Java Programming, Lecture Notes # 96, Revised 10/19/98.
Students in Prof. Baldwin's Intermediate Java Programming classes at ACC are responsible for knowing and understanding all of the material in this lesson.
JDK 1.1 was formally released on February 18, 1997. This lesson was originally written on March 23, 1997 using the software and documentation in the JDK 1.1 download package.
In previous lessons on event handling under JDK 1.1, you have learned how to use the Source/Listener concept embodied in the Delegation Event Model to handle different types of events generated by visual components on the Graphical User Interface.
In this lesson, we take the subject of event handling one step further by teaching you how to create and dispatch events under program control which produce the same response as if the events were caused by actions of the user on visual components. In other words, you will learn how to create and dispatch events under program control which simulate the behavior of a user. For example, you might dispatch an event of a given type if a particular character were read from a file.
Although you may not immediately see the need to create and dispatch events under program control, an understanding of this material is critical to an understanding of Lightweight Components that will be presented in a subsequent lesson. Therefore, it is very important that you gain a solid understanding of the concept of program-generated events under JDK 1.1 before embarking on your studies of Lightweight Components.
In addition, an understanding of this material will give you a much better understanding of what you are really doing when you implement the Delegation Event Model using visual components from the AWT.
The main theme of this lesson is the use of the AWTEventMulticaster class of the Delegation Event Model. This class is used to maintain a list of Listener objects which are registered to be notified whenever an event of a particular type occurs on a particular object and to cause those Listener objects to be notified. In previous lessons, the use of the AWTEventMulticaster class has occurred "under the covers." In this lesson, we will provide some exposure as to how that class does what it does.
As is frequently the case, we will use sample programs to illustrate the concepts. In this lesson, we will take it in two steps.
In order to use program-generated events with the techniques described in this lesson, you will need to define a class that is capable of generating the type of events of interest. In this lesson we concentrate on Action events. However, there is no reason that you could not use similar techniques to generate low-level events such as mouse events and key events.
The class must be a subclass of the Component class and must include the following three members (as a minimum):
This program is designed to be compiled and run under JDK 1.1 This is a bare-bones program that illustrates Program Generated Events under JDK 1.1.
A non-visual object class (named NonVisual) is defined. Objects of this class are capable of generating Action events.
A single object of the NonVisual class is instantiated. A single ActionListener class is also defined. An object of this class is instantiated and registered to listen for Action events on the NonVisual object.
Then the generateActionEvent() method is invoked on the NonVisual object causing an Action event to be trapped and processed by the ActionListener object. The processing is simply to display information identifying the NonVisual object that generated the event.
The output from the program is:
Copyright 1997, R.G.Baldwin actionPerformed() method invoked on NonVisualObject |
The program was tested using JDK 1.1 running under Win95.
The first interesting code fragment is the code in the constructor that
NonVisual nonVisualObject = new NonVisual("NonVisualObject");
nonVisualObject.makeActionListenerList(new AnActionListenerClass());
nonVisualObject.generateActionEvent();
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The next interesting code fragment is the declaration of the
instance variable in the NonVisual class definition that
will refer to a list of registered Listener objects. As
mentioned earlier, this instance variable could just as easily
refer to a single object of the ActionListener type as to
refer to a list of such objects. In fact, in this bare-bones
program it does refer to a single instance of an ActionListener
object as we will discuss in more detail later.
ActionListener actionListener;//list of registered action listener objects |
The next interesting code fragment is the statement that is used to construct the list of registered Listener objects by adding a new object to the list.
The first time this statement is executed in a program, it simply returns a reference to the object being added to the list.
If it is executed a second time, it returns a reference to a list of objects that is being maintained separately (this is explained more fully later).
In the bare-bones program, only one Listener object is
added to the list. If we were to examine the contents of the
reference returned by the AWTEventMulticaster.add()
method, we would find that it is simply a reference to the
object. In a later program, we will see how it refers to a list
when more than one object is added to the list.
actionListener = AWTEventMulticaster.add(actionListener, listener); |
The final interesting code fragment in this bare-bones program is the statement that invokes the actionPerformed() method of the ActionListener object, or more properly invokes that method on all the objects in the list of registered ActionListener objects.
Fortunately, all we have to do is to invoke the method once on the instance variable that refers to the list and the system takes care of invoking the method on all the objects in the list. This is the central feature of the AWTEventMulticaster class.
