AWT Widgets

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AWT Widgets Yangjun Chen Dept. Business
Computing University of Winnipeg
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Out line AWT Widgets

• Graphical components
• - Label
• - TextComponent
• - Button
• - CheckBox
• - Choice
• - List
• - Container
• - Canvas

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Graphics
AWT
AWT Widgets - components for GUIs
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AWT Widgets

• Widgets are components such as
• - buttons, text fields, menus, etc.
• These widgets are used to make up a Graphical
  User Interface (GUI)
• The java.awt package provides a set of classes
  for
• implementing a wide variety of widgets.
• These classes are platform independent, so a menu
  in a Java
• program will look like a Windows menu on a PC
  running
• Windows, a Mac menu on a Macintosh, and a Motif
  menu on
• a Unix machine - all with the same code.

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AWT Widgets

• We construct a GUI by placing components within a
  container.
• - normally, there are only two types of
  containers used
• Frame - creates a complete window
• Panel - a container and a component
• - Several Panels may be within a Frame, and
  within each Panel may be
• several components.

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The Component Class

• The Component class is an abstract class.

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The Component Class
The Component Class

• The Component class is an abstract class.
• public abstract class Component
• paint (Graphics g)
• repaint(int x, int, y, int width, int
  height)
• update(Graphics g)

• The Component class is an abstract class.

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The Component Class

• void paint (Graphics g)
• - this method causes something to be painted in
  the component.
• - in Component class, this method is empty and
  does nothing, it is intended to be overridden
  in a subclass as we have been doing.
• - Example
• public void paint(Graphics g)
• g.setFont (new Font(Serif, Font.BOLD, 36))
• FontMetrics fm g.getFontMetrics( )
• String str counter
• Dimention d getSize()
• int x d.width/2 - fm.stringWidth(str)/2
• g.drawString(str, x, d.height/2)

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The Component Class

• When you want to update the display, you cannot
  directly call the paint( ) method can. But you
  can cause it to be called by calling a method
  called repaint().
• void repaint ()
• void repaint (int x, int y, int width, int
  height)
• - These two methods are used to tell the system
  to redraw the
• Component as soon as possible
• - They instruct the system to schedule a call to
  update()
• - You cant count on repaint() to immediately
  update your screen -
• most of the time it will happen quickly, but if
  you are running
• something time sensitive like animation,
  special techniques must be
• used.

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The Component Class

• void update( Graphics g)
• - like the paint() method, you cant invoke this
  method directly
• - The system calls this method when the screen
  needs to be redrawn.
• For example, whenever repaint( ) is executed.
• - If this method is not overridden, this method
  will clear your
• component and then call the paint method

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import java.applet. import java.awt. import
java.lang. / ltapplet code"DoubleBuffer"
width30 height100gt lt/appletgt / public class
DoubleBuffer extends Applet implements Runnable
int x 5 Thread t Image buffer
null Graphics bufferg
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public void init() //Start thread t new
Thread(this) t.start() //Create
buffer Dimension d getSize() buffer
createImage(d.width, d.height) public
void run() try while(true) repaint()
//Request a repaint
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//Sleep before update Thread.sleep(100)
catch(Exception e) public void
update(Graphics g) paint(g) public
void paint(Graphics g) //Get graphics object
for buffer if (buffer ! null)
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bufferg buffer.getGraphics() //Draw to
buffer Dimension d getSize() bufferg.setCol
or(Color.white) bufferg.fillRect(0, 0,
d.width, d.height) bufferg.setColor(Color.black
) bufferg.fillOval(x, d.height/4, 50,
50) //Update screen g.drawImage(buffer, 0,
0, this) //Increment x x 5 if (x 50
gt d.width) x 0
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Textual Widgets - Labels

• A label is a component that will display a line
  of read-only text on the screen.
• It has three constructors
• - Label()
• center alignment and no text
• - Label( String text)
• a center aligned Label object displaying the
  text given
• - Label( String text, int alignment)
• Label with the given text and alignment
  specified as either
• Label. LEFT, Label. CENTER, and Label. RIGHT
• After you have a Label object, you can use
  methods
• defined in the Label class to get and set
  various attributes.

