItemEvent
System.out.println(e.getSource());
}//end itemStateChanged
}//end class CheckBoxMenuProcessor |
/*File Menu03.java Copyright 1997, R.G.Baldwin
This program is designed to be compiled and run under
JDK 1.1
Illustrates use of checkbox menus.
This application places one menu in a Frame object. The
menu contains three items of type CheckboxMenuItem. An
item of this type in a menu looks like an ordinary menu
item until you select it. When you select it, a checkmark
or other visual indication that it has been selected
appears on the menu. Selecting the item again causes the
checkmark to be removed. These operations cause the
"state" of the item to change. A checked item has a state
of true and an unchecked item has a state of false.
Selection of an item generates an ItemEvent that can
be trapped using a Listener object of type ItemListener
which overrides the method itemStateChanged(ItemEvent e).
When such an event is trapped, several pieces of important
information including the new state, the item label, and
the item name are contained in the ItemEvent object passed
in as a parameter. This information can be used to
identify which item has changed state and to implement the
required action.
In this program, the action of the overridden
itemStateChanged() method is to display a line of text on
the screen showing the information contained in the
ItemEvent object.
Typical screen outputs (with line breaks manually inserted)
when menu items are selected and deselected are:
java.awt.CheckboxMenuItem[
chkmenuitem0,label=First Item,state=true]
java.awt.CheckboxMenuItem[
chkmenuitem1,label=Second Item,state=true]
java.awt.CheckboxMenuItem[
chkmenuitem2,label=Third Item,state=true]
java.awt.CheckboxMenuItem[
chkmenuitem0,label=First Item,state=false]
java.awt.CheckboxMenuItem[
chkmenuitem1,label=Second Item,state=false]
java.awt.CheckboxMenuItem[
chkmenuitem2,label=Third Item,state=false]
A windowClosing() event listener object is instantiated
and registered on the frame to terminate the program when
the frame is closed.
The program was tested using JDK 1.1.3 running under Win95.
**********************************************************/
import java.awt.*;
import java.awt.event.*;
//=======================================================//
public class Menu03 {
public static void main(String[] args){
GUI gui = new GUI();//instantiate a GUI
}//end main
}//end class Menu03
//=======================================================//
class GUI {
public GUI(){//constructor
//Instantiate CheckboxMenuItem objects
CheckboxMenuItem firstMenuItem =
new CheckboxMenuItem("First Item");
CheckboxMenuItem secondMenuItem =
new CheckboxMenuItem("Second Item");
CheckboxMenuItem thirdMenuItem =
new CheckboxMenuItem("Third Item");
//Instantiate an ItemListener object and register it
// on the CheckboxMenuItem objects.
CheckBoxMenuProcessor eventProcessor =
new CheckBoxMenuProcessor();
firstMenuItem.addItemListener(eventProcessor);
secondMenuItem.addItemListener(eventProcessor);
thirdMenuItem.addItemListener(eventProcessor);
//Instantiate a Menu object and add the
// CheckboxMenuItem objects to it
Menu menuA = new Menu("Menu A");
menuA.add(firstMenuItem);
menuA.add(secondMenuItem);
menuA.add(thirdMenuItem);
//Instantiate a MenuBar object and add the Menu
// objects to it
MenuBar menuBar = new MenuBar();
menuBar.add(menuA);
//Instantiate a Frame object and associate the MenuBar
// object with the Frame object. Note that this is
// NOT a typical myFrame.add() method invocation, but
// rather is a special form of method invocation that
// is required to associate a MenuBar object with a
// Frame object.
Frame myFrame =
new Frame("Copyright 1997, R.G.Baldwin");
myFrame.setMenuBar(menuBar);
myFrame.setSize(250,100);
myFrame.setVisible(true);
//Instantiate and register a window listener to
// terminate the program when the Frame is closed.
myFrame.addWindowListener(new Terminate());
}//end constructor
}//end class GUI definition
//=======================================================//
//Class to instantiate an ItemListener object to be
// registered on the menu items.
class CheckBoxMenuProcessor implements ItemListener{
public void itemStateChanged(ItemEvent e){
//Display the menu item that generated the ItemEvent
System.out.println(e.getSource());
}//end itemStateChanged
}//end class CheckBoxMenuProcessor
//=======================================================//
class Terminate extends WindowAdapter{
public void windowClosing(WindowEvent e){
System.exit(0);//terminate the program
}//end windowClosing
}//end class Terminate
//=======================================================// |