Advanced GUIs and Applets

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• Advanced GUIs and Applets

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Inheritance Hierarchy of GUI Classes
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Applets

• Applet a Java program that is embedded within a
  Web page and executed by a Web browser
• Create an applet by extending the class JApplet
• class JApplet contained in package javax.swing

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Members of class JApplet
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Members of class JApplet
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Applets

• No main method
• Methods init, start, and paint guaranteed to be
  invoked in sequence
• To develop an applet
• Override any/all of the methods above

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Applet Methods

• init Method
• Initializes variables
• Gets data from user
• Places various GUI components
• paint Method
• Performs output

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Skeleton of a Java Applet
import java.awt.Graphics import
javax.swing.JApplet public class WelcomeApplet
extends JApplet
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Applet Displaying Welcome Message
//Welcome Applet import java.awt.Graphics impor
t javax.swing.JApplet public class
WelcomeApplet extends JApplet public
void paint(Graphics g)
super.paint(g)
//Line 1 g.drawString(Welcome to
Java Programming,
30, 30) //Line2

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HTML to Run Applet
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Differences Between Applets and GUI Applications

• Applets
• Derived from JApplet
• No main method
• Uses init method
• Displayed by HTML
• Sets title in HTML
• Size set in HTML
• Applet closes when HTML doc closes

• GUI applications
• class extends JFrame
• Invokes main method
• Uses constructors
• Uses method setVisible
• Uses setTitle method
• Uses method setSize
• Closes with Exit button

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Converting a GUI Application to an Applet

• Change JFrame to JApplet
• Change constructor to method init
• Remove method calls such as setVisible, setTitle,
  setSize
• Remove the method main
• If applicable, remove Exit button/all code
  associated with it (e.g. action listener)

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Additional GUI Components

• JTextArea
• JCheckBox
• JRadioButton
• JComboBox
• JList

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JTextArea

• Can collect multiple lines of input from user
• Can display multiple lines of output
• Pressing Enter key separates lines of text
• Each line ends with newline character \n
• Derived from class JTextComponent

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Constructors and Methods of class JTextArea
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Methods Inherited by class JTextArea from Parent
class JTextComponent
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JCheckBox

• User selects from predefined values
• Example of a toggle button
• Clicking JCheckBox generates item event
• Use interface ItemListener and its abstract
  method itemStateChanged to handle event

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Constructors and Methods of class JCheckBox
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Constructors and Methods of class JCheckBox
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Constructors and Methods of class JCheckBox
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JRadioButton

• Created same way as check boxes
• Placed in content pane of applet
• Forces user to select only one radion button at a
  time
• You create a button group to group radio buttons
• Generates an ItemEvent
• interface ItemListener and method
  itemStateChanged used to handle events

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Constructors and Methods of class JRadioButton
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Constructors and Methods of class JRadioButton
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Constructors and Methods of class JRadioButton
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JComboBox

• Commonly known as a drop-down list
• Used to select an item from a list of
  possibilities
• Generates an ItemEvent
• Event monitored by ItemListener
• ItemListener invokes method itemStateChanged

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Constructors of class JComboBox
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Applet with JCheckBox, JComboBox, and JRadioButton
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Constructors of class JList
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Methods of class JList