Java Programming, Second Edition

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Java Programming, Second Edition

• Chapter Nine
• Applets

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In this chapter, you will

• Write an HTML document to host an applet
• Understand simple applets
• Use Labels with simple AWT applets
• Write a simple Swing applet and use a JLabel
• Add JTextField and JButton Components to Swing
  applets

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• Learn about event-driven programming
• Add output to a Swing applet
• Understand the Swing applet life cycle
• Create a more sophisticated interactive Swing
  applet
• Use the setLocation() and setEnabled() methods

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To Write a Java Application

• Write the application in the Java programming
  language, and then save it with a .java file
  extension
• Compile the application into bytecode using the
  javac command. The bytecode is stored in a file
  with a .class file extension
• Use the java command to interpret and execute the
  .class file

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Writing an HTML Document to Host an Applet

• Applets- Programs that are called from within
  another application
• You run applets within
• a page on the Internet
• an intranet
• or a local computer from within another program
  called Applet Viewer
• To view an applet, it must be called from within
  another document written in HTML

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Writing an HTML Document to Host an Applet

• To create an applet
• Write the applet in the Java programming
  language, and save it with a .java file extension
• Compile the applet into bytecode using the javac
  command
• Write an HTML document that includes a statement
  to call your compiled Java class
• Load the HTML document into a Web browser or run
  the AppletViewer program

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Writing an HTML Document to Host an Applet

• Applets are popular because users can execute
  them using a Web browser
• Web browser- A program that allows you to display
  HTML documents on your computer screen
• Internet Explorer
• Netscape Navigator

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Writing an HTML Document to Host an Applet

• Code to run an applet from within an HTML
  document
• ltappletgt
• lt/appletgt
• Applet tag attributes
• CODE is followed by the name of the compiled
  applet you are calling
• WIDTH is followed by the width of the applet on
  the screen
• HEIGHT is followed by the height of the applet
  on the screen

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Applets

• The WIDTH and HEIGHT attributes are measured in
  pixels
• Pixels- Picture elements, or tiny dots that make
  up the image on your video monitor

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Understanding Simple Applets

• To write an applet you must also
• Include import statements to ensure that
  necessary classes are available
• Learn to use some Windows components and applet
  methods
• Learn to use the keyword extends

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Understanding Simple Applets

• Component- A class that defines any object that
  you want to display
• Container- A class that is used to define a
  component that can contain other components

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Understanding Simple Applets

• Most AWT applets contain 2 import statements
• import java.applet.
• import java.awt.
• java.applet- Contains a class named Applet
• Every applet you create is based on Applet
• java.awt- The Abstract Windows Toolkit, or AWT

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Understanding Simple Applets

• Most Swing applets contain 2 import statements
• import javax.swing.
• import java.awt.
• javax.swing- A package that contains classes that
  define GUI components (Swing components)

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Understanding Simple Applets

• Swing classes- part of a more general set of GUI
  programming capabilities that are known as the
  Java Foundation Classes, or JFC
• JFC includes Swing component classes and selected
  classes from the java.awt package

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AWT and Swing Applets

• AWT and Swing applets
• Begin the same way as Java applications
• Must also include
• extends Applet
• extends JApplet
• The extends keyword indicates the applet will
  build upon Applet and JApplet

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Applets

• Four methods in every applet
• public void init()
• public void start()
• public void stop()
• public void destroy()
• Java can create these for you

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Using Labels with Simple AWT Applets

• The java.awt package contains commonly used
  Windows components
• Labels
• Menus
• Buttons
• Label- Built-in class that holds text that you
  can display within an applet

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Using Labels with Simple AWT Applets

• Label class contains fields that indicate font
  and alignment
• You can assign some text to a label with the
  setText() method
• Use the add() method to add a component to an
  applet window

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Writing a Simple Swing Applet and Using a JLabel

• JLabel- Built-in class that holds text that you
  can display within an applet
• The counterpart to the AWT Label

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Writing a Simple Swing Applet and Using a JLabel

• Available constructors include
• JLabel() creates a JLabel instance with no image
  and an empty string for the title
• JLabel(Icon image) creates a JLabel instance with
  the specified image
• JLabel(Icon image, int horizontalAlignment)
  creates a JLabel instance with the specified
  image and horizontal alignment
• JLabel(String text) creates a JLabel instance
  with the specified text
• JLabel(String text, Icon icon, int
  horizontalAlignment) creates a JLabel instance
  with the specified text, image, and horizontal
  alignment
• JLabel(String text, int horizontalAlignment)
  creates a JLabel instance with the specified text
  and horizontal alignment

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Writing a Simple Swing Applet and Using a JLabel

• AWT components are added directly to the Applet
• Swing components must use a content pane
• The content pane is an object of the Container
  class
• A container can be created using the
  getContentPane() method

