Event Driven Programming and Graphical User Interface

1
Event Driven Programming and Graphical User
Interface

• B.Ramamurthy

2
Types of Programs

• Control-driven The order in the program code
  determines the sequence of events.
• The actions are predetermined.
• Event-driven The operation of the program
  depends on what you do with the controls
  presented (usually by the GUI)
• Selecting menu items, pressing buttons, dialog
  interaction cause actions within the program.

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Events

• 1. Actions such as clicking a button, moving a
  mouse, are recognized and identified by the
  operating systems(OS) or JVM.
• 2. For each action, OS/JVM determines which of
  the many currently running programs should
  receive the signal (of the action)
• 3. The signals that the application receives from
  the OS/JVM as result of the actions are called
  events.

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Event Generation
User Interface
Mouse actions, Keystrokes
Operating Systems
JAVA API /Windows
Events
Methods and handlers
Methods and handlers
Application 1
Application 2
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Event Handlers

• An application responds to the events by
  executing particular code meant for each type of
  event.
• Not all events need to be handled by an
  application. For example, a drawing application
  may be interested in handling only mouse
  movements.
• As a designer of an event-driven application you
  will write classes/methods to handle the relevant
  events.

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Event Handling Process

• Source of an event is modeled as an object. Ex
  button clicks object is a button
• Type of the event ActionEvent, WindowEvent,
  MouseEvent etc. Ex An ActionEvent object is
  passed to the application that contains
  information about the action.
• Target of an event Listener object of the event.
  Passing the event to a listener results in
  calling a particular method of the listener
  object.

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JDK1.2 Additions
JFC
java2D(3D)
AWT
DragDrop
Swing
Accessibility
(Speech, Sight API)
Collection classes
Bug fixes
JDK1.2 changes
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Swing Component

• Two distinguishing characteristics are
• 1. It supports pluggable Look and Feel. Besides
  run-anywhere, appearance of a component may be
  changed at run-time to look and feel like the
  system on which it is running.
• 2. Swing components are Lightweight components
  they do not dependent on any native system
  classes.

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Swing Components (controls)

• JApplet
• JButton
• JCheckBox
• JTable
• JTextPane
• JSlider
• JTree
• JPopupMenu

• JToolTip
• JScrollPane
• JDialog
• JMenuBar
• etc.
• Demo jdk1.2/demo/jfc/SwingSet/SwingSetApplet.java
  
• Note J prefix to distinguish it from regular
  AWT components.

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Steps in GUI Application Development

• Extend the application class from JFrame
• Declare all the components needed.
• Define the constructor.
• Instantiate a ContentPane and Define layout.
• Instantiate objects for the components and set
  their values.
• Add them to the ContentPane object.
• Add any listeners/handlers to specific components
  and define action to be performed in
  actionPerformed method.
• Add a main method that instantiate the object
  created above and takes care of Window closing.

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Event Handling

• Three ways
• Frame itself is a (mouse) listener, with an
  action performed method.
• Event handling delegated to an object specially
  instantiated for this purpose.
• Anonymous class/object to handle just one event.
• Note last two methods use inner class. We will
  examine the last two ways in the next example.

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Example
op1
op2
result
Add
Sub
This is a representation of the GUI for Lab2.
Lab2 illustrates all the concepts discussed
above.