Chapter 13 Java AWT

1
Chapter 13Java AWT Part II(Optional)

• Lecture Slides to Accompany
• An Introduction to Computer Science Using Java
  (2nd Edition)
• by
• S.N. Kamin, D. Mickunas, E. Reingold

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Chapter Preview

• In this chapter we will
• discuss the use of Java in animating World Wide
  Web applications
• show how to write Java applets
• introduce Java graphical components (e.g.
  Textfields, Buttons, and Labels)
• show how Java programs can be made to react to
  user actions on Java graphical components
• show how Java applications can be run from inside
  web browsers

3
Frames

• A frame is a window with a title bar and a border
• The Frame class is a subclass of Container class
• Container class objects may have other components
  (e.g. Buttons) added to them using the add method
  
• A typical Java GUI will create and display one or
  more frames
• To make a frame visible the message
  setVisbible(true) must be sent to the frame

4
Layout Managers

• Governs how components are arranged inside a
  Container object
• The Container method setLayout allows the
  programmer to specify which layout manager (e.g.
  FlowLayout) is desired for a particular container
  object
• The size of a Container object should be set
  explicitly by the programmer as well
• Can use pack() to set size to accommodate
  preferred sizes of components)

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Frame Example

• public class TwoButtons
• Frame f
• Button redButton, blueButton
• public TwoButttons()
• f new Frame(Two Buttons Frame)
• redButton new Button(Red)
• blueButton new Button(Blue)
• f.setLayout(new Flowlayout())
• f.add(redButton)
• f.add(blueButton)
• f.pack()
• f.setVisible(true)

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UML Notation

• The filled diamond represents composition
• This shows that the class TwoButtons contains a
  class Frame which contains a class Buttons
• A good UML diagram only shows the classes and
  associations that are important to understanding
  the architecture
• This UML diagram does not include super classes
  like Component or Container

8
Using Inheritance

• It is possible to use inheritance to allow
  TwoButtons to become a frame in its own right
• Note that the UML diagram will show TwoButtons as
  a subclass of Frame
• You will need to use super()to set the frame title

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Frame Inheritance Example

• public class TwoButtons extends Frame
• Button redButton, blueButton
• public TwoButttons()
• super(Two Buttons Frame)
• redButton new Button(Red)
• blueButton new Button(Blue)
• f.setLayout(new Flowlayout())
• f.add(redButton)
• f.add(blueButton)
• f.pack()
• f.setVisible(true)

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Other Simple Java Components

• Label
• contains some text that can be set using a
  constructor or the method setLabel
• TextField
• a box into which the user can type text
• text can be edited (backspace, delete, etc.)
• text can be retrieved using getText()
• text can be set using setText()

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Java AWT Event Model

• Including reactive program components involves
• Having the class header declare itself as
  implementing the ActionListener interface
• Typically in the class constructor the class
  instance registers itself as being interested in
  listening for events from a newly created
  component
• One method (e.g. actionPerformed) of the
  ActiveListener interface is defined

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Frame Inheritance Example

• public class classname extends Frame implements
  ActionListener
• Button buttonname
• public classname()
• buttonname new Button(button label)
• add(buttonname)
• buttonname.ActionListener(this)
• public void actionPerformed(ActionEvent e)
• what to do when button is pushed

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UML Notation for Button Program

• Abstract class indicated by italicizing the class
  name
• Implements association (concrete class) indicated
  by line with a closed arrowhead
• UML stereotype (constructors, accessor, interface
  implementation methods) indicated by enclosing a
  descriptive word inside guillements ltlt gtgt

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Closing the Window

• You must implement the WindowListener and its
  seven required methods
• windowActivated, windowClosed, windowClosing,
  windowDeactivated, windowIconified,
  windowDeiconified, windowOpened
• This is usually done in a programmer defined
  class like ClosableFrame and all classes needing
  these services would extend ClosableFrame

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Using Conditionals with Reactive Components

• public void actionPerformed(ActionEvent e)
• double fahr, cent
• // check for user input in tFahr TextField
• if (e.getSource() tFahr)
• fahr new Double(tFahr.getText()),doubleValue
  ()
• cent 5.0 (fahr 32.0) / 9.0
• tCent.setText(cent ) // update tCent
  TextField
• else
• cent new Double(tCent.getText()),doubleValue
  ()
• fahr 9.0 cent / 5.0 32.0
• tFahr.setText(fahr )

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Checkboxes

• Used to allow user to select one or more items
• Always has a label
• Program often needs to react to the user
  selection(s)
• Declaration example
• Checkbox powerBrakes new Checkbox(Power
  Brakes)
• Checkbox powerSteering new Checkbox(Power
  Steering)
• Checkbox ac new Checkbox(Air Conditioning)
• add(powerBrakes)
• add(powerSteering)
• add(ac)

