Java The KeyEventDispatcher in Java

The KeyEventDispatcher in Java

Java Programming, Notes # 1855


Preface

The focus subsystem

This lesson is part of a series of lessons designed to
teach you how to use several important features the new features of the focus
subsystem.

The first lesson in the series was entitled
Focus
Traversal Policies in Java Version 1.4
.  The previous lesson was
entitled The
Opposite Focus Component
.

Previous topics

Previous lessons in this series have dealt with several aspects of the new focus subsystem,
including:

  • Defining new focus traversal keys
  • How to control focusability at runtime.
  • The ability to query for the currently focused Component.
  • The default Focus Traversal Policy.
  • How to establish a focus traversal policy and modify it at runtime.
  • How to control the focus programmatically.
  • Opposite components.

This lesson will show you how to use the KeyEventDispatcher interface.

What do we mean by focus?

One of the ways to describe focus is as follows.  Among all of
the applications showing on the desktop at any point in time, only one will
respond to the keyboard.  We will refer to that application as the
active
application.

If the active application is a Java application, only one component
within that application’s graphical user interface (GUI) will respond to
the keyboard.  That is the component that has the focus at that
point in time.

A Java component that has the focus also has the ability to fire KeyEvents when it responds to the keyboard.

What does Sun have to say?

Before getting into the technical details, I will provide some quotations
from Sun that describe a KeyEventDispatcher.  In discussing the changes to the
focus subsystem, Sun has this to say:

"While the user’s KeyEvents should generally be delivered to the
focus owner, there are rare cases where this is not desirable. …

A KeyEventDispatcher is a lightweight interface that allows client
code to pre-listen to all KeyEvents in a particular context.

Instances of classes that implement the interface and are registered with
the current KeyboardFocusManager will receive KeyEvents before
they are dispatched to the focus owner, allowing the KeyEventDispatcher
to retarget the event, consume it, dispatch it itself, or make other changes.

… if a KeyEventDispatcher reports that it dispatched the
KeyEvent
, regardless of whether it actually did so, the
KeyboardFocusManager
will take no further action with regard to the
KeyEvent
."

Viewing tip

You may find it useful to open another copy of this lesson in a separate
browser window.  That will make it easier for you to scroll back and
forth among the different figures and listings while you are reading about
them.

Supplementary material

I recommend that you also study the other lessons in my extensive collection
of online Java tutorials.  You will find those lessons published at
Gamelan.com.  However,
as of the date of this writing, Gamelan doesn’t maintain a consolidated index
of my Java tutorial lessons, and sometimes they are difficult to locate there. 
You will find a consolidated index at www.DickBaldwin.com.

Preview

In this lesson, I will show you how to use KeyEventDispatcher objects
to cause a KeyEvent fired by one component to be dispatched to a
different component for processing.

Using a sample program, characters typed into one text field in a GUI will
appear in another text field belonging to the same GUI.  Those characters
will not appear in the text field into which they are typed.

Discussion and Sample
Code

Description of the program

This lesson presents and explains a sample program named KeyEventDispatch01.

This program illustrates the use of a KeyEventDispatcher as an
alternative to the default dispatching of key events.

The program GUI

The program causes a single JFrame object to appear on the screen as
shown in Figure 1.



Figure 1 Program GUI.

The JFrame object contains three JTextField objects. The initial text in
the three JTextField objects is respectively:

  • Field A
  • Field B
  • Field C

Two KeyEventDispatcher objects

Two different KeyEventDispatcher objects are
registered on the current KeyboardFocusManager
in the following order (this is one case in Java where the order of
registration is important):

  • AlternateDispatcherA
  • AlternateDispatcherB

When a KeyEvent occurs …

When a KeyEvent occurs, the current KeyboardFocusManager delivers that event to
the AlternateDispatcherA object for dispatching.

(Events are delivered to multiple registered KeyEventDispatcher objects in the
order in which they are registered.)

