Java Applet and GUI Basics

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Java Applet and GUI Basics

• K.P. Chow
• University of Hong Kong

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Java Applets - Example

• import java.applet.Applet
• import java.awt.Graphics
• public class Simple extends Applet
• stringBuffer buffer
• public void init ()
• buffer new StringBuffer()
• addItem(initializing ...)
• public void start ()
• addItem(starting ...)

public void stop () addItem(stopping
...) public void destroy ()
addItem( prepare for unloading ...)
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Java Applets - Example (cont)

• void addItem(String newWord)
• System.out.println(newWord)
• buffer.append(newWord)
• repaint()
• public void paint (Graphics g)
• g.drawRect(0,0,size().width-1,size().height-1)
• g.drawString(buffer.toString(),5,15)

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Applet Execution in a Browser
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Applet Execution in a Browser (cont)
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Applet Execution in Details
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Applet - Drawing Graphics

• import java.applet.
• import java.awt.
• public class GApplet extends Applet
• static final String message
• Hello World
• private Font font
• public void init()
• font new Font(Helvetica,
• Font.BOLD,48)

public void paint(Graphics g) g.setColor(Color.
pink) // pink oval g.fillOval(10,10,330,100)
g.setColor(Color.red)// red outline g.drawOval(1
0,10,330,100) g.drawOval(9,9,330,100) g.drawOv
al(8,8,330,100) g.drawOval(7,7,330,100) g.set
Color(Color.black) g.setFont(font) g.drawStrin
g(message,40,75)
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Applet - Drawing Graphics (cont)
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Applet - Reading Applet Parameters

• HTML for the following applet
• ltAPPLET codeColorScribble.class width300
  height300gt
• ltPARAM nameforeground value0000FFgt
• ltPARAM namebackground valueFFCCCCgt
• lt/APPLETgt
• public class ColorScribble extends Scribble
• public void init()
• super.init()
• Color fggetColorParameter(foreground)
• Color bggetColorParameter(background)
• if ( fg ! null ) this.setForeground(fg)
• if ( bg ! null ) this.setBackground(bg)

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Applet - Reading Applet Parameters (cont)

• protected Color getColorParameter(String name)
• String valuethis.getParameter(name)
• int intvalue
• try
• intvalueInteger.parseInt(value,16)
• catch ( NumberFormatException e )
• return null
• return newColor(intvalue)

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Applet - Reading Applet Parameters (cont)

• public String getParameterInfo()
• // used by web browser for info about params
• String info
• foreground,hexidecimal color value,
  foreground color ,
• background,hexidecimal color value,
  background color
• return info
• public String getAppletInfo()
• // used by About dialog box
• return Scribble v.0.02

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Exercise - Applet Reading Parameters

• Implement StaticLabel class which takes a text
  string and displays it centered in an applet.
  The font and label can be passed as parameters
  from HTML. Following is an example HTML file and
  the corresponding display.
• lttitlegtStatic Lablelt/titlegt
• lthrgt
• ltapplet codeStaticLabel.class width200
  height80gt
• ltparam namelabel valueHello, worldgt ltparam
  namefontname valueCouriergt
• ltparam namefontslant valueitalicgt ltparam
  namefontweight valueboldgt
• ltparam namefontsize value24gt
• lt/appletgt lthrgt

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Applet Debugging

• Put a main method in any class, even a class that
  runs as an applet or a class that isnt the main
  routine of your application.
• Test individual classes using the main
• Use main as different entry point to different
  program
• Cut down number of files
• at least 3 files .java, .class, .html
• Use appletviewer

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JAR Files

• Group together a number of files, called JAR
  (Java ARchive)
• Convenience a program may consist of several
  class files, jpg files and audio files
• Efficiency reduce overhead of sending several
  files
• PKZIP was used in JDK 1.0.2
• Use utility jar to create JAR file, e.g.
• jar cvf myJarFile.jar .class .jpg
• HTML tag
• ltAPPLET ARCHIVEmyJarFile.jar CODEmyapplet.class
• WIDTH600 HEIGHT250gt lt/APPLETgt

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JAR Files (cont)

• Java applet to extract noncode files
• import
• public class view extends Applet
• URL MyURL
• Image MyImg
• ImageProducer MyImageProd
• public void init()
• Toolkit toolToolkit.getDefaultToolkit()
• MyURLgetClass().getResource(titan.jpg)
• try
• MyImgProd (ImageProducer) MyURL.getContent()
• catch

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GUI Libraries

• Basic areas of functionalities
• Create user interface controls or components,
  such as scrollbars, buttons
• Support for giving behavior to the controls by
  GUI events
• Support for grouping and arranging the controls
  on the screen
• Support for accessing window manager facilities,
  such as which window has input focus, reading
  JPEG, etc
• Related graphics libraries provide support for
  graphics operations such as draw an arc, fill a
  polygon, etc
• Before JDK 1.1
• Java GUI operations supported by AWT (Abstract
  Window Toolkit)
• Support portability give GUI programmer a common
  binary Windowing interface (API) with different
  native window systems
• GUI methods make calls to native code library,
  uses underlying native window system to
  manipulate GUI objects

