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Chapter 27 JavaBeans
and Bean Events
Chapter 16 Applets and Multimedia
Chapter 27 JavaBeans and Bean
Events
Chapter 28 Containers, Layout
Managers, and Borders
Chapter 29 Menus, Toolbars,
Dialogs, and Internal Frames
Chapter 30 MVC and Swing Models
Chapter 31 JTable and JTree
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
1
Objectives
To
know what a JavaBeans component is
(§27.2).
To discover the similarities and differences
between beans and regular objects (§27.2).
To understand JavaBeans properties and naming
patterns (§27.3).
To review the Java event delegation model
(§27.4).
To create custom event classes and listener
interfaces (§27.5 optional).
To develop source components using event sets
from the Java API or custom event sets (§27.6).
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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2
What is JavaBean?
A JavaBeans component is a serializable public class with a
public no-arg constructor.
Every GUI class is a JavaBeans component, because (1) it
is a public class; (2) it has a public no-arg constructor; (3) It
is an extension of java.awt.Component, which implements
java.io.Serializable.
class
Data members
Methods
Constructors
JavaBeans Component
public class
public no-arg constructor
serializable
may have accessor/mutator methods
may have registration/deregistration methods
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Minimum
requirement
Optional
requirement
3
Why JavaBeans?
The JavaBeans technology was developed to enable the
programmers to rapidly build applications by assembling
objects and test them during design time, thus making
reuse of the software more productive.
JavaBeans is a software component architecture that
extends the power of the Java language by enabling wellformed objects to be manipulated visually at design time in
a pure Java builder tool, such as JBuilder and NetBeans.
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JavaBeans Properties and
Naming Patterns

The get method is named
get<PropertyName>(),
which takes no parameters and returns an object of the type
identical to the property type.
 For a property of boolean type, the get method should be named
is<PropertyName>(),
which returns a boolean value.
 The set method should be named
set<PropertyName>(newValue),
which takes a single parameter identical to the property type and
returns void.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Properties and Data Fields
Properties describe the state of the bean. Naturally, data fields are
used to store properties. However, a bean property is not necessarily
a data field. For example, in the MessagePanel class in Example
12.5 in Chapter 13, you may create a new property named
messageLength that represents the number of the characters in
message. The get method for the property may be defined as
follows:
public int getMessageLength() {
return message.length();
}
NOTE: A property may be read-only with a get method but no set
method, or write-only with a set method but no get method.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Bean Events
A bean may communicate with other beans. The Java
event delegation model provides the foundation for beans
to send, receive, and handle events. When something
happens to a bean, such as a mouse click on a
javax.swing.JButton bean, an event object is created to
encapsulate information pertaining to the event. The bean
passes the event object to the interested beans for the
event to be processed.
Events are typically generated by Java GUI components,
such as javax.swing.JButton, but are not limited to GUI
components. This section introduces the development of
custom events and the beans that can generate events.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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The Event Delegation Model
Register by invoking
source.addXListener(listener);
Trigger an event
source: SourceClass
User
Action
listener: ListenerClass
+addXListener(XListener listener)
Keep it a list
XListener
event: XEvent
Invoke
listener1.handler(event)
listener2.handler(event)
…
listenern.handler(event)
listener1
listener2
…
listenern
+handler(XEvent event)
Internal function of the source object
+handler(
XEvent
Figure 14.2
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Predefined Event Pairs
(Event Classes and Listener Interface)
EventObject
AWTEvent
ChangeEvent
ActionEvent
ContainerEvent
AdjustmentEvent
FocusEvent
ComponentEvent
InputEvent
ItemEvent
PaintEvent
TextEvent
WindowEvent
HyperLinkEvent
TableColumnEvent
TableModelEvent
InternalFrameEvent
TreeModelEvent
MouseEvent
KeyEvent
ListSelectionEvent
TreeSelectionEvent
TreeExpansionEvent
javax.swing.event package
Examples:
ActionEvent/ActionListener
AdjustmentEvent/AdjustmentListener
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Examples of Event Pairs
java.util.EventObject
+EventObject(source: Object)
+getSource(): Object
java.awt.AWTEvent
java.util.EventListener
java.awt.event.ActionListener
+actionPerformed(ActionEvent e): void
java.awt.event.ActionEvent
actionCommand: String
modifier: int
when: long