If the focus owner is Field A …

If the
owner of the focus at that point in time (the source of the event) is
Field A, the AlternateDispatcherA object
re-dispatches that event to Field C and returns
true. This causes keystrokes entered into Field A to appear in Field C instead
of appearing in Field A.

Returning
true prevents any further dispatching of the
event by the KeyboardFocusManager.

If the source of the event is not Field A …

If the source of the event is not Field A, the AlternateDispatcherA object
returns false. This causes the current KeyboardFocusManager to deliver the event to
AlternateDispatcherB for dispatching.

If the source of the event is Field B …

If the
owner of the focus at that point in time is
Field B, the AlternateDispatcherB object
re-dispatches that event to Field C and returns
true. This causes keystrokes entered into
Field B to appear in Field C instead of appearing in Field B.

Returning
true prevents any further dispatching of the
event.

If the source of the event is not Field B …

If the owner of the focus at that point in time
is not Field B, the AlternateDispatcherB object
returns false.

This causes the current KeyboardFocusManager to dispatch the event to
the component that owns the focus, which is what normally happens in the absence
of registered KeyEventDispatcher objects.

Must be Field C

In this
program, that would have to be Field C
because it has already been determined that
neither Field A nor Field B owns the focus.

This causes keystrokes entered into Field C to
appear in Field C as normal.

All keystrokes appear in Field C

Thus, all keystrokes entered into any of the
three fields in the GUI in Figure 1 will appear in Field C.

This program was tested using J2SE 1.4.2 under WinXP.

The state of the GUI

Figure 1 shows the state of the GUI after the characters a, b, and c, were entered respectively into fields A, B, and C, in that order.  Note
that all three characters, abc, appear in Field C.

Will discuss in fragments

I will discuss this program in fragments.  A complete listing of the
program is provided in Listing 10 near the end of the lesson.

Listing 1 shows the class named KeyEventDispatch01, including
the main method.

class KeyEventDispatch01{
  public static void main(String[] args){
    new GUI();
  }//end main
}//end class KeyEventDispatch01

Listing 1

In this case, the main method simply instantiates an object of the
GUI
class.  The object of the GUI class appears in Figure 1.

The GUI class

Listing 2 shows the beginning of the GUI class, which extends the
JFrame
class.  Thus, a GUI object is also a JFrame
object.

class GUI extends JFrame{
  JTextField fieldA =
                    new JTextField("Field A",12);
  JTextField fieldB =
                    new JTextField("Field B",12);
  JTextField fieldC =
                    new JTextField("Field C",12);

  KeyboardFocusManager manager;

Listing 2

Listing 2 declares and initializes the three JTextField objects shown
in Figure 1.

Listing 2 also declares a reference variable named manager that will
be used to hold a reference to the current KeyboardFocusManager object.

The constructor

The constructor for the GUI class begins in Listing 3.

  GUI(){//constructor
    getContentPane().setLayout(new FlowLayout());
    getContentPane().add(fieldA);
    getContentPane().add(fieldB);
    getContentPane().add(fieldC);

    setSize(390,100);
    setTitle("Copyright 2004, R.G. Baldwin");
    setDefaultCloseOperation(
                           JFrame.EXIT_ON_CLOSE);
    setVisible(true);

Listing 3

The code in Listing 3 is completely straightforward.  Code like this has
been included in hundreds of previous tutorials to create a graphical user
interface and to cause it to appear on the screen.

The current KeyboardFocusManager

Continuing with the constructor, the statement in Listing 4 is new to this lesson.

    manager = KeyboardFocusManager.
                getCurrentKeyboardFocusManager();

Listing 4

Listing 4 invokes the static getCurrentKeyboardFocusManager method of
the KeyboardFocusManager class to gain
access to the current KeyboardFocusManager object.

The object’s
reference is saved in the variable named manager that was declared in
Listing 2.

What is a KeyboardFocusManager object

According to Sun,

"The KeyboardFocusManager is responsible for managing the active
and focused Windows, and the current focus owner.

