Lecture Summary 5

Events and Listeners

• A event is an object that represents some occurance which we may want to handle.
  • Often generated by user actions: keyboard, mouse, etc.
  • Can also be generated by other programs (GUI components such as butons or sliders).
  • There are different types of events represented by different classes: MouseEvent, ActionEvent, etc.
  
  
• A listener is an object that waits for and responses to events.
  • To respond to an event, we must do two things:
    • create a listener object for the event of interest
    • add that listener to the graphical component that may generate the event
  • For example, to respond to a mouse event,
    • we must create a MouseListener object
    • to do that, we define a class that implements MouseListene interface
    • we add the object to the graphical component that may generate the mouse event
    Eg., an applet can generate mouse events, so we can call its addMouseListener method that takes a MouseListener as parameter, to add a MouseListener to the applet.
    • the listener object has methods that are automatically called when an event occurs
    • each method receives a MouseEvent object as parameter that represents the event.
    • we can then invoke a service of MouseEvent object. Eg., invoke getPoint in mousePressed method by using event.getPoint() to get the point where the mouse event occured.
  • To create a listener object we define a class that implement a listener interface.
  • Components that can generate certain events have methods that can be called to add a listener.
  • There are different types of listeners represented by different listener interfaces: MouseListener , ActionListener,etc.
  • A listener class should implement one of the listener interfaces.
  
  
• Interface MouseListener
• void mousePressed(MouseEvent event)
• void mouseReleased(MouseEvent event)
• void mouseClicked(MouseEvent event)
• void mouseEntered(MouseEvent event)
• void mouseExited(MouseEvent event)


• Class MouseEvent
  • Point getPoint()
  • int getX()
  • int getY()
  • int getClickCount()
• 
  Example: Dots.java,   DotsMouseListener.java   [Dots.html]
  Notes: (1) We can remove init() method and include all its statements in paint method. This wpuld ork just fine.
      (2) Try to modify the program (in a version similar to RubberLines.java) to have only one class that extends Applet and implements MouseListener.
  Example: DotsAppletModified.java,   DotsAppletModified.html
  
  
  
• Interface MouseMotionListener
  • void mouseMoved(MouseEvent event)
  • void mouseDragged(MouseEvent event)
• Example: RubberLines.java [RubberLines.html]
  
  
• Interface KeyListener
  • void keyPressed(KeyEvent event)
  • void keyReleased(KeyEvent event)
  • void keyTyped(KeyEvent event)
  
  
  
• Class KeyEvent
  • int getKeyCode()
  • constants for all keys
• Example: Direction.java [Direction.html]

Animation

• An animation is a constantly changing series of pictures or images that create the illusion of movement
• We can create animations in Java by changing a picture slightly over time
• The speed of a Java animation is usually controlled by a Timer object
• Timer class
  • Timer(int delay, ActionListener listener)
  • void addActionListener(ActionListener listener)
  • boolean isRunning()
  • void start()
  • void stop()
• Example: Rebound.java [Rebound.html]
  • This applet requires Java 2. Some browsers does not support Java 2. You can run this applet using appletviewer.