graphical user interface (gui) programming ii. lecture objectives understand the event-handling...
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Graphical User Interface (GUI) Programming II
Lecture Objectives
• Understand the Event-Handling Processes in Java
Caution Note!!!
This lecture presentation contains a number of "hidden" slides. Feel free to read, study and learn from them on your own!!!
Outline for Today's Fun
• Events: What is an event? Simple (?) Example
• Swing Components: JFrames JComponents An example Swing Component Design (MVC/UI-Delegate??)
Events• Behind the scenes, the Java runtime environment
is monitoring many things
• When any of a number of things happen an event is said to occur. Sometimes the terminology is “an event gets fired“
• Examples of the types of things that can "fire" events: Pressing a key on the keyboard Clicking on a component (like a button) Entering a component with the mouse pointer Have a timer "time-out"
• Moving the mouse around any reasonably complicated GUI can literally cause hundreds if not thousands of events to occur.
• Events will be ignored except for the ones that you tell Java that you are interested in doing something about.
• Java maintains a data structure of all the events that you have decided to handle and looks up events and does what you tell it to do.
Events (Cont’d)
Code from the Past: Remember???
import java.awt.*;
public class HelloGUI {
public static void main (String[ ] args) {
System.out.println("About to make GUI");
Frame f = new Frame ("Hello GUIs");
f.setSize( 200, 200 );
f.show();
System.out.println("Finished making GUI");
}// main
}// class HelloGUI
What didn't work???
Code from the Past: Remember???
Making It Work (MIT Approach)!• Determine which event occurs when the "Close
the Window" button is pressed: The API is your friend The lecture notes are your friend Hint: In this case it's an event called "Window Closing"
• You decide what class is going to handle this event It might be the actual class which has the window. It can be any other class.
Making It Work (MIT Approach)!
• Write the method (and class?) that will handle the event: When this event occurs, Java is going to go to the class that you identify as the event handler or Listener. It will look for a method called:
public void windowClosing(WindowEvent e)
• Java will report an error to you if this class doesn't have this method. How might the designers of Java be guaranteed that you will implement this method?
Interfaces: Back to School!
// Note: found in java.awt.event
public interface WindowListener extends EventListener {
void windowActivated(WindowEvent e);
void windowClosed(WindowEvent e);
void windowClosing(WindowEvent e);
void windowDeactivated(WindowEvent e);
void windowDeiconified(WindowEvent e);
void windowIconified(WindowEvent e);
void windowOpened(WindowEvent e);
}
So we could write a class as shown below:import java.awt.*;
import java.awt.event.*;
public class Handler implements WindowListener {
public void windowActivated(WindowEvent e) {}
public void windowClosed(WindowEvent e) {}
public void windowClosing(WindowEvent e) {
Window w = e.getWindow();
w.setVisible(false);
w.dispose();
System.exit(0);
}
public void windowDeactivated(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowIconified(WindowEvent e) {}
public void windowOpened(WindowEvent e) {}
}
Using Event InterfacesOutline:
Import
Class definition
Implementation
Dummy (lazy) implementation
• Register the listener with Java. That is, tell Java in which class the method will be located to run when the Window Closing Event occurs.
Using Event Interfaces (Cont’d)
Event Handler Registrationimport java.awt.*;
public class HelloGUI {
public static void main (String[] args) {
Handler h = new Handler();System.out.println ("About to make GUI");
Frame f = new Frame ("Hello GUIs");
f.setSize( 200, 200 );
f.addWindowListener(h);f.show();
System.out.println("Finished making GUI");
} // main
} // class HelloGUI
Demonstration
Class Pool
class HelloGUI main { Frame f Handler h }
class Frame
class Handler
FrameInstance
HandlerInstance
Interface WindowListener
Event Handling: Key Ideas
1. Determine which event occurs
2. Decide what class is going to handle this event
3. Write the method (and class?) that will handle the event.
4. Register the listener with Java.
Today’s First Wisdom
Very important that you understand this simple example to
understand the concepts that follow.
Potential Points of Confusion• What exactly is the listener? Is it the component
getting clicked on? No, the listener is the object that contains the method
that Java will call when the event happens. You must tell the component getting clicked which object
that is by registering: addWindowListener... As we will see it could be the same!!!