The focus owner is defined as the Component in an application that
will typically receive all KeyEvents generated by the user.

The focused Window is the Window that is, or contains, the
focus owner. Only a Frame or a Dialog can be the active Window.

(Note that JFrame extends Frame, and JDialog
extends Dialog – Baldwin.)

The native windowing system may denote the active Window or its
children with special decorations, such as a highlighted title bar. The active
Window is always either the focused Window, or the first Frame
or Dialog that is an owner of the focused Window.

The KeyboardFocusManager is both a centralized location for client
code to query for the focus owner and initiate focus changes, and an event
dispatcher for all FocusEvents, WindowEvents related to focus, and
KeyEvents.
"

Many methods are available

A KeyboardFocusManager object provides more than fifty different
methods that can be used by client code for "managing the active and focused
Windows
, and the current focus owner."
 

I will be using several of those methods in this lesson.  The first
such method that I will use is the static getCurrentKeyboardFocusManager
method shown in Listing 4.  As explained earlier, this method returns a
reference to the current KeyboardFocusManager object.

In addition, I will also be using the following methods in subsequent code:

  • addKeyEventDispatcher
  • redispatchEvent

Registering KeyEventDispatcher objects

This program defines two different classes that implement the
KeyEventDispatcher
interface:

  • AlternateDispatcherA
  • AlternateDispatcherB

Continuing with the constructor, the code in Listing 5 registers one object of each of those
two classes on the
KeyboardFocusManager
.  Registration makes it possible to "pre-listen to all KeyEvents" and to
take appropriate action when a KeyEvent occurs.

    manager.addKeyEventDispatcher(
                        new AlternateDispatcherA(
                         fieldA,fieldC,manager));
    manager.addKeyEventDispatcher(
                        new AlternateDispatcherB(
                         fieldB,fieldC,manager));

  }//end constructor
}//end class GUI

Listing 5

I will explain what I mean by appropriate action later.

End of the GUI class

Listing 5 also signals the end of the constructor and the end of the GUI
class.

The AlternateDispatcherA class

One of the classes that implement the KeyEventDispatcher interface is
named AlternateDispatcherA.

Listing 6 shows the beginning of the class named AlternateDispatcherA

class AlternateDispatcherA
                   implements KeyEventDispatcher{
  JTextField fieldA;
  JTextField fieldC;
  KeyboardFocusManager manager;

Listing 6

(An object instantiated from the AlternateDispatcherA class is registered on the
KeyboardFocusManager in
Listing 5.)

Listing 6 declares three instance variables that will be populated by the
constructor.

References to the three text fields shown in Figure 1, as well as a
reference to the KeyboardFocusManager are required to make it possible
for an object of this class to provide the appropriate action mentioned
earlier.

The constructor

Listing 7 shows the constructor for the AlternateDispatcherA class. 

  AlternateDispatcherA(
                   JTextField fieldA,
                   JTextField fieldC,
                   KeyboardFocusManager manager){
    this.fieldA = fieldA;
    this.fieldC = fieldC;
    this.manager = manager;
  }//end constructor

Listing 7

The code in the constructor receives the incoming references and saves them
in the instance variables declared in Listing 6.

The dispatchKeyEvent method

The KeyEventDispatcher interface declares a single method named
dispatchKeyEvent
.  All classes that implement the interface must
provide a concrete definition of this method.  This is the method whose
behavior performs the appropriate action mentioned earlier.

What does Sun have to say?

Here is part of what Sun has to say about the dispatchKeyEvent method.

"This method is called by the current KeyboardFocusManager
requesting that this KeyEventDispatcher dispatch the specified event on
its behalf. This KeyEventDispatcher is free to retarget the event,
consume it, dispatch it itself, or make other changes.

This capability is typically used to deliver KeyEvents to
Components
other than the focus owner.

(That is what is done in this sample program – Baldwin.)