As you can see, this code instantiates an object of type ActionEvent
and passes that object as a parameter to the actionPerformed()
method thus satisfying the signature requirements of the actionPerformed()
method.
actionListener.actionPerformed( new ActionEvent(this, ActionEvent.ACTION_PERFORMED, ID)); |
The parameters for the ActionEvent constructor are
shown below.
| public ActionEvent(Object source, int id, String
command) Constructs an ActionEvent object with the specified source object. Parameters: |
So there you have the essential ingredients of program-generated events reduced to code fragments. A complete listing of the program is contained in the next section.
This section contains a complete listing of the bare-bones
program. Refer to previous sections for an operational
description of the program.
/*File Event25.java Copyright 1997, R.G.Baldwin
This program is designed to be compiled and run under JDK 1.1
This is a bare-bones program that illustrates Program Generated
Events under JDK 1.1.
The output from the program is:
Copyright 1997, R.G.Baldwin
actionPerformed() method invoked on NonVisualObject
The program was tested using JDK 1.1 running under Win95.
*/
//=======================================================================
import java.awt.*;
import java.awt.event.*;
//=======================================================================
public class Event25 {
public static void main(String[] args){
new Event25();//instantiate an object of this type
}//end main
//-------------------------------------------------------------------------
public Event25(){//constructor
System.out.println("Copyright 1997, R.G.Baldwin");
NonVisual nonVisualObject = new NonVisual("NonVisualObject");
nonVisualObject.makeActionListenerList(new AnActionListenerClass());
nonVisualObject.generateActionEvent();
}//end constructor
}//end class Event25
//========================================================================
//Class to respond to action events
class AnActionListenerClass implements ActionListener{
public void actionPerformed(ActionEvent e){
System.out.println(
"actionPerformed() method invoked on " + e.getActionCommand());
}//end actionPerformed
}//end class AnActionListenerClass
//========================================================================
//Class to create object capable of generating Action events.
//-------------------------------------------------------------------------
class NonVisual extends Component {
String ID; // The ID of the object
ActionListener actionListener;//list of registered action listener objects
//-----------------------------------------------------------------------
public NonVisual(String ID) {//Constructs a NonVisual object
this.ID = ID;
}//end constructor
//-----------------------------------------------------------------------
public void makeActionListenerList(ActionListener listener) {
actionListener = AWTEventMulticaster.add(actionListener, listener);
}//end makeActionListenerList()
//-----------------------------------------------------------------------
public void generateActionEvent() {
actionListener.actionPerformed(
new ActionEvent(this, ActionEvent.ACTION_PERFORMED, ID));
}//end generateActionEvent
}//end class NonVisual
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The next program is more substantial and is designed to illustrate the ability of the AWTEventMulticaster class to dispatch events to more than one Listener object in a list of registered Listener objects.
This program is designed to be compiled and run under JDK 1.1 and illustrates Program Generated Events under JDK 1.1.
A non-visual object class (named NonVisual) is defined. Objects of this class are capable of generating Action events. The mechanism for causing an object of this type to generate an Action event is to invoke the generateActionEvent() method on an object of the class.
Two objects of the NonVisual class are instantiated. Also, two different ActionListener classes are defined.
An ActionListener object of one of these classes is instantiated and registered to listen for Action events on both of the objects of the NonVisual class.
In addition, an object of the other ActionListener class is instantiated and registered to listen for Action events on only one of the NonVisual objects.
Thus, one NonVisual object is registered with only one ActionListener object. The other NonVisual object is registered with two different ActionListener objects.
The registration of two different ActionListener objects on a single NonVisual object requires the AWTEventMulticaster class to dispatch Action events to two different Listener objects and illustrates the central feature of the AWTEventMulticaster class.
After the NonVisual objects are instantiated and the ActionListener objects are registered on the NonVisual objects, the generateActionEvent() method is invoked on each of the NonVisual objects. This causes Action events to be generated and to be trapped and processed by the respective ActionListener objects.
Numerous statements are included to explain what is happening along the way. All of the output from the program goes to the standard output device.
The output from the program for one particular run was as
shown below, Some hard return characters were manually inserted
to force the text to fit on the printed page.
