Cup 6: AWT Event Handling

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Cup 6 AWT Event Handling

• Special Topics Java

Dr. Tim Margush Department of Mathematics and
Computer Science The University of Akron
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Event Handling Process

• Operating System monitors keystrokes, mouse
  actions, and other events
• events are sent to the program that owns the
  window in which the event occurred
• Events in Java programs cause calls to the AWT
  library routines which create Event objects
• These objects are then passed to methods in your
  program to handle the events

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Listeners

• Objects that must respond to events generally
  register one or more listeners
• An add_X_Listener method does this
• Components can handle their own events

• A Listener is simply a class that implements
  certain methods
• The methods are defined in an interface
• Listeners extend java.util.EventListener

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Event Classes

• Low-Level Events
• keyboard, mouse, window actions
• Semantic Events
• component-related (button click, scrollbar
  adjustment, menu operation)
• Some system events cause multiple Java events
• click button - causes both types of events

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Low-Level Event Classes

• FocusEvent
• occurs when a component gets or loses the focus
• MouseEvent
• movement of the mouse or pressing mouse button

• KeyEvent
• keyboard
• WindowEvent
• activating, deactivating, closing, iconifying,
  etc.
• related to Window objects (and subclasses)

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Event Hierarchy

• java.util.EventObject
• java.awt.AWTEvent
• java.awt.event.ComponentEvent
• All events are subclassed from ComponentEvent and
  are located in the java.awt.event package
• All events inherit getSource() from the top level

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Low-Level Listeners

• All listeners are in java.awt.event
• There are Listener classes for each event type
• WindowListener
• MouseListener
• MouseMotionListener
• KeyListener
• FocusListener

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Adapter Classes

• A class that implements all the methods of a
  Listener interface with empty bodies
• By subclassing, you need only override methods
  you need to implement

• There are adapter classes for all of the
  low-level listener interfaces
• FocusAdapter
• MouseAdapter
• WindowAdapter
• MouseMotionAdapter
• KeyAdapter

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Semantic Event Classes

• ActionEvent
• Button, List, MenuItem
• TextEvent
• TextArea, TextField
• AdjustmentEvent
• Scrollbar

• ItemEvent
• Checkbox
• CheckboxMenuItem
• Choice
• List

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Semantic Listeners

• Each semantic event has a corresponding Listener
  interface
• ActionListener
• ItemListener
• TextListener
• AdjustmentListener
• Each of these listener interfaces has only one
  method to be implemented