… Note that if a KeyEventDispatcher dispatches the KeyEvent
itself, it must use redispatchEvent to prevent the current
KeyboardFocusManager
from recursively requesting that this
KeyEventDispatcher
dispatch the event again.

If an implementation of this method returns false, then the
KeyEvent
is passed to the next KeyEventDispatcher in the chain,
ending with the current KeyboardFocusManager.

If an implementation returns true, the KeyEvent is assumed
to have been dispatched (although this need not be the case), and the current
KeyboardFocusManager
will take no further action with regard to the
KeyEvent
."

Definition of the dispatchKeyEvent method

Listing 8 shows the definition of the dispatchKeyEvent method in the
AlternateDispatcherA class.

  public boolean dispatchKeyEvent(KeyEvent e){
    if(e.getSource() == fieldA){
      manager.redispatchEvent(fieldC,e);
      return true;
    }else{
      return false;
    }//end else
  }//end dispatchKeyEvent
}//end class AlternateDispatcherA

Listing 8

(Note that the dispatchKeyEvent method receives a reference to the
KeyEvent
object.
)

Is Field A the source of the event?

The code in Listing 8 begins by testing to see if the source of the KeyEvent is
the JTextField object referred to by fieldA.

(This is the text field identified as Field A in Figure 1.)

If so, the code in Listing 8 invokes the redispatchEvent method to
re-dispatch the event to the JTextField object referred to by fieldC

(This is the text field identified as Field C in Figure 1.)

This causes Field C to respond to the event instead of Field A.

The normal response of a
text field to a KeyEvent is to display the character associated with the key that was
pressed to
cause the KeyEvent to be fired.

As a result, characters typed into Field A in Figure 1 appear in Field C instead of
Field A.

The redispatchEvent method

What does it mean to dispatch an event to a component?  One way
to think of this is to deliver the event object to the component and to ask the
component to deal with it in whatever way is appropriate for that component. 

For example, when a KeyEvent of a particular type is dispatched to a
text field, the appropriate behavior is for the text field to extract the
character from the event object and to display that character in the text field. 
That character is also combined with other characters and saved in such a way
that it is possible later to get the character content of the text field by
invoking a method on the text field.

(Experimentation indicates that this happens for a KeyEvent of
type keyPressed, as opposed to type keyReleased or type
keyTyped
.)

Figure 2 contains some of what Sun has to say about the redispatchEvent
method of the KeyboardFocusManager class.

public final void redispatchEvent(
                                        Component target,
AWTEvent
 e)

Re-dispatches an AWTEvent in such a way that
the AWT event dispatcher will not recursively request that the
KeyboardFocusManager
, or any installed KeyEventDispatchers,
dispatch the event again.

Client implementations of dispatchEvent and client-defined
KeyEventDispatchers must call redispatchEvent(target, e)
instead of target.dispatchEvent(e) to dispatch an event.

Parameters:

  • target – the Component to which the event should be
    dispatched
  • e – the event to dispatch

Figure 2
 

Given the above discussion, Figure 2 should be relatively self-explanatory.

The return value

Returning now to the code in Listing 8, after re-dispatching the KeyEvent
to Field C, the dispatchKeyEvent method returns true
This causes the KeyboardFocusManager to assume that the KeyEvent
has been dispatched.  It will take no further action with respect to that
KeyEvent.

If the source of the KeyEvent in Listing 8 was not Field A, the
dispatchKeyEvent method returns false.  This causes the
KeyboardFocusManager
to pass the KeyEvent to the
KeyEventDispatcher
of type AlternateDispatcherB that was also
registered on the KeyboardFocusManager.

(Recall that events are delivered to multiple registered
KeyEventDispatcher
objects in the order in which they were registered on
the KeyboardFocusManager.)