Copyright 1997, R.G.Baldwin Instantiate two NonVisual objects with the ability to generate Action events. Name of first NonVisual object: NonVisualObjA Name of second NonVisual object: NonVisualObjB Register ActionListener objects on the NonVisual objects addActionListener() method invoked NonVisualObj A: Listener to add is: FirstActionListener@1cc728 Invoke AWTEventMulticaster.add() to get reference to ActionListener NonVisualObj A: Ref to ActionListener is: FirstActionListener@1cc728 addActionListener() method invoked NonVisualObj B: Listener to add is: FirstActionListener@1cc761 Invoke AWTEventMulticaster.add() to get reference to ActionListener NonVisualObj B: Ref to ActionListener is: FirstActionListener@1cc761 addActionListener() method invoked NonVisualObj B: Listener to add is: SecondActionListener@1cc783 Invoke AWTEventMulticaster.add() to get reference to ActionListener NonVisualObj B: Ref to ActionListener is: java.awt.AWTEventMulticaster@1cc799 Invoke generateActionEvent() method on the object named NonVisualObjA which has only one registered ActionListener object. In generateActionEvent() method, dispatching ACTION_PERFORMED event to FirstActionListener@1cc728 for NonVisualObj A In actionPerformed() method of FirstActionListener object actionPerformed() method invoked on NonVisualObj A Invoke generateActionEvent() method on the object named NonVisualObjB which has two registered ActionListener objects. In generateActionEvent() method, dispatching ACTION_PERFORMED event to java.awt.AWTEventMulticaster@1cc799 for NonVisualObj B In actionPerformed() method of FirstActionListener object actionPerformed() method invoked on NonVisualObj B In actionPerformed() method of SecondActionListener object actionPerformed() method invoked on NonVisualObj B |
The program was tested using JDK 1.1 running under Win95.
Much of the code in this program replicates code that you have seen in previous programs. In addition, much of the code consists of System.out.println() statements that are there to explain what is happening when you view the output. For the most part, we will skip code that you have seen before and concentrate only on the code that is germane to the objective of this program -- illustration of program-generated events with particular emphasis on the AWTEventMulticaster class.
The first interesting code fragment instantiates two objects
of the NonVisual class with labels and assigns names to
the objects. When you assign names in this manner, you should
make certain that you assign a unique name to each object.
NonVisual aNonVisual = new NonVisual("NonVisualObj A");
aNonVisual.setName("NonVisualObjA");
NonVisual bNonVisual = new NonVisual("NonVisualObj B");
bNonVisual.setName("NonVisualObjB");
|
The next interesting code fragment registers ActionListener
objects on the two NonVisual objects as described
earlier. Note that unlike the bare-bones program, this program
uses the conventional name of addActionListener() for the
method that generates the list of registered ActionListener objects.
aNonVisual.addActionListener(new FirstActionListener()); bNonVisual.addActionListener(new FirstActionListener()); bNonVisual.addActionListener(new SecondActionListener()); |
The next interesting code fragment causes each of the NonVisual
objects to generate an Action event. Note that some
uninteresting code was deleted between these two statements shown
below.
aNonVisual.generateActionEvent(); ... bNonVisual.generateActionEvent(); |
This is followed in the program by two relatively-standard ActionListener class definitions which won't be repeated here.
The next interesting code fragment is the beginning of the NonVisual
class that extends the Component class and the
declaration of two instance variables of the class. (Note that
some comments were removed from the code fragment.)
class NonVisual extends Component {
...
String ID; // The ID of the object
ActionListener actionListener;
|
The first instance variable in the above code fragment is a reference to the identification of the object which is passed in as a parameter when the object is instantiated. In our program, this is a String object provided as a parameter to the constructor when the NonVisual object is instantiated.
The constructor consists of a single assignment statement which assigns its incoming parameter to this instance variable, so we won't show it here.
The next instance variable with the identifier of actionListener is vital to this program.
Once ActionListener objects are registered on this NonVisual object, this instance variable will either
In the second case, when we later invoke the actionPerformed() method on the instance variable named actionListener, we will in fact be invoking that method on an object of type AWTEventMulticaster which will, in turn, invoke the method on all of the objects of type ActionListener contained in the list of ActionListener objects registered on the NonVisual object. As mentioned earlier, this is the central feature of the AWTEventMulticaster class.
The next interesting code fragment is the code which constructs the list of objects registered as Listener objects on a specific NonVisual object.
New objects are added to the list by calling the static add() method of Class java.awt.AWTEventMulticaster and passing to it the instance variable which references the list along with the new Listener object to be added to the list.
When the first object is added to the list, a reference to the Listener object itself is returned. Hence, in the case of a list containing a single Listener object, the reference to the list is simply a reference to the Listener object.
When additional Listener objects are added to the list,
a reference to an object of type java.awt.AWTEventMulticaster
is returned by the add() method of class AWTEventMulticaster.