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Label Class
- String getText() returns a string containing
this labels text - void setText( String
text) sets the text for this label to the value
of the argument - int getAlignment() returns an
integer corresponding to the alignment of the
label 0. is Label. LEFT 1. is Label.
CENTER 2. is Label. RIGHT - void setAlignment(
int alignment) sets the alignment of the label
to the specified argument
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Label Class
Example import java. awt. public class
LabelExample extends java. applet.
Applet public void init() setLayout( new
GridLayout( 1, 1)) add( new Label( left,
Label. LEFT)) add( new Label( center,
Label. CENTER)) add( new Label( right,
Label. RIGHT)) //end of init //end of
LabelExample
row number
column number
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init() Method

• The init() method is somewhat like the paint()
  method in
• that its not called by any method in our class.
• The init() method belongs to the Applet class and
  is called
• when the applet is first loaded.
• - This is where we would want to do any onetime
  initialization
• - Very similar to the main( ) method for Java
  application

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Text Component Class

• There are two textual widgets TextField and
  TextArea that are very similar to each other, so
  the data and methods that are common to both were
  removed and placed into the TextComponent class
• So TextField and TextArea are both subclasses of
  Text
• Component
• There are quite a few methods that are inherited
  by
• TextField and TextArea
• - String getText()
• returns a copy of the current contents of the
  component
• - void setText( String text)
• sets the current text to what is specified in
  by the argument

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TextField Class

• Text fields are a box into which a single line of
  text may be
• placed.
• Text fields are different from labels in that
  they can be
• edited.

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TextField Class

• There are four constructors to TextField objects
• - TextField()
• creates an empty text field that is 0
  characters wide
• - TextField( int columns)
• creates an empty text field wide enough to
  hold columns characters
• - TextField (String Text)
• creates a text field initialized with the
  given string
• - TextField( String text, int columns)
• creates a text field with content and width
  given by text and columns

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TextArea Class

• Text areas are like text fields except they can
  handle larger
• amounts of text.
• Text areas can be given any width and height and
  have
• scroll bars by default.

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TextArea Class

• There are five constructors for a TextArea
  object
• - TextArea()
• creates an empty TextArea object
• - TextArea( int rows, int columns)
• creates an empty text area with the given
  number of rows and
• columns( chars)
• - TextArea( String text)
• creates a default sized text area containing
  the argument string
• - TextArea( String text, int rows, int columns)
• creates a text area displaying the given
  string with the size specified

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TextArea Class
- TextArea( String text, int rows, int columns,
int scrollbars) - Acts like the preceding
constructor, in addition it has an argument
to specify whether or not the text area will have
scrollbars. - There are 4 constant
arguments that can be used TextArea.
SCROLLBARS_ BOTH TextArea. SCROLLBARS_
NONE TextArea. SCROLLBARS_ HORIZONTAL_
ONLY TextArea. SCROLLBARS_ VERTICAL_
ONLY - The default is for both horizontal and
vertical scrollbars to be displayed.
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TextArea Class

• An example
• import java. awt.
• public class TextAreaExample extends java.
  applet. Applet
• public void init()
• String str This is an example for a
  TextArea that \n
• spans multiple lines. \n
• This is for course 91.3930/ 3 at the \n
• University of Winnipeg!\n
• add( new TextArea( str))
• //end of int
• //end of TextAreaExample

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Button Class

• Buttons are a simple UI component that triggers
  some action
• There are two constructors for a Button object
• - Button()
• creates an empty button with no label
• - Button( String label)
• creates a button with the given string as a
  label
• After creating a Button object, you can get the
  value of the
• Buttons label by using the getLabel() method
  which
• returns a String.
• Using the setLabel() method, you can set the
  label of the
• button by passing in a String value.

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Button Class

• An Example
• import java. awt.
• public class ButtonExample extends java.
  applet. Applet
• public void init()
• add( new Button( Rewind))
• add( new Button( Fast Forward))
• add( new Button( Play))
• add( new Button( Stop))
• add( new Button( Pause))
• //end of init
• //end of ButtonExample

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Checkbox Class

• Check boxes are UI components that have two
  states on
• and off (or checked and unchecked, or true and
  false)
• unlike buttons, check boxes dont trigger a
  direct action,
• but are used to indicate optional features of
  some other
• action.
• Check boxes can be used in two ways
• - Nonexclusive
• given a series of checkboxes, any of them can
  be selected
• can be checked or unchecked independently of
  each other
• - Exclusive
• given a series of checkboxes, only one can be
  selected at any one time
• also known as radio buttons or checkboxgroups

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Checkbox Class

• There are five constructors
• - Checkbox()
• creates a Checkbox with no label and false
• - Checkbox( String label)
• creates a Checkbox with the given label and
  initialized to false
• - Checkbox( String label, boolean state)
• creates a Checkbox with the given label and
  state
• - Checkbox( String label, boolean state,
  CheckboxGroup group)
• - Checkbox( String label, CheckboxGroup group,
  boolean state)
• The last two create a Checkbox with the label
  and state specified as well as specifying a
  group that this Checkbox will belong to.
• There are a number of methods in this class that
  allow you
• to get and set the label, state and
  CheckboxGroup.

