AWT

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AWT

• Responding to Events

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Terminology

• Delegation Event Model A source generates an
  event and notified the listeners that registered
  and requested to be notified when the event
  occurs. You only receive events you want to
  process.
• Event A user interaction with the system. An
  Event object described a state change in the
  Source object that generated of the event.
• Listener An object that wants to be notified
  with a certain event occurs. A Listener must
  register with a Source to receive notification of
  an event.
• Source An object that generates an event.

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AWTEvent class
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Event Handling

• Every time the user types a character or pushes a
  mouse button, an event occurs.
• Any object can be notified of the event. All it
  has to do is implement the appropriate interface
  and be registered as an event listener on the
  appropriate event source.
• Swing components can generate many kinds of
  events. You will see a few examples on the next
  few slides.

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Event Driven Programs
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Event Handling Process
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Event Handling Process

• To detect when the user clicks an on-screen
  button (or does the keyboard equivalent), a
  program must have an object that implements the
  ActionListener interface.
• The program must register this object as an
  action listener on the button (the event source),
  using the addActionListener method.
• When the user clicks the on-screen button, the
  button fires an action event. This results in the
  invocation of the action listener's
  actionPerformed method (the only method in the
  ActionListener interface).
• The single argument to the method is an
  ActionEvent object that gives information about
  the event and its source.

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Event Handling Process
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Event Handling
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Event Handling

• Each event is represented by an object that gives
  information about the event and identifies the
  event source.
• Event sources are typically components, but other
  kinds of objects can also be event sources.
• As the following figure shows, each event source
  can have multiple listeners registered on it.
  Conversely, a single listener can register with
  multiple event sources.
• From Suns Java Tutorial

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Event Source Event Generated
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Low-Level Event Classes

• There are four kinds of low-level events that you
  can elect to handle in your programs.
• They are represented by the following classes in
  the java.awt.event package

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Low-Level Event Classes
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Looks Like Times Up!

• What so soon?