According to the JDK 1.1 documentation on class AWTEventMulticaster,
"This class will manage the structure of a chain of event listeners and dispatch events to those listeners." |
When the actionPerformed() method is later invoked on the reference to the list, the method will be invoked either on a single object of type ActionListener or will be invoked on an object of type AWTEventMulticaster. In the second case, the actionPerformed() method of class AWTEventMulticaster assumes responsibility for invoking the actionPerformed() method on all the objects in the list.
With that long introduction, the interesting code fragment is
not too complex:
actionListener = AWTEventMulticaster.add(actionListener, listener); |
Of particular interest to us is the output produced by the program when two different ActionListener objects are registered on a single NonVisual object. In this case, pay particular attention to the identification (highlighted in boldface) of the Listener object passed in as a parameter to the addActionListener() method and the identification of the object ultimately referenced by the instance variable which references the list.
When the first Listener object was added to the list, the reference was set to the object itself. When the second Listener object was added to the list, the reference was set to java.awt.AWTEventMulticaster@1cc799.
A portion of the output from the program illustrating this
behavior is shown below (with some hard returns manually
inserted).
addActionListener() method invoked NonVisualObj B: Listener to add is: FirstActionListener@1cc761 Invoke AWTEventMulticaster.add() to get reference to ActionListener NonVisualObj B: Ref to ActionListener is: FirstActionListener@1cc761 addActionListener() method invoked NonVisualObj B: Listener to add is: SecondActionListener@1cc783 Invoke AWTEventMulticaster.add() to get reference to ActionListener NonVisualObj B: Ref to ActionListener is: java.awt.AWTEventMulticaster@1cc799 |
The next interesting code fragment occurs in a method named generateActionEvent().
The purpose of this method is to instantiate an ActionEvent
object and to invoke the actionPerformed() method on the
reference to the list of ActionListener objects, passing
the ActionEvent object as a parameter.
actionListener.actionPerformed(new ActionEvent( this, ActionEvent.ACTION_PERFORMED, ID)); |
As mentioned earlier, a lot of output statements were included in this program so that the output would contain an explanation of what is happening as the program executes. This section has highlighted only the code necessary to implement program-generated events with multicasting. A complete listing of the program along with the output produced by the program is contained in the next section.
This section contains a complete listing of the program with
additional comments. The comments include the output produced by
the program. See the previous sections for an operational
description of the program.
/*File Event24.java Copyright 1997, R.G.Baldwin
This program is designed to be compiled and run under JDK 1.1
This program illustrates Program Generated Events under JDK 1.1.
The output from the program for one particular run was:
Copyright 1997, R.G.Baldwin
Instantiate two NonVisual objects with the ability to generate
Action events.
Name of first NonVisual object: NonVisualObjA
Name of second NonVisual object: NonVisualObjB
Register ActionListener objects on the NonVisual objects
addActionListener() method invoked
NonVisualObj A: Listener to add is: FirstActionListener@1cc728
Invoke AWTEventMulticaster.add() to get reference to ActionListener
NonVisualObj A: Ref to ActionListener is: FirstActionListener@1cc728
addActionListener() method invoked
NonVisualObj B: Listener to add is: FirstActionListener@1cc761
Invoke AWTEventMulticaster.add() to get reference to ActionListener
NonVisualObj B: Ref to ActionListener is: FirstActionListener@1cc761
addActionListener() method invoked
NonVisualObj B: Listener to add is: SecondActionListener@1cc783
Invoke AWTEventMulticaster.add() to get reference to ActionListener
NonVisualObj B:
Ref to ActionListener is: java.awt.AWTEventMulticaster@1cc799
Invoke generateActionEvent() method on the object named NonVisualObjA
which has only one registered ActionListener object.
In generateActionEvent() method, dispatching ACTION_PERFORMED event to
FirstActionListener@1cc728 for NonVisualObj A
In actionPerformed() method of FirstActionListener object
actionPerformed() method invoked on NonVisualObj A
Invoke generateActionEvent() method on the object named NonVisualObjB
which has two registered ActionListener objects.
In generateActionEvent() method, dispatching ACTION_PERFORMED event to
java.awt.AWTEventMulticaster@1cc799 for NonVisualObj B
In actionPerformed() method of FirstActionListener object
actionPerformed() method invoked on NonVisualObj B
In actionPerformed() method of SecondActionListener object
actionPerformed() method invoked on NonVisualObj B
The program was tested using JDK 1.1 running under Win95.