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GUI Programming

• Declare controls declare buttons, menus, etc.
• Event handling implement an interface to provide
  the event handler that responds to control
  activity
• Layout add the controls to container, containers
  are what you display on the screen

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Event Handling

• Examples of events
• a mouse is clicked
• an item on a scrollable list is selected
• a button is pressed
• some text is entered into a text field or text
  area
• Event handling mechanisms
• Old model use inheritance to tie together your
  code and the event mode
• JDK 1.1 event model use delegation-based model,
  register a callback with event-handling classes

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Old Model Event Handling
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Old Model - using action()

• import java.applet.Applet
• import java.awt.
• public class evui extends Applet
• Panel botPanel
• public void init ()
• setLayout(new FlowLayout())
• botPanel new Panel() add(botPanel)
• botPanel.add(new TextField(TextField))
• botPanel.add(new Button(Button 1))
• botPanel.add(new Button(Button 2))

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Old Model - using action() (cont)

• public boolean action(Event e, Object o)
• if ( e.target instance of Button )
• String str (String) o
• if ( str.equals(Button 1))
• showStatus(First Button!)
• else if (str.equals(Button 2))
• showStatus(Second Button!)
• else
• return false
• return true
• else if ( e.target instanceof TextField)
• showStatus(TextArea (String) o)
• else return false
• return true

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JDK 1.1 Event Handling

• 2 key tasks
• register an event listener (object)
• implement an event handler (method)
• An event listener is an object of a class that
  implement one or more event listener interfaces
  from package java.awt.event
• Delegation event model an event handler is a
  method that is automatically called in response
  to a particular type of event

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JDK 1.1 Event Handling Example

• import java.applet.Applet
• import java.awt.
• import java.awt.event.
• import javax.swing.
• public class MyButtons extends JApplet
• private JButton pushButton1, pushButton2
• public void init()
• pushButton1 new JButton(Button 1)
• ActionListener alnew ActionListener()
• public void actionPerformed(ActionEvent e)
• applet.showStatus(You pressed
• e.getActionCommand())
• pushButton1.addActionListener(al)
• add(pushButton1)

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JDK 1.1 Event Handling Example (cont)

• pushButton2 new JButton(Button 2)
• ActionListener al2new ActionListener()
• public void actionPerformed(ActionEvent e)
• applet.showStatus(You pressed
  e.paramString())
• pushButton2.addActionListener(al2)
• add(pushButton2)

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Exercise - Alice in Wonderland

• Program an Alice in Wonderland applet a panel
  with 2 buttons, one of which makes the panel grow
  larger, the other smaller.
• Hint
• Use ActionerListener
• Use resize(int width, int height) which resize a
  Container.
• resize(d.width10,d.height10) and
  resize(d.width-10,d.height-10)

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Event Handling Basic Framework

• Interface SomethingListener has one or more
  methods that is called when the corresponding
  SomethingEvent occurs
• Write a class that implements a SomethingListener
  interface
• Declare an object, called, say myHandler, of your
  class
• On your component, called the addSomethingListener
  (myHandler) method

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Event Handling Basic Framework (cont)
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Event Handler Interface
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Event Handler Interface (cont)
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Event Handling - example with text

• import java.applet.Applet
• import java.awt.
• import java.awt.event.
• import javax.swing.
• public class MyTextfield extends JApplet
• private JTextField text1, text2
• private JTextFieldHandler handler
• public void init ()
• setBackground(Color.lightGray)
• handler new TextFieldHandler(this)
• text1 new JTextField() // textfield with
  default sizing
• text1.addActionListener(handler)
• add(text1)
• text2 new JTextField(Enter text) //
  textfield with default text
• text2.addActionListener(handler)
• add(text2)

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AWT Event Handling - action listener with text
(cont)

• class TextFieldHandler implements ActionListener
  
• private JApplet applet
• public TextFieldHandler(Applet a) applet a
  
• public void ActionPerformed (ActionEvent e)
• applet.showStatus(Text is
  e.getActionCommand())

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Exercise - Form

• Program an applet that will display a form for
  telephone directory enquiry that can capture the
  following information
• Name (20 characters)
• An indication of whether it is an exact search or
  partial search
• Address (30 characters)
• An indication of whether it is an exact search or
  partial search
• A list of districts, e.g. Lai Chi Kok, Cheung Sha
  Wan, Tai Po, Yau Ma Tai, North Point, Peak,
  Aberdeen
• Selection for one of the districts HK, KLN, NT
• 2 buttons Submit Query and Clear (which clear
  all input)
• Number type phone number, fax number, pager
  number, mobile number
• Directory type residential, business, government