• What type of listener do I use? There are only so many. See the Java API. Lecture/Instructor/Lab/WebCT/Classmates/Friends. Try yourself and experience the excitement!
Potential Points of Confusion (Cont’d)• What about all those other window things (e.g.
windowActivated) We actually did implement them (with empty bodies) –
Lazy or dummy implementation! We said: Don't do anything!
• Events: What is an event? Simple (?) Example
• Swing Components: JFrames JComponents An example Swing Component Design (MVC/UI-Delegate)
Outline for Today's Fun: Reminder
A Call from the Past!
Earlier, we cautioned aboutthe existence of two toolkitsin Java for creating GUIs:
AWT
Swing
Today, we examine a few Swing components.
The goal is to learn how Swing components in general are designed, so that you can make better use of the API.
1.
2.
Java’s GUI Capabilities
Java provides essentially two related toolkits for making GUIs:
The Abstract Windowing Toolkit (“AWT”), and
The Java Foundation Classes (“Swing”)
Swing is merely a new, improved version of the AWT, and still uses some of AWT’s features.
1.
2.
Java’s GUI Capabilities
Java provides essentially two related toolkits for making GUIs:
The Abstract Windowing Toolkit (“AWT”), and
The Java Foundation Classes (“Swing”)
Swing is merely a new, improved version of the AWT, and still uses some of AWT’s features.
Swing: Sun’s Response to Microsoft?
• In 1997, Sun announced a new graphical toolkit for Java called the “Java Foundation Classes”, or “JFC”.
• This is usually called “Swing”.
• The JFC/Swing classes provide well designed, powerful widgets for GUI developers.
• Let’s take a look . . .
Welcome to JFC/Swing Planet
“Now it gets interesting . . . “
Historical Problems with AWT
• All AWT components required runtime peer resources: Slow on some platforms (notably Windows) Portability problems (slightly different look, some behaviors different) Least common denominator phenomenon: If one OS (e.g., Windows) did not
support a widget, the entire AWT had to suffer without it.
• Limited AWT widget library Addressed somewhat by JDK 1.1b3+, which allowed subclassing of components,
or “lightweights” (not covered in this course)
java.awt.Button
WinNTButtonPeer
MacOSButtonPeer
MotifButtonPeer
ButtonPeer
AWT components requirednative “peer” methods torender and operate--many steps!
CLICK ME
File Edit Help
Slow & Inflexible!
• Developers avoided a few AWT limitations through: Pervasive use of lightweights (again, not covered in cs1312). e.g., tooltip simulation through threads/windows/components extensive coding around rigid look use of layered gifs
CLICK ME
File Edit Help
Tooltip
(Tooltips requiredthreads,window subclasses &extensive event handling)
(Image buttons requiredcomponent subclassing,methods to handle ‘click’look, as well as eventhandlers)
(Menu bars limited;no images possiblewithout *extensive*simulation throughcomponents)
(Layering of componentsrequires layoutsubclasses)Bottom line: Making stuff look cool
or satisfying a client’s requestcould be a nightmare!
Bottom line: Making stuff look coolor satisfying a client’s requestcould be a nightmare!
Fixing AWT Problems
Introducing Swing/JFC• Sun addressed these problems by producing Java Foundation
Classes (JFC) i.e., Swing
• Key elements: No reliance on native peers; the JVM does the work, and is faster Swing completely controls look and feel of all components:
• PLAF, or “pluggable look and feel” Superior design: MVC-esque (Model View Control)
javax.swing.JButton
Fast, flexible, extensible!
Fast, flexible, extensible!
CLICK ME
File Edit Help
javax.swing.*javax.swing.*
Swing Packages• All the new Swing components and classes need a home. Where? A subject of great debate!
• For JDK 1.1, Sun used “com.sun.java.swing”; developers revolted.
• Problem: developers complained this “com” designation was not appropriate for “core” class--something part of language.
Solution:
Denotes ‘extension’ package thathas migrated to core status
Why “javax”?