+ActionEvent(source: Object, id: int, command: String)
+getActionCommand(): String
+getModifier(): int
+getWhen(): long
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Source Components
Source Component
Register listener method
Deregister listener method
Detect events
A vector (stores the listener objects)
Fire and process event by invoking the event
handler from each listener in the vector
The source component detects events and processes
the events by invoking the event listeners' handler.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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11
Listener Components
Listener Component
Source Component
JButton
Listener vector
Generate an
event
Invoke listener’s
actionPerformed
method
MyListener class implements
ActionListener
Process event
actionPerformed(ActionEvent e)
addActionListener(ActionListener l)
removeActionListener(ActionListener l)
Test Class
JButton jbt = new JButton(); // Create a source object
MyListener listener = new MyListener (); // Create a listener object
jbt.addActionListener(listener); // Register listener to the source
The listener is registered with the source, and the source
invokes the listener's handler to process the event.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
12
Creating Custom Event Pairs
You have already used event sets (e.g., ActionEvent/ActionListener) and
event source components (JButton) in Java GUI programming. You can
create your own event sets and source components.
A custom event class must extend java.util.EventObject or a subclass of
java.util.EventObject. Additionally, it may provide constructors to create
events, data members and methods to describe the event.
A custom event listener interface must extend java.util.EventListener or a
subinterface of java.util.EventListener, and define the signature of the
handlers for the event. By convention, the listener interface should be
named <Event>Listener for the corresponding event class named
<Event>.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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13
Example: Creating a Custom
Event Set
Problem: This example creates a custom event named TickEvent for
describing tick events, and its corresponding listener interface TickListener
for defining a tick handler.
java.util.EventObject
java.util.EventListener
TickEvent
TickListener
-tickCount: long
+handleTick(TickEvent e): void
-tickInterval: long
+TickEvent(source: Object)
+getTickCount(): long
TickEvent
+getTickInterval(): long
+setTickCount(tickCount: long): void
+setTickInterval(tickInterval: long): void
TickListener
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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14
Creating Custom Source Components
Unicast Registration Methods:
A source component must have the appropriate registration and
deregistration methods for adding and removing listeners.
Events can be unicasted (only one object is notified of the
event) or multicasted (each object in a list of listeners is
notified of the event). The naming pattern for adding a
unicast listener is
public void add<Event>Listener(<Event>Listener l)
throws TooManyListenersException;
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Creating Custom Source Components
Multicast Registration Methods:
The naming pattern for adding a multicast listener is the same, except that it
does not throw the TooManyListenersException.
public void add<Event>Listener(<Event>Listener l)
The naming pattern for removing a listener (either unicast or multicast) is:
public void remove<Event>Listener(<Event>Listener l)
A source component contains the code that creates an event object and
passes it to the listening components by calling a method in the listener's
event listener interface. You may use a standard Java event class like
ActionEvent to create event objects or may define your own event
classes if necessary.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Example: Creating a Source
Component
Problem: This example creates a custom source component that generates
a tick event at every specified time interval in milliseconds. Create a
custom source component that is capable of generating a tick event at a
variant time interval. The component contains the properties tickCount,
tickInterval, maxInterval, minInterval, and step. The component adjusts
the tickInterval by adding step to it after a tick event occurs. If step is 0,
tickInterval is unchanged. If step > 0, tickInterval is increased. If step <
0, tickInterval is decreased. If tickInterval > maxInterval or tickInterval
< minInterval, the component will no longer generate tick events.
NOTE: You learned to use javax.swing.Timer to control the animation in
Chapter 19, “Multithreading.” The Timer class generates a timer at a
fixed time interval. This new component can generate a tick event at a
variant time interval as well as a fixed time interval.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
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Example: Creating a Source
Component
java.io.Serializable
Tick
Tick
-tickCount: long
JavaBeans property for tickCount (default 0).
