Download World Wide Web

6. Event-Driven Programming
6.1 The World Wide Web and Applets
6.2 Paint Events
6.3 Drawing Text
6.4 Using Color
6.5 Interfaces
6.6 Event Listeners
Objectives
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Learn the uses of protocols
Learn basic HTML tags
Write and test applets
Understand the idea of event-handling code
Respond to paint events
Take a first look at inheritance
Know AWT classes needed for drawing
Draw in color
The World Wide Web
• The Internet gives us the capability to exchange
information
• The World Wide Web (WWW, or The Web) makes
the exchange of information easy
• Computers have rules of interactions, called
protocols, specifying how they interact
• Common protocols include HTTP (Hypertext
Transfer Protocol), FTP (File Transfer protocol),
SMTP (Simple Mail Transfer Protocol), POP3
(Post Office Protocol - version 3), etc.
The World Wide Web
• For web programs, the most important protocol is
HTTP
• Hypertext - text augmented with links to other
files
• Hypermedia - a hypertext file that also links to
images, and other resources
• Two computers can use HTTP to communicate
and transfer hypermedia
• The computer sending hypermedia is called a web
server
The World Wide Web
• The program running on the other computer that
receives and presents the hypermedia is a browser,
e.g. Netscape, Internet Explorer, Mosaic
• Browsers load and interpret documents that have
been formatted using the HyperText Markup
Language (HTML)
• The Java language has very intimate relationship
with Web processing since links to Java programs
can be embedded in HTML documents and
executed through Web browsers
Uniform Resource Locator
• Information on the Web is found by
identifying an URL
• Three components: the protocol, domain
name, and path
http://java.sun.com/applets/index.html
http - the protocol
java.sun.com - domain name of the server (a
unique address of that computer)
applets/index.html - a path to the resource
HTML Documents
• WebPage.html
• Comments in HTML documents starts with
<!-- and ends with -->
• The title displays at the top of the frame, not in the
document itself
• The text between the h1 tags has the largest size
• The em tag causes the text to be displayed in
italics
HTML Documents
• Each item of an unordered list is preceded by a bullet
• <a href="java.sun.com">Sun's Home Page</a>
specifies a link to the page
• <img src="gittleman.gif"> specifies to include an
image which is a .gif file
• <applet code="Sort.class" width=300 height=200>
specifies the applet code to execute and the size of the
page that it paints
<br>
Break to the next line.
<p>
New paragraph (after a blank line).
<em> ... </em>
Emphasize the text.
<strong>...</strong>
Strongly emphasize the text.
<title>... </title>
Title, displayed separately from text.
<h1>... </h1>
Top-level header.
<h3>... </h3>
Third-level header., (lowest is sixth).
<u1> ... </u1>
An unordered list.
<1i>
Element of a list.
<a>... </a>
An anchor, a hypertext link.
<img>
An image.
<applet>...</applet>
A Java applet.
Figure 6.2 Some HTML tags
More on Applets
• An applet is a small program that is intended not
to be run on its own, but rather to be embedded
inside another application
• The Applet class provides a standard interface
between applets and their environment
• Applets can be executed by a Web browser or
appletviewer in JDK.
• HelloCount.java
The Applet Method
– public void init( ) - Initialize the applet. Called just
after the applet is loaded
– public void start( ) - Starts the applet. Called just after
the applet is made active
– public void stop( ) - Stops the applet. Called just after
the applet is made inactive
– public void destroy( ) - Destroys the applet. Called
when the browser is exited
– public void paint(Graphics g) - Inherits from the
Container class. Called automatically when an applet
needs to be redrawn
The Applet Method
– getCodeBase( ) - Returns the base URL
– getDocumentBase( ) - Returns an absolute URL
naming the directory of the document in which the
applet is embedded
– getImage(URL url) - Returns an Image object that
can then be painted on the screen
– getImage(URL url, String name) - Returns an Image
object that can then be painted on the screen
– getLocale( ) - Gets the Locale for the applet, if it has
been set
– getParameter(String name) - Returns the value of
the named parameter in the HTML tag
The Applet Method
– getSize( ) - Returns the size of this applet in the form
of a Dimension object
– getWidth( ) - Return the current width of this applet
– getX( ), getY( ) - Return the current x or y coordinate
of the components origin
– play(URL url) - Plays the audio clip at the specified
absolute URL
– resize(Dimension d) - Requests that this applet be
resized
– resize(int width, int height) - Requests that this applet
be resized
The Graphic Class
– drawArc(int x, int y, int width, int height, int
startAngle, int arcAngle)
Draws the outline of a circular or elliptical arc covering
the specified rectangle
– drawLine(int x1, int y1, int x2, int y2)
Draws a line, using the current color, between the
points (x1, y1) and (x2, y2) in this graphics context's
coordinate system
– drawOval(int x, int y, int width, int height) Draws the
outline of an oval
The Graphic Class
– drawPolygon(int[ ] xPoints, int[ ] yPoints,
int nPoints)
Draws a closed polygon defined by arrays of x and y
coordinates
– drawPolyline(int[ ] xPoints, int[ ] yPoints,
int nPoints)
Draws a sequence of connected lines defined by arrays
of x and y coordinates
– drawRect(int x, int y, int width, int height) Draws the
outline of the specified rectangle
The Graphic Class
– drawRoundRect(int x, int y, int width,
int height, int arcWidth, int arcHeight)
Draws an outlined round-cornered rectangle using this
graphics context's current color
– drawString(String str, int x, int y)