* logical grouping
* minimizes transitioncosts
* most developershappy with it
* helps maintainexisting JDK 1.1 code (cf. MFC lib breaks)
Overview of JFC/Swing Packages
• javax.swing.plaf
• javax.swing.plaf.basic
• javax.swing.plaf.metal
• javax.swing.plaf.multi
• javax.swing.text
• javax.swing.text.html
• javax.swing.text.html.parser
• javax.swing.text.rtf
• javax.swing
• javax.swing.table
• javax.swing.tree
• javax.swing.border
• javax.swing.colorchooser
• javax.swing.filechooser
• javax.swing.event
• javax.swing.undo
Overview of the Overview
Overview of J FC/S w ing P ac kagesOverview of J FC/S w ing P ac kages
• javax.swing.plaf
• javax.swing.plaf.basic
• javax.swing.plaf.metal
• javax.swing.plaf.multi
• javax.swing.text
• javax.swing.text.html
• javax.swing.text.html.parser
• javax.swing.text.rtf
• javax.swing
• javax.swing.table
• javax.swing.tree
• javax.swing.border
• javax.swing.colorchooser
• javax.swing.filechooser
• javax.swing.event
• javax.swing.undo
Text-based widgets(including html/rtf display) New event packages
Components, including“aggregate” or complexcomponents
Packages tocontrol the“look and feel”of Swing
Short Examples
1.1.
Widget Example: JButtons• The java.swing.JButton class implements a “state version” of a
java.swing.AbstractButton. Many methods come from the abstract class:
A variety of constructors (Strings, Icons, etc.);
setRolloverEnabled(boolean);
setIcon(Icon);
setRolloverIcon(Icon);
setActionCommand(String); -- an extra String tacked onto the event that
gets fired!
setContentAreaFilled(boolean) -- transparency for icons!
setModel(ButtonModel); -- sets the ‘type’ of Button (you can define your
own Button behaviors!)
setMnemonic(char/int); -- set mnemonics for button
Lesson: Check the APIfor useful behaviors.
import javax.swing.*;
public class HelloWorld2 extends JFrame {
public JButton bNew;
public JPanel p;
public HelloWorld2() {
bNew = new JButton("New Document", new
ImageIcon("Document.gif"));
bNew.setRolloverEnabled(true);
bNew.setRolloverIcon(new ImageIcon("New.gif"));
p = new JPanel();
p.add(bNew);
getContentPane().add(p);
this.pack();
}
public static void main(String arg[ ]) {
new HelloWorld2().show(); }
}// class HelloWorld2
Cool Buttons
Sets icon androllover Icon.
Note: Icon constructortook String argument,and automaticallyloaded image
Images from disk:
Why getContentPane() ?
• The HelloWorld example required us to call getContentPane() before “add()”ing an object to the JFrame:
Usually“this” E.g.,
“myJButton”
• This differs from traditional AWT container additions, where we simply call “add”, passing in the component.
• Let’s cut a JFrame open to find out why . . .
Required ofJFrame, JDialogand JInternalFrameinstances
myFrameInstance.getContentPane().add(myComponent);
A JFrame Autopsy
click
A java.awt.Frame is composed of a single container--the Frame object itself. It is flat as a pop tart.
click
JLayeredPane
JPanelContentPane
Menu
A javax.swing.JFrame is composed of a transparent “glassPane” surface, and an inner JPanel with Contents and Menu
“The Pop Tart / Sandwich Duality”
JFrame Class View
JRootPane JMenuBar
JLayeredPane
JFrame
Container
Component
ContentPane
GlassPane
manages
manages
contains
cont
ains
cont
ains
JComponent
Frame
The JRootPane is a container with a JLayeredPane (holding the Menu and ContentPane) and a Component GlassPane. It serves as base for numerous classes.
cont
ains
cont
ains
AWTAWT
JFCJFC
JRootPane: The Content Pane
The JRootPane contains only two components:
the JLayeredPane and
the Component GlassPane
Its layout manager ignores all attempts to add new components.
Instead, one must add to the JRootPane’s ContentPane, found inside the JLayeredPane.
A call to getContentPane() returns an instance of the ContentPane.
JFrame Blue PrintWe can use the top “glassPane”as a drawing area. Since it spans the entire JFrame, we can draw on top of menu bars, and every component.
