Object%20Oriented%20Programming%20in%20Java%20(95-707)%20Advanced%20Topics

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Lecture 9Object Oriented Programming in Java

• Advanced Topics
• Abstract Windowing Toolkit (AWT)

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Todays Lecture

• AWT Fundamentals
• http//developer.java.sun.com/developer/onlineTrai
  ning/

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History

• AWT in JDK 1.0
• AWT JDK 1.1
• JFC/Swing

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Purpose of AWT

• Build Graphical User Interfaces
• Keep native platform look
• Uses native platform API

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Hierarchy of GUI Components

• superclass to all classes is Component
• Component
• Button
• Canvas
• Checkbox
• Choice
• Label
• TextField
• Container
• Panel
• SCrollPan
• Window
• Frame
• List
• ScrollBar

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Containers

• containers group widgets or other containers
• Containers can embed other containers
• containers can have a particular layout
• examples
• Panel
• Applet
• Frame

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Widgets

• Button
• Canvas
• Label
• Checkbox
• Choice
• List
• TextField
• TextArea
• examples
• Button b new Button()
• Label l new Label(OK)

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Event Handlers

• Event handlers are registered with Components and
  handle events that are generated
• mouse clicks
• keyboard keys
• windows events
• Event handlers often implement ActionListener
• example
• ActionListener theListener new MyListener()
• Button b new Button()
• b.addActionListener(theListener)

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How to build a GUI

• Add components to a Container
• Create Event handler classes
• Setup event handlers to respond to clicks,...
• Display the GUI
• Therefore you always need
• one or more containers
• one or more Components
• one or more Event Handler

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Extend the Applet class

• import java.awt.Button
• import java.applet.Applet
• public class TheApplet extends Applet
• public void init()
• // add() one or more widgets to the applet
  itself
• // create an event handler for a widget

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Add a widget to the Applet

• Example
• //Create the widget
• Button aButton new Button(some text)
• // add(Component c) is a method of Container
• add(aButton)

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Create an ActionListener

• import java.awt.event.ActionListener
• import java.awt.event.ActionEvent
• import java.awt.Component
• public class Beeper implements ActionListener
• public void actionPerformed(ActionEvent
  event)
• // get the conponent which initiated the event
• Component c (Component)event.getSource()
• // beep
• c.getToolkit().beep()

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Create HTML

• lthtmlgt
• ltapplet codeAButton.class width100 height100gt
• lt/appletgt
• lt/htmlgt

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View the file

• Use the appletviewer
• or use your browser to bring up the html file

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Standalone GUI

• Create a class with a main method
• Create a Frame f new Frame(frame name)
• Create a Button and add it to the frame
• Create an ActionListener class (reuse Beeper)
• Register the event handlers with the button
• Bring up the frame f.pack() f.show()

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Enhanced standalone GUI

• Use Button class setActionCommand(String s)
  method
• This method gives a name to the click action of
  the button
• example
• b.setActionCommand(QUIT)
• Add a few more buttons to your frame and give
  them various actions

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Enhanced Action Listener

• Within the actionPerformed(ActionEvent event)
  method, check for which action command is
  performed
• 1) Use the ActionEvent method getActionCommand()
• 2) check to see which event was raised
• 3) when event QUIT perform a System.exit(0)

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Layouts

• Layouts define how the components are laid within
  the containers
• You have used a default layout all along
• You can assign a layout explicitly
• example
• Frame f new frame(frame name)
• f.setLayout(new GridLayout(3,2))

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Types of layouts

• example
• GridLayout
• FlowLayout
• CardLayout
• BorderLayout
• GridBagLayout

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Nested Classes

• Nested Classes are a powerful feature of Java
  starting with JDK 1.1
• with nested classes, Java lets you define a class
  as a member of another class. Such a class is a
  nested class
• Nested class can see all of the class members of
  its enclosing class, even the private ones

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Nested Class

• Class EnclosingClass
• class InnerClass
• ...
• An inner class is a nested class whose instance
  exists within an instance of its enclosing class
  and has direct access to the instance members of
  its enclosing instance

EnclosingClass
InnerClass
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Use Inner Classes for Listeners

• Create a class that extends Frame
• Add Components to the frame in the constructor
• Define an inner class which extends
  ActionListener
• Instantiate the inner class within the frames
  constructor the inner class now has access to
  the Frames variables

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Example of Inner Class

• import java.awt.
• import java.awt.event.
• public class AppFrame extends Frame
• Button b1, b2
• public AppFrame(String title)
• super(title)
• // instantiate buttons
• // instantiate inner class
• ActionListener al new MyHandler()
• b1.addActionListener(al)
• class MyHandler extends ActionListener
• public void actionPerformed(ActionEvent event)
  
• // get the conponent which initiated the event
• Component c (Component)event.getSource()
• if (cb1) //do this
• if (cb2) // do that...