The AlternateDispatcherB class

The AlternateDispatcherB class is shown in Listing 9.  This class
is very similar to the AlternateDispatcherA class shown in Listings 6, 7,
and 8.

class AlternateDispatcherB
                   implements KeyEventDispatcher{
  JTextField fieldB;
  JTextField fieldC;
  KeyboardFocusManager manager;

  //Constructor
  AlternateDispatcherB(
                   JTextField fieldB,
                   JTextField fieldC,
                   KeyboardFocusManager manager){
    this.fieldB = fieldB;
    this.fieldC = fieldC;
    this.manager = manager;
  }//end constructor

  public boolean dispatchKeyEvent(KeyEvent e){
    if(e.getSource() == fieldB){
      manager.redispatchEvent(fieldC,e);
      return true;
    }else{
      return false;
    }//end else
  }//end dispatchKeyEvent
}//end class AlternateDispatcherB

Listing 9

The main difference between the AlternateDispatcherA class and the
AlternateDispatcherB
class is:

The dispatchKeyEvent method in the AlternateDispatcherB
class re-dispatches the event to Field C if the source of the
KeyEvent
is Field BIn contrast, the
dispatchKeyEvent
method in the AlternateDispatcherA class
re-dispatches the event to Field C if the source of the KeyEvent
is Field A

This causes characters entered into Field B to appear in Field C
instead of appearing in Field B.

The end of the program

Listing 9 signals the end of the program.

Run the Program

I encourage you to copy, compile, and run the program provided in this lesson. 
Experiment with it, making changes and observing the results of your changes.

Summary

I showed you how to use KeyEventDispatcher objects to cause a
KeyEvent
fired by one component to be dispatched to a different component
for processing.  As a result, characters typed into one text field appear
in another text field, and do not appear in the text field into which they are
typed.

What’s Next?

Future lessons will discuss other important features of the focus subsystem including the
following:

  • The KeyEventPostProcessor interface
  • FocusEvent and WindowEvent
  • Event delivery
  • Temporary focus events
  • Focus and PropertyChangeListener
  • Focus and VetoableChangeListener

The next lesson in the series will explain the use of the KeyEventPostProcessor interface
to cause a final stage of customized event processing to take place after a
KeyEvent
has already been dispatched to a component for default processing.

Complete Program Listing

A complete listing of the program discussed in this lesson is provided below.

/*File KeyEventDispatch01.java
Copyright 2004,R.G.Baldwin
Revised 07/27/04

This program illustrates the use of an
alternate KeyEventDispatcher.

A JFrame object appears on the screen containing
three JTextField objects.  The initial text in
the three JTextField objects is respectively:

Field A
Field B
Field C

Two alternative KeyEventDispatcher objects are
registered on the current KeyboardFocusManager
in the following order:

AlternateDispatcherA
AlternateDispatcherB

When a key event occurs, the current
KeyboardFocusManager delivers that event to
AlternateDispatcherA for dispatching.  If the
owner of the focus at that point in time is
Field A, the AlternateDispatcherA object
re-dispatches that event to Field C and returns
true.  That causes keystrokes entered into
Field A to appear in Field C instead.  Returning
true prevents any further dispatching of the
event.

If the owner of the focus at that point in time
is not Field A, the AlternateDispatcherA object
returns false.  This causes the current
KeyboardFocusManager to deliver the event to
AlternateDispatcherB for dispatching.  If the
owner of the focus at that point in time is
Field B, the AlternateDispatcherB object
re-dispatches that event to Field C and returns
true.  That causes keystrokes entered into
Field B to appear in Field C instead.  Returning
true prevents any further dispatching of the
event.

If the owner of the focus at that point in time
is not Field B, the AlternateDispatcherB object
returns false.  This causes the current
KeyboardFocusManager to dispatch the event to
the component that owns the focus.  In this
simple program, that would have to be Field C
because it has already been determined that
neither Field A nor Field B own the focus.

That causes keystrokes entered into Field C to
appear in Field C as is normally the case.

Thus, all keystrokes entered into any of the
three fields in the GUI will appear in field C.