The JPanel has a remarkable layering feature, allowing us to stack and shuffle components.
public Component getGlassPane();public void setGlassPane(Component);
public JPanel getContentPane();public JPanel getContentPane();
JRootPane: The Glass Pane
JFrame Disposal• JFrame allows you to configure how it responds to closure.
Default: hides JFrame on closure attempt.
To modify: invoke setDefaultCloseOperation(). E.g.,:
MyJFrameInstance.setDefaultCloseOperation(WindowConstants.DO_NOTHING_ON_CLOSE);
/* behaves just like java.awt.Frame */
other constants in javax.swing.WindowConstants: HIDE_ON_CLOSE - invokes any registered WindowListener object, then hides.
This is default behavior. DISPOSE_ON_CLOSE - invokes any registered WindowListener object, and
then disposes.
JComponent: The Generic Widget
• The JComponent provides the basis for all Swing components.
• JComponent extends java.awt.Container, making all Swing components large, powerful widgets. (Also, all Swing components are also containers--even if you wouldn’t normally place things in them. E.g., JButton)
• In turn, JComponent is subclassed by numerous widgets. Thus, composition is favored over inheritance for widget manipulation.
Jcomponent (Cont’d)
Since JComponent is the basis for most Swing components, all Swing widgets have the following general features:
Borders-- JComponent derived classes can have borders
Accessibility -- JComponents use Swing’s accessibility features to provide additional information about the widget.
Tooltips -- JComponents can have time sensitive tooltips.
Double Buffering -- By default, Swing components have double buffering built in
Serialization -- Ability to save state to a file.
Events
Here, we review event handling one more time.
To understand how events work in Java, we have to look closely at how we use GUIs.
When you interact with a GUI, there are many events taking place each second. Only a few of these, however, may actually be ‘delivered’ to the application.
Java uses a “delegation” event model found in many other toolkits.
Under the delegation model, components fire events, which can be caught and acted on by listeners. A listener is linked to a component through a registration process.
The delegation event model is contrasted to an event filtration model where all events are delivered to target components regardless of whether they asked for them.
Events (Cont’d)
Events: General Overview
ExplanationExplanationWhen the Java VM createdour Frame, it entered into akind of ‘infinite loop’,waiting for input andevents. (This is commonof graphical toolkits.)
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
while(true){ //get user input // handle event}
Since we didn’twrite any eventhandlers, not eventhe “windowdisposal” x works.
Recall our first consideration of events, where our first frame would not close, even when the end of main() was reached.
We explained this behavior by thinking of our program as entering an “infinite loop” when the graphics are shown. This ‘infinite loop’ is actually an event-driven cycle, but we can think of it as a “while (true)” structure that periodically polls for user input.
The Real Story
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
We usually think of our program as a single, linear set of steps being executed. But something special happens when we create graphical objects.
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
When Java sees that you’ve created a GUI, your program gets a second “set” of linear instructions.
This is actually a separate “thread”, but don’t worry if that’s unclear for now. We can think of this as a second part of our program than handles special graphics-related tasks (such as drawing the window, etc.)
The Real Story (Cont’d)
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
This model is very important to understand because as it turns out, when an event occurs--such as mouse click, it happens in the “graphics side” of the model.
Mouse Clickoccurs
The code trappingthis event appears
in the graphics thread
Actually, there’s a separate “event queue” that handles incoming events. But this is already complicated enough. Let’s just generalize and imagine that all events arrive in the ‘graphics side’ of things.
The Real Story (Cont’d)
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
Since the event arrived in the ‘graphics half’ of our program, we need a way to have it call a method in our program. This is known as a “call back”.
callbackcallback
Our eventhandling
code
The code trappingthis event appears
in the graphics thread
The Real Story: “Call backs”
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
callbackcallback
So Java needs to call some event handling code that we write. The trouble is, how will Java know what we called out method? We can name them anything we want, and Java won’t necessarily know what methods handle events.
But Wait!But Wait!
We can useWe can useinterfaces, right?interfaces, right?
The Real Story: How?
Event InterfacesJava uses interfaces as its primary event handling scheme. If you implement an event-related interface, Java will know which methods to call. This is because the contract nature of interfaces requires all methods to appear in the implementing class.
