Event Handling 2

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Event Handling 2
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General Idea

• You can register just about any component to
  respond to events of some kind
• May want to review previous slides before
  continuing
• Common components
• Buttons
• Applets
• TextFields and TextAreas
• Menus, ChoiceLists, CheckBoxes, etc...

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The Event Classes

• Mother class EventObject - defined in java.util
  package
• Below that is AWTEvent - defined in java.awt
  package
• Below that are the classes youve seen before
• MouseEvent, ActionEvent, TextEvent, KeyEvent,
  WindowEvent, etc...

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Things in EventObjectThat you inherit

• getSource( ) gets the object that originated the
  event (such as a button or textfield)
• getID( ) returns the ID of an event such as
  MOUSE_CLICKED.
• Not used as often, because we rely on methods to
  determine the event
• Was more popular in 1.0 event model

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List of Event Types

• ActionEvent occurs by activation of components
  (like a button, textfield or menu)
• AdjustmentEvent works with scrollbars
• ComponentEvent high-level event thats rarely
  used
• ContainerEvent when components are added to a
  container
• InputEvent high-level event from input

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List of Event Types

• ItemEvent event when an item is selected
• KeyEvent event from the keyboard
• MouseEvent event from the mouse
• PaintEvent event when components are painted
  (rarely used)
• TextEvent when textfields are changed
• WindowEvent from window activity

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How to Listen for Events

• Has this format
• ltcomponentgt.addXListener (ltlistenergt)

This is the thing thats responsible for handling
the event. It must implement the interface. If
we added a mouseListener to a componenent, then
the listener must implement mouseListener
This is where you add a particular listener,
where X is a kind of listener. Example mouseListe
ner
This is the component thats interested
in reacting to events. Example a
button, textfield, menus, applets, frames, etc
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More Abstraction

• We can create separate classes as listener
• class MouseHandler implements MouseMotionListener
• public void mouseMoved (MouseEvent e)
• System.out.println (Mouse was moved)
• public void mouseDragged (MouseEvent e)
• System.out.println (Mouse was dragged)

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Now that we have a MouseHandler...

• Applet no longer has to implement the
  MouseMotionListener interface
• We can pass a new MouseHandler as the one
  responsible for handling MouseEvents
• Example
• MouseHandler x new MouseHandler( )
• this.addMouseMotionListener (x)

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The Listener Interfaces

• Important Ones
• ActionListener, AdjustmentListener, ItemListener,
  KeyListener, MouseListener, MouseMotionListener,
  TextListener, WindowListener

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AWTEvent Subclasses

• Important Ones ActionEvent, AdjustmentEvent,
  ItemEvent, KeyEvent, MouseEvent, TextEvent,
  WindowEvent

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Adapter Classes

• Its a pain to have to override methods when
  youre only concerned with one thing
• MouseListener 5 methods
• WindowListener 7 methods
• Adapter classes are classes that have overridden
  the methods already
• Can extend and Adapter class that has already
  implemented this interface!
• Is there an ActionAdapter?

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Guts of an Adapterby Example

• class MouseAdapter implements MouseListener
• public void mousePressed (MouseEvent e)
• public void mouseReleased(MouseEvent e)
• public void mouseClicked (MouseEvent e)
• public void mouseEntered(MouseEvent e)
• public void mouseExited (MouseEvent e)
• //end class

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More about Adapters

• class MyMouseHandler extends MouseAdapter
• public void mouseEntered (MouseEvent e)
• System.out.println (Mouse entered
  the component)
• //end class

Note now you can use MyMouseHandler to listen to
events. It has inherited 5 methods and overridden
the one that it wanted!
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Adapter Example Revisited

• class MyFrame extends Frame
• MyFrame(String title)
• super (title)
• init( )
• public void init( )
• this.addWindowListener (new WindowAdapter( )
• public void windowClosing (WindowEvent e)
• System.exit (1)
• )

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