Draws the text given by the specified string, using this
graphics context's current font and color
– fill3DRect(int x, int y, int width, int height, boolean
raised)
Paints a 3-D highlighted rectangle filled with the
current color
The Graphic Class
– fillOval(int x, int y, int width, int height)
Fills an oval bounded by the specified rectangle with the
current color
– fillPolygon(int[ ] xPoints, int[ ] yPoints, int nPoints)
Fills a closed polygon defined by arrays of x and y
coordinates
– fillRect(int x, int y, int width, int height)
Fills the specified rectangle
– fillRoundRect(int x, int y, int width, int height,
int arcWidth, int arcHeight)
Fills the specified rounded corner rectangle with the
current color
The Graphic Class
– getColor( )
Gets this graphics context's current color
– getFont( )
Gets the current font
– getFont(Font f)
Gets the font metrics for the specified font
– setColor(Color c)
Sets this graphics context's current color to the
specified color
– setFont(Font f)
Sets this graphics context's font to the specified font
Center this string
n
w
Figure 6.8 Centering a string
Drawing Text
• Text.java
– The three arguments to the Font constructors are the
font name, the style, and the point size. Styles are
constants of the Font class
– The FontMetrics class has many methods to provide
data about the font. Each font has its own FontMetrics
object
– The getName method returns the name of the font
– The stringWidth method of the FontMetrics returns
the width in pixels of its string argument as drawn in
this font
Using Color
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ColorChips.java
MoreColor.java
SnowMan.java (extra)
Bullseye.java (extra)
public interface Drivable {
public static final int LEFT = 0;
public static final int RIGHT = 1;
public void start();
public void stop();
public void accelerate();
public void decelerate();
public void turn(int direction);
}
Figure 6.12 The Drivable interface
public static void goForward(Drivable d) {
d.start();
d.accelerate();
d.decelerate();
}
Figure 6.13 Using Drivable
Interfaces
• SportsCar.java
• Van.java
• TestDrivable.java
Events
• An event is an object that represents some activity
to which we may want to respond
• For example, we may want our program to
perform some action when the following occurs:
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the mouse is moved
a mouse button is clicked
the mouse is dragged
a graphical button is clicked
a keyboard key is pressed
a timer expires
Events
• Often events corresponds to user actions, but not
always
• The Java standard library contains several classes
that represent typical events
• Certain objects, such as an applet or a graphical
button, generate (fire) an event when it occurs
• Other objects, called listener, respond to events
• We can write listener objects to do whatever we
want when an event occurs
Listener Interfaces
• We can create a listener object by writing a class
that implements a particular listener interface
• The Java standard class library contains several
interfaces that correspond to particular event
categories
• For example, the MouseListener interfcae
contains methods that correspond to mouse events
• After creating the listener, add the listener to the
component that might generate the event to set up
a relationship between the generator and listener
Mouse Events
• The following are mouse events:
– mouse pressed - the mouse button is pressed down
– mouse released - the mouse button is released
– mouse clicked - the mouse button is pressed and
released
– mouse entered - the mouse pointer is moved over a
particular component
– mouse exited - the mouse pointer is moved off of a
particular component
• Any given program can listen for some, none, or
all of these
Mouse Motion Events
• The following are mouse motion events:
– mouse moved - the mouse is moved
– mouse dragged - the mouse is moved while the mouse
button is held down
• There is a corresponding MouseMotionListener
interface
• One class can serve as both a generator and a
listener
• One class can serve as a listener for mutiple event
type
Examples
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Dots.java (extra)
DotsMouseListener.java (extra)
RubberLines.java (extra)
TicketEvent.java, TicketListener.java,
Student.java, Scalper.java, Promoter.java,
TicketTest.java
The student registers with the promoter as a TicketListener.
The scalper registers with the promoter as a TicketListener.
The promoter creates a TicketEvent.
The promoter passes the TicketEvent to the student's
concertComing method.
The student's concertComing method decides whether to buy
tickets.
The promoter passes the TicketEvent to the scalper's
concertComing method.
The scalper's concertComing method decides whether to buy
tickets.
Figure 6.14 Student, scalper, and promoter interactions
Key Events
• The following are called key events:
– key pressed - a keyboard key is pressed down
– key released - a keyboard key is released
– key typed - a keyboard key is pressed and released
• The KeyListener interface handles key events
• Listener classes are often implemented as inner
class, nested within the component that they are
listening to
• Direction.java (extra)
Animations
• An animation is a constantly changing series of
pictures or images that create the illusion of
movement
• We can create animation in Java bu changing a
picture slightly over time
• The speed of a Java animation is usually
controlled by a Timer object
• The Timer class is defined in the javax.swing
package
Animations
• A Timer object generates ActionEvent every n
milliseconds (where n is set by the object creator)
• The ActionListener interface contains an
actionPerformed method
• Whenever the timer expires (generates an
ActionEvent) the animation can be updated
• Rebound.java (extra)