AWT, Swing and GUIs in Java

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AWT, Swing and GUIs in Java
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Overview

• Applets and Applications
• Class Hierarchy of AWT and Swing
• JPanels and Contexts
• Assembling the GUI
• Event Generators and Event Listeners
• Required Methods

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Event Driven Programming

• Event Driven Programming
• Enter data, then generate action event
• User options available
• Traditional or Stop and Wait Programming
• Loops (or lack of) in Event Driven Programming

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Applets and Applications

• Applets run in a browser (i.e., IE or Netscape)
• Applet code can also be run in an appletviewer
• Applets can not read or write local files, check
  for the existence of files, execute programs on
  the local machine, etc.
• These restrictions are for security reasons
• Applications can provide the same interface but
  are not subject to the above security restrictions

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JApplet Methods

• init()
• Describes the GUI
• You write this
• start() //can be ignored or removed
• stop() //ditto
• destroy() //ditto

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Class Hierarchy of AWT and Swing

• See Koffman and Wolz page 443
• Component //abstract
• Container //abstract
• Panel
• Applet
• JApplet
• Jcomponent //abstract

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Class Hierarchy of AWT and Swing

• JComponent (an abstract class)
• Jlabel //for label
• JOptionPane
• Jpanel //holds other components
• AbstractButton
• Jbutton //a button to push
• JToggleButton
• JCheckBox //a box to put a check mark in
  either true or false
• JRadioButton //used for selecting one from a
  group of options
• JTextComponent
• JTextField //for input
• JTextArea //for output

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Jpanels and Contexts
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Assembling the GUI

• JLabel
• JTextField
• JTextArea
• JButton
• JPanel
• Example put a JLabel and a JTextField onto a
  JPanel, and then put the JPanel onto the Applet

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Event Generators and Event Listeners

• Need to import java.awt.event.
• Constitutes the Java "event model"
• GUI components "generate" events
• Could be a button click
• Could be a CR or check box click
• Could be a mouse movement, etc.
• Applet or JFrame serves as "listener"
• Waits till some event occurs
• Then can determine what occurred (e.g., which
  button pushed)
• Can then determine what to do

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Event Generators and Event Listeners

• init() constructs GUI
• Within init() registers Applet or JFrame as a
  listener to a particular GUI object
• myDeleteButton.addActionListener(this)
• myChangeButton.addActionListener(this)
• When either myDeleteButton or myChangeButton is
  pushed, the method actionPerformed() is
  automatically executed

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Writing the actionPerformed() method

• Determining which button was pressed
• Public void actionPerformed(Action Event aE)
• Object sourceOfEvent aE.getSource()
• If (sourceOfEvent myDeleteButton)
• code for what you want to do when delete
  pressed
• Else if (sourceOfEvent myChangeButton)
• code for what you want to do when change
  pressed

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Writing the actionPerformed() method

• Determining values showing in various GUI objects
• Public void actionPerformed(Action Event aE)
• String priceStr priceInputJTextField.getText()
  //get value
• double price Double.parseDouble(priceStr)
  //convert to double

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Writing the actionPerformed() method

• Displaying values back to GUI objects
• Public void actionPerformed(Action Event aE)
• String priceStr priceInputJTextField.getText()
  //get value
• double price Double.parseDouble(priceStr)
  //convert to double
• DecimalFormat df new DecimalFormat(".00")
• priceInputJTextField.setText(df.format(price))
  //formats output
• outputJTextArea.setText("num tickets is "
  numTickets)

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Methods You Will Find in Your GUI

• init() //creates the layout
• actionPerformed() //responds to events
• itemStateChanged() //if check boxes used
• Look at samples on the course web site and in
  Chapter 7

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Converting an Applet to an Application

• Extends JFrame and not JApplet
• Still implements ActionListener
• init() contents go into constructor
• Need a main method to construct an instance of
  the class itself
• See page 464 for sample code