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
callbackcallbackpublic void actionPerformed (ActionEvent e) { // code doing something }
This method This method MUSTMUSTbe there, so Java knowsbe there, so Java knowsit can “callback” to itit can “callback” to it
ActionListener
Why “Registration”?We are told that “event registration” must occur before event handling will occur. What does this mean?
Well, since we can have any class handle events, we need to tell Java which object implements the proper event handling interface.
This “registers” the component as being interested in receiving callbacks.
import java.awt.*;public class HelloGUI { public static void main (String[ ] arg) { System.out.println (“About to make GUI”); Frame f = new Frame (“Hello GUIs”); f.setSize( 200, 200 ); f.show(); System.out.println (“Finished making GUI”); }// main}// class HelloGUI
Where toWhere tocallback?callback?
An Examplepublic class DemoFrame extends Frame {
public DemoFrame( ) {
super (“A poor use of inheritance, but simple”);
Handler2 h = new Handler2();
this.setSize(400,400);
this.setLayout(new FlowLayout());
Button b = new Button (“Click me”);
this.add(b);
this.show();
} // Constructor
public static void main(String[] args) {
DemoFrame df;
df = new DemoFrame();
} // main
} // DemoFrame
Another Examplepublic class Handler2 implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.out.println (“Button was clicked”);
}
} // Handler2
Why doesn’t this work?
public class DemoFrame extends JFrame {
public DemoFrame( ) {
super (“A poor use of inheritance, but simple”);
Handler2 h = new Handler2();
this.setSize(400,400);
this.setLayout(new FlowLayout());
JButton b = new JButton (“Click me”);
b.addActionListener(h);
add(b);
show();
} // Constructor
public static void main(String[] args) {
DemoFrame df;
df = new DemoFrame();
} // main
} // DemoFrame
Another Example (Cont’d)
Question???
We said we had to have a Listener to handle the event and it had to be an object. Does it have to be
a separate object?
Another Examplepublic class DemoFrame extends Frame implements ActionListener {
public DemoFrame( ) {
super (“A poor use of inheritance, but simple”);
Handler2 h = new Handler2();
this.setSize(400,400);
this.setLayout(new FlowLayout());
Button b = new Button (“Click me”);
b.addActionListener(this);
this.add(b);
this.show();
} // Constructor
public void actionPerformed(ActionEvent e) {
System.out.println (“Button was clicked”);
}
public static void main(String[] args) {
DemoFrame df;
df = new DemoFrame();
} // main
} // DemoFrame
Is There Any Other Event There?
Anything can be an event. Including general protection faults.
But for the most part, good programming dictates that handled events should come from the following area of input:
Keyboard events
Timing events
Mouse events
Other user action inputs
a
b
c
d
Java Event Handling Strategies
With this basic understanding, we can investigate theFOUR primary means of event handling in Java
Event Adapters
Semantic Events
Event Listeners
Inheritance-based event handling
1
2
3
4
Very similar
Verygeneral
Very old
We’ll not talk about this one…
Listeners Strategy
StrategyNo. 1No. 1
From the discussion about callbacks, we notedthat interfaces were the primary mechanism for structuring our event handling.
There are numerous event interfaces we can implement, roughly divided around categories of events.
The next slide lists many of them. Don’t freak out because there are so many.
We’ll highlight the most commonly used ones. . .
Most commonlyused in this course
Listeners: So Many Choices
Package java.awt.event features:
ActionListener
MouseListener
MouseMotionListener
AdjustmentListener
ComponentListener
FocusListener
ContainerListener
ItemListener
KeyListener
WindowListener
TextListener
As it turns out, the ActionListener is part of the “semantic” event group, even though it’s an interface. So let’s focus on simple events like MouseListener...
