6. Event-Driven Programming

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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

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Objectives

• 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

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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.

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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

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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

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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

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HTML Documents

• WebPage.html
• Comments in HTML documents starts with lt!-- and
  ends with --gt
• 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

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HTML Documents

• Each item of an unordered list is preceded by a
  bullet
• lta href"java.sun.com"gtSun's Home Pagelt/agt
  specifies a link to the page
• ltimg src"gittleman.gif"gt specifies to include an
  image which is a .gif file
• ltapplet code"Sort.class" width300 height200gt
  specifies the applet code to execute and the size
  of the page that it paints

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Figure 6.2 Some HTML tags
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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

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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

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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

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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

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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

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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

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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

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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

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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

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Center this string
n
w
Figure 6.8 Centering a string
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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

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Using Color

• ColorChips.java
• MoreColor.java
• SnowMan.java (extra)
• Bullseye.java (extra)

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• 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
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• public static void goForward(Drivable d)
• d.start()
• d.accelerate()
• d.decelerate()

Figure 6.13 Using Drivable
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Interfaces

• SportsCar.java
• Van.java
• TestDrivable.java

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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
• 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

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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

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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

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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

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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

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Examples

• Dots.java (extra)
• DotsMouseListener.java (extra)
• RubberLines.java (extra)
• TicketEvent.java, TicketListener.java,
  Student.java, Scalper.java, Promoter.java,
  TicketTest.java

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• 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
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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)

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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

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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)