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Programming Checkboxes

• Program is declared with implements ItemListener
• Checkbox is registered by calling addItemListener
  
• Event is handled using itemStateChanged argument
  type ItemEvent
• The ItemEvent argument is used to tell which item
  triggered the event by calling getSource

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Radio Buttons

• A group of Checkboxes in which only one item can
  be selected at a time
• Implemented using a Java CheckboxGroup
• Items are declared initially as selected (true)
  or unselected (false)
• Example
• CheckboxGroup gender
• Checkbox maleCheck
• new Checkbox(Male, gender, true)
• Checkbox femaleCheck
• new Checkbox(Female, gender, true)

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Processing Radio Buttons

• public void compute()
• boolean female
• (gender.getSelectedCheckbox() femaleCheck)
• if ((bodyMassIndex gt 27.8)
• (female (bodyMassIndex gt 27.3))
• lHigh.setText(This is considered high)
• else
• 1High.setText(this is not considered high)

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Drawing in a Frame

• To draw in a Frame you need to the override the
  frames paint method
• public void paint(Graphics g)
• Graphics objects are defined by the Java runtime
  system and are used for drawing operations
• The Frame should be considered to be a grid with
  upper left coordinates (0,0) and positive
  coordinates (x,y) for the lower right

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Typical Drawing Code

• public class MyDrawing extends ClosableFrame
• public MyDrawing()
• public void paint(Graphics g)
• g.drawLine()
• g.drawImage()

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Repaint and Update

• The paint method requires an argument and
  actionPerformed does not know what to supply when
  called
• The method repaint will clear the Frame and then
  call paint along with supplying the needed
  missing argument
• You can prevent clearing the Frame when using
  repaint by overriding the method update before
  calling repaint

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Panel and Canvas Classes

• The Panel class is container subclass that is
  used to reserve a rectangular portion of a Frame
  to place other components
• The Canvas class is not a Container subclass, but
  does allow you to reserve a portion of a Frame to
  draw in

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Comparing Layout Managers

• FlowLayout
• Default frame layout
• Components are placed on new line only when
  needed
• GridLayout
• Frame is declared as a grid of fixed size (e.g.
  two rows and three columns)
• Components are placed in the grid left to right
  and top to bottom
• BorderLayout
• Frame is divided into north, south, east, west,
  and center
• Components are placed by the programmer in the
  desired location using the add method

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Typical GUI Frame

• Panel p1 new Panel()
• p1.setLayout(new GridLayout(2,1))
• p1.add(component)
• p1.add(component)
• Panel p2 new Panel()
• p2.setLayout()
• p2.add(component)
• setLayout(new BorderLayout())
• add(North, p1)
• add(Center, p2)

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Scrollbars

• The leftmost position of a horizontal scrollbar
  corresponds to some integer value and the
  rightmost position corresponds to a larger
  integer value
• Scrollbars can be displayed vertically
• User movement options
• Clicking either end button causes bubble to move
  in unit increments
• Clicking the are between bubble and end button
  causes movement in 10 unit increments
• Clicking and dragging the bubble in the desired
  direction

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Hypertext Markup LanguageHTML Skeleton

• ltHTMLgt
• ltHEADgt
• ltTITLEgtPage Titlelt/TITLEgt
• lt/HEADgt
• ltBODYgt
• Text and graphics to display in browser
• lt/BODYgt
• lt/HTMLgt

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HTML Text Displays

• Displays a centered heading in browser window
• ltCENTERgt
• ltH1gtNaploean Ilt/H1gt
• lt/CENTERgt
• To identify text paragraphs to be displayed
• ltPgt
• In my spare time I like to do origami
• and spend quiet evenings at home with my wife
  Josephine.
• lt/Pgt

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HTML Using Pictures and Links

• Displays a centered picture
• ltCENTERgt
• ltIMG SRCjojo.jpggt
• lt/CENTERgt
• To display a text link to another web page
• ltA HREFjojo.htmlgtJosephinelt/Agt
• To display a picture link to another web page
• ltA HREFjojo.htmlgtltIMG SRCjojo.jpggtlt/Agt

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Applet Tag and HTML

• To include a Java applet as part of an HTML
  document
• ltAPPLET CODEBodyMass.class WIDTH0 HEIGHT0gt
• lt/APPLETgt
• Java applet shell for BodyMass.class
• import java.applet.
• public class BodyMass extends Applet
• public_html void start()
• new BodyMass()

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Transforming a Java Program into an Applet

• Set the size you want the applet to be in the
  HTML file
• Add the import statement to the Java code
  java.applet.
• Change header from extends Frame to extends
  Applet
• Change constructor heading to
• public void start()
• Eliminate any Java code that is meaningless in
  applets