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Checkbox Class

• An Example
• import java. awt.
• public class CheckboxExample extends java.
  applet. Applet
• public void init()
• setLayout( new FlowLayout( FlowLayout. LEFT))
• add( new Checkbox( Red))
• add( new Checkbox( Orange))
• add( new Checkbox( Yellow))
• add( new Checkbox( Green))
• add( new Checkbox( Blue))
• add( new Checkbox( Indigo))
• add( new Checkbox( Violet))
• //end of init
• //end of CheckboxExample

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CheckBoxGroups

• A CheckboxGroup is a collection of Checkboxes in
  which
• only one of them can be selected at one time -
  also known
• as radio buttons
• To create a series of radio buttons, first create
  an instance
• of CheckboxGroup
• - ChecboxGroup cbg new CheckboxGroup()
• Then create and add the radio buttons into the
  group
• - use either of the following two methods
• Checkbox( String label, boolean state,
  CheckboxGroup group)
• Checkbox( String label, CheckboxGroup group,
  boolean state)

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CheckBoxGroups

• An Example
• import java. awt.
• public class CheckboxGroupExample extends java.
  applet. Applet
• public void init()
• setLayout( new FlowLayout( FlowLayout. LEFT))
• CheckboxGroup cbg new CheckboxGroup()
• add( new Checkbox( One, false, cbg))
• add( new Checkbox( Two, false, cbg))
• add( new Checkbox( Three, true, cbg))
• add( new Checkbox( Four, false, cbg))
• //end of init
• //end of CheckboxGroupExample

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CheckBoxGroups

• Since CheckboxGroups only allow one button to be
  selected
• at one time, the last Checkbox to be added to
  the group with
• the state being true will be the one selected by
  default.
• So in the previous example, radio button with the
  label three would be selected.

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Choice Class

• Choice menus are more complex than labels,
  buttons, or
• checkboxes
• Choice menus are pop-up or pull-down lists in
  which an
• item can be selected.
• To create a Choice menu, we first need an
  instance of the
• Choice class and then add items to it.
• The items added are enumerated from top to bottom
• starting with the index 0.
• One of the methods in this class to make note of
  is
• - String getSelectedItem()
• returns the text of the current selection

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Choice Class

• An Example
• import java. awt.
• public class ChoiceExample extends java. applet.
  Applet
• public void init()
• Choice c new Choice()
• c. add( Red)
• c. add( Orange)
• c. add( Yellow)
• c. add( Green)
• c. add( Blue)
• c. add( Indigo)
• c. add( Violet)
• add( c)
• //end of init
• //end of ChoiceExample

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List Class

• A List is functionally similar to a Choice menu
  in that it
• lets you choose from a list, but Lists do not
  pop-up, they
• are permanently displayed on the screen.
• You can choose more than one item on the List if
  that
• capability is enabled.
• List items are enumerated from top to bottom
  starting at
• index 0 just like a Choice menu.

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List Class

• There are three constructors for a List object
• - List()
• creates a default sized List object in single
  selection mode
• - List( int rows)
• creates a List object in single selection mode
  with the number of rows given
• - List( int rows, boolean multipleselection)
• creates a List object with the specified
  number of rows. The List is in multiple
  selection mode if the boolean argument is true
  and single selection otherwise.

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List Class

• An Example
• import java. awt.
• public class ListExample extends java. applet.
  Applet
• public void init()
• List mylist new List( 7, true)
• mylist. add( Red)
• mylist. add( Orange)
• mylist. add( Yellow)
• mylist. add( Green)
• mylist. add( Blue)
• mylist. add( Indigo)
• mylist. add( Violet)
• add( mylist)
• //end of init
• //end of ListExample

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List Class

• There are many methods in this class, we will
  look at a few
• of them
• - String getItem( int index)
• returns the item string at the given index
• - int getItemCount()
• returns the number of items in the list
• - int getSelectedIndex()
• returns the index of the current selection
• returns -1 if no item is selected or more than
  1 item is selected
• - int getSelectedIndexes()
• returns an array of index positions (for
  multiple selection lists)

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List Class
- String getSelectedItem() returns the
selected item as a string - String
getSelectedItems() returns an array of strings
containing all selected items - void select( int
index) selects the item at the specified index