MouseListenerThe MouseListener interface has several methods we have to code:
public void mouseClicked(MouseEvent e) { }-- a timing-based determination; else
the events are processed as pressed/releases
public void mouseEntered(MouseEvent e) { }-- entry into component
public void mouseExited(MouseEvent e) { }-- exit from component
public void mousePressed(MouseEvent e) { }-- simply a press . . .
public void mouseReleased(MouseEvent e){ }-- ... the corresponding release
import java.awt.*;import java.awt.event.*;
public class MouseFrame implements MouseListener{Color highlight, normal;boolean bHighlight = true;Frame fr;public MouseFrame () {
fr = new Frame(“For demonstration only”);highlight = Color.red;normal = Color.gray;frame.setSize(400,400);Button b = new Button("Click");b.addMouseListener(this);fr.setBackground(normal);fr.setLayout(new FlowLayout());fr.add(b); fr.show();
}
public static void main(String[] args) {new MouseFrame();
}
To keep it simple,we ignore
WindowEvents
Note that whenwe run this theconstructor willrun and terminate
MouseListener: An Example
public void mouseReleased(MouseEvent e){System.out.println ("Changing color");if (bHighlight)
frame.setBackground(highlight);else
frame.setBackground(normal);bHighlight = !bHighlight;
}
public void mouseClicked(MouseEvent e) {}public void mouseEntered(MouseEvent e) {}public void mouseExited(MouseEvent e) {}public void mousePressed(MouseEvent e) {} } // MouseFrame “click”
“click”
MouseListener: An Example (Cont’d)
Event Listener Summary
We need a class that implements the appropriate listener type.
We need to “register” a component as interested in receiving events:
addXYZXYZListener ( <listener instance> );
Whatever listener we’re working with. E.g.:
addMouseListener(this);addMouseMotionListener(myEventHandler);
There’s another strategy using “adapters”, using inheritance that could have saved us some trouble...
The “WindowListener” interface required numerous methods.
But only one was important to us.
All the rest were coded as “no-op” or no operation methods.
1111
2222
3333
Event Listener: Observations
Adapters
public class MouseAdapter implements MouseListener { public void mouseClicked(MouseEvent e) {} public void mouseEntered(MouseEvent e) {} public void mouseExited(MouseEvent e) {} public void mousePressed(MouseEvent e) {} public void mouseReleased(MouseEvent e) {}}
Java has built-in classes called “event adapters” that implement each of the various event listeners.
But all of these methods are “no-ops”.
WHY???
Strategy
StrategyNo. 2No. 2
Key to Adapters: Inheritance
Why a bunch of no-op methods?
Well, if you subclass the adapter, your class IS-A type of event listener.
And you then only have to override the one or two methods you care about. The rest can be inherited as “no-ops”
MouseFrame
MouseAdapter
Parentclass takes
care ofthese
import java.awt.*;import java.awt.event.*;public class MouseFrame extends MouseAdapterextends MouseAdapter implements MouseListener{ Color highlight, normal; boolean bHighlight = true; Frame frame; public MouseFrame () { frame = new Frame(“For demonstration only”);
highlight = Color.red;normal = Color.gray;frame.setSize(400,400);Button b = new Button("Click");b.addMouseListener(this);frame.setBackground(normal);frame.setLayout(new FlowLayout());frame.add(b);
frame.show(); } public void mouseClicked(MouseEvent e) {} public void mouseEntered(MouseEvent e) {} public void mouseExited(MouseEvent e) {} public void mousePressed(MouseEvent e) {}
Event Adapters (Cont’d)
public void mouseReleased(MouseEvent e){System.out.println ("Changing color");if (bHighlight) frame.setBackground(highlight);else frame.setBackground(normal);bHighlight = !bHighlight;
} public static void main(String[] args) {
new MouseFrame(); } } // MouseFrame
We overridethe one or
two methodswe care about
Same behavior; less code;but we use up our single inheritance
Event Adapters (Cont’d)
import java.awt.*;import java.awt.event.*;public class MouseFrame extends MouseAdapter implements MouseListener{
Color highlight, normal;boolean bHighlight = true;Frame frame;public MouseFrame () {
frame = new Frame(“For demonstration only”);highlight = Color.red;normal = Color.gray;frame.setSize(400,400);Button b = new Button("Click");b.addMouseListener(this);frame.setBackground(normal);frame.setLayout(new FlowLayout());frame.add(b); frame.show();
}public void mouseReleased(MouseEvent e) {
System.out.println ("Changing color");if (bHighlight)
frame.setBackground(highlight);else
frame.setBackground(normal);bHighlight = !bHighlight;
}public static void main(String[] args) {
new MouseFrame(); }} // MouseFrame
public class MouseAdapter implements MouseListener { public void mouseClicked(MouseEvent e) {} public void mouseEntered(MouseEvent e) {} public void mouseExited(MouseEvent e) {} public void mousePressed(MouseEvent e) {} public void mouseReleased(MouseEvent e) {}}
This comes with Java!