*/
//=======================================================================
import java.awt.*;
import java.awt.event.*;
//=======================================================================
public class Event24 {
public static void main(String[] args){
new Event24();//instantiate an object of this type
}//end main
//-------------------------------------------------------------------------
public Event24(){//constructor
System.out.println("Copyright 1997, R.G.Baldwin");
System.out.println("Instantiate two NonVisual objects with the "
+ "ability to generate Action events.");
NonVisual aNonVisual = new NonVisual("NonVisualObj A");
aNonVisual.setName("NonVisualObjA");
NonVisual bNonVisual = new NonVisual("NonVisualObj B");
bNonVisual.setName("NonVisualObjB");
System.out.println("Name of first NonVisual object: " +
aNonVisual.getName());
System.out.println("Name of second NonVisual object: " +
bNonVisual.getName());
//Register cross-linked ActionListener
// objects on the NonVisual objects.
// One NonVisual object is registered on a
// single ActionListener object.
// The other NonVisual object is registered on
// two different ActionListener
// objects, one of which is of the same class
// as the object registered on
// the first NonVisual object.
System.out.println(
" ActionListener objects on the NonVisual objects");
aNonVisual.addActionListener(new FirstActionListener());
bNonVisual.addActionListener(new FirstActionListener());
bNonVisual.addActionListener(new SecondActionListener());
//Now cause each of the NonVisual objects to generate an Action event.
System.out.println(
"Invoke generateActionEvent() method on the object named "
+ aNonVisual.getName());
System.out.println(
"which has only one registered ActionListener object.");
aNonVisual.generateActionEvent();//do it
System.out.println();//blank line
System.out.println(
"Invoke generateActionEvent() method on the object named "
+ bNonVisual.getName());
System.out.println(
"which has two registered ActionListener objects.");
bNonVisual.generateActionEvent();//do it
}//end constructor
}//end class Event24
//========================================================================
//The following two classes are standard ActionListener classes. Objects
// of these classes simply trap Action events and display some information
// about them.
//First class to respond to action events
class FirstActionListener implements ActionListener{
public void actionPerformed(ActionEvent e){
System.out.println(
"In actionPerformed() method of FirstActionListener object");
System.out.println(
"actionPerformed() method invoked on " + e.getActionCommand());
}//end actionPerformed
}//end class FirstActionListener
//========================================================================
//Second class to respond to action events
class SecondActionListener implements ActionListener{
public void actionPerformed(ActionEvent e){
System.out.println(
"In actionPerformed() method of SecondActionListener object");
System.out.println(
"actionPerformed() method invoked on " + e.getActionCommand());
}//end actionPerformed
}//end class FirstActionListener
//////////////////////////////////////////////////////////////////////////
// The following class produces a NonVisual object capable of generating
// Action events.
//========================================================================
class NonVisual extends Component {
//The state of a NonVisual object at any time is defined by the following
// instance variables.
String ID; // The ID of the object
ActionListener actionListener;//Refers to a list of ActionListener
// objects to be notified when an Action
// event occurs. (See addActionListener
// description below).
//-----------------------------------------------------------------------
public NonVisual(String ID) {//Constructs a NonVisual object
this.ID = ID;
}//end constructor
//-----------------------------------------------------------------------
//The behavior of a NonVisual object is defined by the following
// instance methods.
//-----------------------------------------------------------------------
/*
The following method adds ActionListener objects passed in as parameters
to the list of ActionListener objects designated to be notified of
action events from a NonVisual object.
Notification takes place in a different method by invoking the
actionPerformed() method of each of the ActionListener objects on
the list.
New objects are added to the list by calling the static add() method
of Class java.awt.AWTEventMulticaster and passing to it the instance
variable which references the list along with the new listener object
to be added.
For the first listener object added to the list, a reference to the
Listener object itself is returned. Hence, in that case the reference
to the list is simply a reference to the Listener object.
When additional listener objects are added to the list, a reference
to an object of type java.awt.AWTEventMulticaster is returned by the
add() method of Class AWTEventMulticaster.
According to the JDK 1.1 documentation on Class AWTEventMulticaster,
"This class will manage the structure of a chain of event
listeners and dispatch events to those listeners."
When the actionPerformed() method is later invoked on the reference to
the list, either the actionPerformed() method is invoked on a single
object, or the AWTEventMulticaster object assumes responsibility for
invoking the actionPerformed() method on all of the Listener objects
that it is maintaining in its list of Listener objects.
