Basic Graphical User Interface Components

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Basic Graphical User Interface Components
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Component Descriptions

• Label - uneditable text is displayed
• Button - triggers an event when clicked
• TextField - user keys in text displays text
• Choice - drop-down list click selects item
• Checkbox - boolean selected true
• List - item list click selects item double
  click generates an action event
• Panel - container (analog of parentheses)

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Part of AWT class hierarchy
Object
CheckboxGroup
TextComponent
TextField
Component
Button
FlowLayout
Label
Object is in java.lang package
BorderLayout
Checkbox
GridLayout
List
Choice
Container
Panel
Applet
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Label Class Some Methods

• public Label()
• Constructs empty Label
• public Label(String s)
• Constructs Label displaying String s
• public Label(String s, int align)
• Alignment values
• Label.LEFT, Label.CENTER, Label.RIGHT

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Label Class Some Methods ...

• public String getText()
• Returns Label text
• public void setText(String s)
• Sets Label text
• public void setAlignment(int align)
• Sets Label alignment
• Label.LEFT, Label.CENTER, Label.RIGHT

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Label Class Example

• See Label entry on Lectures page

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Part of AWT class hierarchy
Object
CheckboxGroup
TextComponent
TextField
Component
Button
FlowLayout
Label
BorderLayout
Checkbox
GridLayout
List
Choice
Container
Panel
Applet
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Button Class Events

• GUIs are event-driven
• Activated when an event occurs
• Events are initiated by the user
• Press enter, a mouse button, or a key
• To process an event, you must
• register (aka delegate) an event listener
• implement that event listener interface

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Part of EventListener interface hierarchy
EventListener is in java.util package
EventListerner
Object
ActionListener
WindowListener
AdjustmentListener
TextListener
ComponentListener
MouseMotionListener
ContainerListener
MouseListener
FocusListener
KeyListener
ItemListener
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Button Class Example

• See Button entry on Lectures page
• We defined the Button event handler as a separate
  class.
• The event handler may be reused elsewhere
• getActionCommand is a method of the ActionEvent
  class.

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Part of AWT class hierarchy
Object
CheckboxGroup
TextComponent
TextField
Component
Button
FlowLayout
Label
BorderLayout
Checkbox
GridLayout
List
Choice
Container
Panel
Applet
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TextField Class Methods

• public TextField()
• Constructs a TextField object
• public TextField( int c )
• Constructs a TextField with c columns
• public TextField( String s )
• Constructs a TextField displaying s.
• public TextField( String s, int c )
• Establishes both

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TextField Class Methods ...

• public void setEchoChar( char c )
• Set display character when user enters text.
• public void setEditable( boolean b )
• Set whether or not TextField can be modified
• true yes false no.
• public String getText()
• Return contents of TextField
• public synchronized void setText( String s )
• Set contents of TextField
• There may be many ActionListeners, their set
  actions must be synchronized

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TextField Class Example

• See TextField entry on Lectures page

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Part of AWT class hierarchy
Object
CheckboxGroup
TextComponent
TextField
Component
Button
FlowLayout
Label
BorderLayout
Checkbox
GridLayout
List
Choice
Container
Panel
Applet
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Choice Class Methods

• A Choice button is a drop-down list.
• The Choice class implements the ItemSelectable
  interface
• Methods include
• public Choice()
• Constructs a Choice drop-down list (empty)
• public String getItem( int index )
• Returns the item at index

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Choice Class Methods ...

• public synchronized void add( String s )
• Adds an item to drop-down list.
• public synchronized String getSelectedItem()
• Returns the selected item.
• public int getSelectedIndex()
• Returns the index of the selected item.
• public synchronized String insert(String s, int
  i)
• Inserts a new item s at index i
• public synchronized void remove( String s )
• Removes the 1st occurrence of s from the list.

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Choice Class Example

• See Choice entry on Lectures page