Big Picture Time
So far, we’ve tinkered with different ways of codingvery low-level event handling.
But what if our event handling needs are very general.
Consider this simple dialog box:
cancel
Are you sure you wishto proceed ?
There’s not much interactionthat needs to be supported.
Mouse entry/exit might notbe needed at all.
ok
Event Listeners(interfaces)
Event Adapters(inheritance)
Costs Benefits
Must code allmethods; wastefulno-ops result
Uses up singleinheritanceopportunity
Keep allevents insingle class
Good abstraction;override those methodsyou need
Event Handling Options: How to Decide?
Debugging Event Handlers
• Debugging an event-driven program (whether applet or graphical application) is more tricky than debugging a non-event-driven program.
• With an event-driven Java program, you don't explicitly code any kind of event-handling loop that "polls" for occurring events, then calls the appropriate handler(s) for those events.
• Instead, the Java internals handle this polling action for you. Debugging becomes trickier because now you have to make sure that your event handling code works correctly.
• You also have to make sure you're handling the correct events in the first place! For example, your code for mouseEntered( ) may work perfectly, but if you're expecting it to get called when the user clicks a mouse button, it won't be!
So, in debugging event-driven programs written with Java, the steps are:
• Be sure you're handling the appropriate events: Map out on paper what events get thrown from what components, and what class(es) handle them.
• Handle the events appropriately: This is the kind of debugging you're already familiar with: Once you're sure the appropriate events are getting handled, the rest is being sure the event-handling code (and the code that the event handlers call) work.
Debugging Event Handlers (Cont’d)
To compare the three event handling techniques, let’s see a *brief* example how all three might work on a common problem.
Events: A Short Example
My Program
BUTTON
TEXT AREA
Panel subclass
Goal: Create asimple Frame thatholds a TextAreaand Button.
The Button togglesthe ability to editthe TextArea
The Panel holding the Button and TextArea is placed in a Frame subclass, which handles its own disposal
import java.awt.*;import java.awt.event.*;public class MyFrame extends Frame implements WindowListener{
public static final int iWidth = 300, iHeight = 500;public MyFrame() {
this.setSize(iWidth, iHeight);this.addWindowListener(this);BorderLayout border = new BorderLayout();this.setLayout(border);
}public void windowClosing (WindowEvent e) {
e.getWindow().setVisible(false);e.getWindow().dispose();System.exit(0);
}public void windowActivated(WindowEvent e) {}public void windowClosed(WindowEvent e) {}public void windowDeactivated(WindowEvent e) {}public void windowDeiconified(WindowEvent e) {}public void windowIconified(WindowEvent e) {}public void windowOpened(WindowEvent e) {}
}// class MyFrame
Constructor
WindowListener
Frames are not self-disposing!(Setting Frame
invisible firsteliminate flicker.)
Events: A Short Example (Cont’d)
import java.awt.*;import java.awt.event.*;public class MyFrame extends Frame { public static final int
iWidth = 300,iHeight = 500;
public MyFrame() {
this.setSize(iWidth, iHeight);this.addWindowListener (new WindowAdapter() {
public void windowClosing (WindowEvent e) {
e.getWindow().setVisible(false);e.getWindow().dispose();System.exit(0);
} });BorderLayout border = new BorderLayout();this.setLayout(border);
}}// class MyFrame
Advanced feature:“Anonymous Inner Class”:
used as a shortcut. For your
code, use listeners
Frames are not self-disposing!(Setting Frame
invisible firsteliminate flicker.)
Events: A Short Example (Cont’d)
import java.awt.*;public class Driver{ public static void main (String arg[]){ Notepad note = new Notepad(); MyFrame f = new MyFrame(); f.add(note, BorderLayout.CENTER); f.show(); }//main}//class Driver
Events: A Short Example (Cont’d)