-tickInterval: long
JavaBeans property for tickInterval (default 100).
-maxInterval: long
JavaBeans property for maxInterval (default 5000).
-minInterval: long
JavaBeans property for minInterval (default 1).
-step: long
JavaBeans property for step (default 0).
-e: TickEvent
Tick event created from the Tick object.
-tickListenerList: ArrayList
Stores the TickEvent listeners.
-suspended: boolean
Indicates whether tick is suspended.
-timer: javax.swing.Timer
Timer for controlling the tick.
+Tick()
Creates a Tick object with default properties.
+Tick(tickInterval: int, maxInterval: int,
minInterval: int, step: int)
Creates a Tick object with the specified properties.
+resume(): void
Resumes the tick.
+suspend(): void
Suspends the tick.
+addTickListener(l: TickListener): void
Adds a new listener to this object.
+removeTickListener(l: TickListener): void
Removes a listener from this object.
-processEvent(e: TickEvent): void
Processes the event.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Example: Using the TickEvent
Class
Problem: Write a program that displays a moving message.
Solution: You can write the code using the Thread.sleep(millis)
method or the Timer class to control the animation (See Exercise
19.11). This example uses the Tick class to display the message
periodically.
JApplet
TickEvent
DisplayMovingMessage
Tick
-tick: Tick
-messagePanel: MovingMessage
TickListener
MovingMessage
+DisplayingMessage()
+handleTick(TickEvent e)
DisplayMovingMessage
Run
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Interaction Between Source and
Listener Components
Listener Component
Source Component
Tick
Listener vector
Generate an
event
Invoke listener’s
handleTick
method
MovingMessage class
implements TickListener
Process event
handleTick(TickEvent e)
addTickListener(TickListener l)
removeTickListener(TickListener l)
DisplayMovingMessage
Tick tick = new Tick(); // Create a source object
MovingMessage messagePanel = new MovingMessage(); // Create a listener object
tick.addTickListener(messagePanel); // Register listener to the source
The listener messagePanel is registered with the source tick, and the
source invokes
the listener's handler handleTick to process the event.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
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Working with Existing Event Sets
TickEvent and TickListener is a new event pair. Most of
the time you don't need to create your own event pairs
unless you want to encapsulate information not available
in the existing event classes, as in the case of the
TickEvent class that contains tick count and tick interval.
If you don't need the tick count and tick interval contained
in a tick event. There is no need to create a TickEvent
class; instead you can use java.awt.ActionEvent and let the
Tick class generate an ActionEvent instance when a tick
event occurs.
Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All
rights reserved. 0-13-148952-6
21
Example: Developing a Source
Component Using Existing Event Sets
Problem: This example presents a new component that generates an
ActionEvent when a tick event occurs rather than using a
TickEvent. Use this new component to rewrite the preceding
example to display a moving message.
JApplet
ActionEvent
DisplayingMessageUsingActionEvent
TickUsingActionEvent
-tickUsingActionEvent1:
TickUsingActionEvent
-messagePanel: MovingMessageNew
+DisplayingMessageUsingActionEvent()
DisplayingMessageUsingActionEvent
ActionListener
MovingMessageNew
+actionPerformed(ActionEvent e)
TickUsingActionEvent
Run
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Interaction Between Source and Listener
Components
Listener Component
Source Component
TickUsingActionEvent
Listener vector
Generate an
event
Invoke listener’s
actionPerformed
method
MovingMessageNew class
implements ActionListener
Process event
actionPerformed(ActionEvent e)
addActionListener(ActionListener l)
removeActionListener(ActionListener l)
DisplayingMessageUsingActionEvent
TickUsingActionEvent tick = new TickUsingActionEvent (); // Create a source object
MovingMessageNew messagePanel = new MovingMessageNew(); // Create a listener object
tick.addActionListener(messagePanel); // Register listener to the source
The listener messagePanel is registered with the source tick, and the source invokes
the listener's handler actionPerformed to process the event.
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23
Note to the Instructor
You may cover Chapter 30 now.
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