*/
public void addActionListener(ActionListener listener) {
System.out.println();//blank line
System.out.println("addActionListener() method invoked");
System.out.println(ID + ": Listener to add is: " + listener);
System.out.println("Invoke AWTEventMulticaster.add() to get "
+ "reference to ActionListener");
actionListener = AWTEventMulticaster.add(actionListener, listener);
System.out.println(ID + ": Ref to ActionListener is: " +
actionListener);
System.out.println();//blank line
}//end addActionListener()
//-----------------------------------------------------------------------
//The purpose of this method is to invoke the actionPerformed() method
// on all the Listener objects that are contained in a list of Listener
// objects that are registered to listen for Action events being
// generated by this NonVisual object. This is accomplished by invoking
// the actionPerformed() method on the reference to the list. When this
// is done, an ActionEvent object is instantiated and passed
//as a parameter.
public void generateActionEvent() {
if(actionListener != null) {
//confirm that an ActionListener is registered
System.out.println("In generateActionEvent() method, dispatching "
+ "ACTION_PERFORMED event to ");
System.out.println(actionListener + " for " + ID);
actionListener.actionPerformed(new ActionEvent(
this, ActionEvent.ACTION_PERFORMED, ID));
}//end if on actionListener
}//end paint
}//end class NonVisual
//===========================================================
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Q - Without viewing the solution that follows, write a Java application that meets the specifications provided in the comments to the following application.
A - See solution below:
/*File SampProg129.java Copyright 1997, R.G.Baldwin
This program illustrates Program Generated Events
under JDK 1.1.
A non-visual object class (named NonVisual) is defined.
Objects of this class are capable of generating Action
events.
One object of the NonVisual class is instantiated.
One ActionListener class is defined. An object of this
class is instantiated and registered to listen for
Action events on the NonVisual object mentioned above.
The user is prompted to enter some characters at
the keyboard. Whenever the user enters the character 'x'
the generateActionEvent() method is invoked on the
NonVisual object causing an Action event to occur and to
be trapped and processed by the ActionListener object.
This causes the following message to be displayed on the
screen:
actionPerformed() method invoked on NonVisualObject
The program was tested using JDK 1.1.3 running under Win95.
*/
//=========================================================
import java.awt.*;
import java.awt.event.*;
import java.io.*;
//=========================================================
public class SampProg129 {
NonVisual nonVisualObject; //reference to a NonVisual obj
//-------------------------------------------------------
public static void main(String[] args){
//instantiate an object of this type
SampProg129 thisObj = new SampProg129();
int data = 0;
System.out.println("Enter some characters");
System.out.println(
"Enter an 'x' to generate an Action Event");
System.out.println("Enter Ctrl-z to terminate");
try{
while((data = System.in.read()) != -1){
if( (char)data == 'x')
//When the user enters an 'x', cause an Action
// Event to be generated on the NonVisual object
// referred to by the instance variable of this
// class named nonVisualObject.
thisObj.nonVisualObject.generateActionEvent();
}//end while loop
}catch(IOException e){}
}//end main
//---------------------------------------------------------
public SampProg129(){//constructor
System.out.println("Copyright 1997, R.G.Baldwin");
nonVisualObject = new NonVisual("NonVisualObject");
nonVisualObject.makeActionListenerList(
new AnActionListenerClass());
}//end constructor
}//end class SampProg129
//=========================================================
//Class to respond to action events
class AnActionListenerClass implements ActionListener{
public void actionPerformed(ActionEvent e){
System.out.println(
"actionPerformed() method invoked on " +
e.getActionCommand());
}//end actionPerformed
}//end class AnActionListenerClass
//=========================================================
//Class to create object capable of generating
// Action events.
//--------------------------------------------------------
class NonVisual extends Component {
String ID; // The ID of the object
//list of registered action listener objects
ActionListener actionListener;
//-------------------------------------------------------
public NonVisual(String ID) {//Construct a NonVisual obj
this.ID = ID;
}//end constructor
//-------------------------------------------------------
//Method to construct a list of registered listeners
public void makeActionListenerList(
ActionListener listener) {
actionListener =
AWTEventMulticaster.add(actionListener, listener);
}//end makeActionListenerList()
//-------------------------------------------------------
//Method to dispatch an action event to the registered
// listeners.
public void generateActionEvent() {
actionListener.actionPerformed(
new ActionEvent(
this, ActionEvent.ACTION_PERFORMED, ID));
}//end generateActionEvent
}//end class NonVisual
//=========================================================
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-end-