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Changing a JLabels Font

• Font object- Holds typeface and size information
• setFont() method requires a Font object argument
• To construct a Font object you need 3 arguments
• Typeface
• Style
• Point size

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Changing a JLabels Font

• To construct a Font object you need 3 arguments
• Typeface
• String representing a font
• Common fonts are Arial, Courier, and New Times
  Roman
• Is only a request
• Style- applies an attribute to displayed text
• Font.PLAIN
• Font.BOLD
• Font.ITALIC
• Point size
• Integer that represents 1/72 of an inch
• Printed text is usually 10- or 12 points

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Adding JTextField Components to Swing Applets

• JTextField- Component into which a user can type
  a single line of text data
• JText field can be constructed from
• public JTextField() constructs a new JTextField
• public JTextField(int numColumns) constructs a
  new empty JTextField with a specified number of
  columns
• public JTextField(String text) constructs a new
  JTextField initialized with the specific text
• public JTextField(String text, int columns)
  constructs a new JTextField with the specified
  text and columns

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Other JTextField Methods

• setText() method- Allows you to change the text
  in a JTextField that has already been created
• getText() method- Allows you to retrieve the
  string of text in a JTextField

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Other JTextField Methods

• Keyboard focus- When the user clicks within the
  JTextField, the JTextField has focus, which means
  the next entries from the keyboard will be at
  that location
• requestFocus() method- To have the insertion
  point appear automatically within the TextField
  without requiring the user to click in it first

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Other JTextField Methods

• Editable- The capacity for a field to accept
  keystrokes
• setEditable() method- Used to change the editable
  status of a JTextField

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Adding JButton Components to Swing Applets

• JButton- Creates a button
• JButton can be constructed from
• public JButton() constructs a button with no set
  text
• public JButton(Icon icon) creates a button with
  an icon of type Icon or ImageIcon
• public JButton(String text) creates a button with
  the specific text
• public JButton(String text, int columns)
  constructs a new JTextField with the specified
  text and columns

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Adding JButton Components to Swing Applets

• setLabel() method
• To change a JButtons label
• readyJButton.setLabel(Dont press me again!)

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Adding Multiple Components to a JApplet

• To add multiple components in a container use a
  layout manager
• To control component positioning
• Default behavior is to use a border layout
• Border layouts
• Flow layouts

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Adding Multiple Components to a JApplet

• Border layouts
• Created by the BorderLayout class
• Divide a container into 5 sections
• North, South, East, West, and center
• Created with the BorderLayout() or
  BorderLayout(int, int) methods

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Adding Multiple Components to a JApplet

• Flow Layouts
• Places components in a row, and when a row is
  filled, it automatically spills components onto
  the next row
• Default positioning of the row of components is
  centered in the container

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Learning about Event-Driven Programming

• Event- Occurs when someone using your applet
  takes action on a component
• Procedural- Programmers dictate the order in
  which events occur
• Event-driven programs- The user can initiate any
  number of events in any order
• Source- Component on which an event is generated
• Listener- Object that is interested in an event

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Preparing Your Swing Applet to Accept Event
Messages

• Prepare your applet to accept mouse events by
• importing the java.awt.event package
• adding the phrase implements ActionListener to
  the class header
• ActionListener is an interface
• Interface- A set of specifications for methods
  that you can use with event objects

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Telling Your Swing Applet to Expect Events to
Happen

• addActionListener() method
• To tell the applet to expect ActionEvents
• aButton.addActionListener(this)

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Telling Your Swing Applet How to Respond to Any
Events That Happen

• actionPerformed(ActionEvent e) method
• When a JApplet has registered as a listener with
  a JButton, and a user clicks the JButton the
  actionPerformed method executes

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Adding Output to a Swing Applet

• You can add components to an applet using the
  add() method
• You can also remove components from an applet
  using the remove() method
• Remove(answer)

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Understanding the Swing Applet Life Cycle

• Override- When you write a method that has the
  same method header as an automatically provided
  method

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Understanding the Swing Applet Life Cycle

• start() method- Executes after the init() method
• Executes every time the applet becomes active
  after it has been inactive
• stop() method- When a user leaves a web page
• You do not usually write your own stop() methods

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Understanding the Swing Applet Life Cycle

• destroy() method- When the user closes the
  browser or AppletViewer
• You do not usually write your own destroy()
  methods

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Using the setLocation() and setEnabled() Methods

• setLocation() method- Allows you to place a
  component at a specific location within the
  AppletViewer window
• X-axis- Horizontal position in a window
• X-coordinate- Value increases as you travel from
  left to right across the window
• Y-axis- Vertical position in the window
• Y-coordinate- Value increases as you travel from
  top to bottom in the window

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The setEnabled() Method

• setEnabled() method- To make a component
  unavailable and, in turn, to make it available
  again
• True if you want to enable a component
• False if you want to disable a component
• If (yLoc280) pressButton.setEnabled(false)