Tested using J2SE 1.4.2 under WinXP.
************************************************/
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class KeyEventDispatch01{
  public static void main(String[] args){
    new GUI();
  }//end main
}//end class KeyEventDispatch01
//=============================================//

class GUI extends JFrame{
  JTextField fieldA =
                    new JTextField("Field A",12);
  JTextField fieldB =
                    new JTextField("Field B",12);
  JTextField fieldC =
                    new JTextField("Field C",12);

  KeyboardFocusManager manager;

  GUI(){//constructor
    getContentPane().setLayout(new FlowLayout());
    getContentPane().add(fieldA);
    getContentPane().add(fieldB);
    getContentPane().add(fieldC);

    setSize(390,100);
    setTitle("Copyright 2004, R.G. Baldwin");
    setDefaultCloseOperation(
                           JFrame.EXIT_ON_CLOSE);
    setVisible(true);
    manager = KeyboardFocusManager.
                getCurrentKeyboardFocusManager();

    manager.addKeyEventDispatcher(
                        new AlternateDispatcherA(
                         fieldA,fieldC,manager));
    manager.addKeyEventDispatcher(
                        new AlternateDispatcherB(
                         fieldB,fieldC,manager));

  }//end constructor
}//end class GUI
//=============================================//

class AlternateDispatcherA
                   implements KeyEventDispatcher{
  JTextField fieldA;
  JTextField fieldC;
  KeyboardFocusManager manager;

  //Constructor
  AlternateDispatcherA(
                   JTextField fieldA,
                   JTextField fieldC,
                   KeyboardFocusManager manager){
    this.fieldA = fieldA;
    this.fieldC = fieldC;
    this.manager = manager;
  }//end constructor

  public boolean dispatchKeyEvent(KeyEvent e){
    if(e.getSource() == fieldA){
      manager.redispatchEvent(fieldC,e);
      return true;
    }else{
      return false;
    }//end else
  }//end dispatchKeyEvent
}//end class AlternateDispatcherA
//=============================================//

class AlternateDispatcherB
                   implements KeyEventDispatcher{
  JTextField fieldB;
  JTextField fieldC;
  KeyboardFocusManager manager;

  //Constructor
  AlternateDispatcherB(
                   JTextField fieldB,
                   JTextField fieldC,
                   KeyboardFocusManager manager){
    this.fieldB = fieldB;
    this.fieldC = fieldC;
    this.manager = manager;
  }//end constructor

  public boolean dispatchKeyEvent(KeyEvent e){
    if(e.getSource() == fieldB){
      manager.redispatchEvent(fieldC,e);
      return true;
    }else{
      return false;
    }//end else
  }//end dispatchKeyEvent
}//end class AlternateDispatcherB

Listing 10


Copyright 2004, Richard G. Baldwin.  Reproduction in whole or in
part in any form or medium without express written permission from Richard
Baldwin is prohibited.

About the author


Richard Baldwin

is a college professor (at Austin Community College in Austin, TX) and
private consultant whose primary focus is a combination of Java, C#, and
XML. In addition to the many platform and/or language independent benefits
of Java and C# applications, he believes that a combination of Java, C#,
and XML will become the primary driving force in the delivery of structured
information on the Web.

Richard has participated in numerous consulting projects, and he frequently
provides onsite training at the high-tech companies located in and around
Austin, Texas.  He is the author of Baldwin’s Programming Tutorials,
which has gained a worldwide following among experienced and aspiring programmers.
He has also published articles in JavaPro magazine.

In addition to his programming expertise, Richard has many years of
practical experience in Digital Signal Processing (DSP).  His first
job after he earned his Bachelor’s degree was doing DSP in the Seismic Research
Department of Texas Instruments.  (TI is still a world leader in DSP.) 
In the following years, he applied his programming and DSP expertise to other
interesting areas including sonar and underwater acoustics.

Richard holds an MSEE degree from Southern Methodist University and
has many years of experience in the application of computer technology
to real-world problems.

[email protected]

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