CS 162 Spring 2005

1
CS 162 - Spring 2005

• Chapter 10 (12 in 2nd Edition)
• Event Handlers

2
Events

• Events are things like a mouse press, a timer, a
  window changing characteristics
• Tracked by the Java run-time system
• When detected, the program must be notified

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How to Handle Events

• Events have properties similar to those we
  discussed in Chapter 9
• It is easy for the run-time system to tell when
  an event has occurred, but each application
  responds differently.
• Solution we introduced in Chapter 9, interfaces
  and polymorphism

4
Problem of Binding Times Again

• The windowing system was written by a programmer
  at Sun a long long time ago. No idea what a mouse
  press means to you
• You write a new application today. You give
  meaning to mouse press, etc.
• How does the windowing system, which does not
  know your application, make it easy for you to
  provide meaning ?

5
Polymorphism

• Reminder meaning of Polymorphism. Poly many,
  Morph form, many forms.
• A variable declared as one type (an interface
  type) but is in reality a different type (an
  application type).
• For events, combined with the idea of Listeners

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An Example, Mouse Listener

• Here is the interface class MouseListener
• public interface MouseListener
• void mousePressed(MouseEvent event)
• void mouseReleased(MouseEvent event)
• void mouseClicked(MouseEvent event)
• void mouseEntered(MouseEvent event)
• void mouseExited(MouseEvent event)

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Event Listener

• User creates a class that implements
  MouseListener
• The user hands this object to the run-time system
• The runtime system "thinks" it is dealing with a
  MouseListener, in reality it is dealing with your
  new application class

8
Solving Binding Time Problem

• The Windowing library knows that it is dealing
  with MouseListener. Works for any MouseListener.
  Written once long ago.
• You create your new application today, give new
  meaning to MouseListener, but old Windowing
  system continues to work.

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Creating the Application Listener

• Public class MouseSpy implements MouseListener
• public void mousePressed(MouseEvent event)
• System.out.println("Mouse Pressed at "
  event.getX() " " event.getY())

10
Attaching the Listener

• Public class MouseApplet extends Applet
• public MouseApplet()
• MouseListener listener new MouseSpy()
• addMouseListener(listener)

11
Inner classes and Listeners

• Often the listener is an inner class, so it can
  modify the surrounding application variables.
• The MouseListener interface has many methods,
  often only interested in one or two, so there is
  an adapter named MouseAdapter.

12
MouseAdapter

• Class MouseAdapter implements MouseListener
• public void mousePressed(MouseEvent event)
• public void mouseReleased(MouseEvent event)
  
• public void mouseClicked(MouseEvent event)
• public void mouseEntered(MouseEvent event)
  
• public void mouseExited(MouseEvent event)
• // notice each method has an empty implementation

13
Using MouseAdapter

• You use inheritance to derive from MouseAdapter,
  and modify only what you want
• class MouseSpy extends MouseAdapter
• public void mouseClicked(MouseEvent event)

14
Graphical Applications

• Normally you want to catch the event, do
  something, then update the graphical display
• To illustrate this we need first a quick
  introduction to the Java graphical model
• We will use applications, not applets

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The Java Graphical model

• The Java (2E) Graphical model is something like
  the following
• You create a frame (a window with title and
  menus) which surrounds a Panel (a drawing
  surface).
• The run-time system tells your panel to display
  by calling paintComponent.

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Creating a Window

• public class Life
• public static void main(String args)
• JFrame frame new JFrame()
• frame.setTitle(Game of Life)
• frame.setSize(400, 300)
• frame.setDefaultCloseOperation(3) //
  EXIT_ON_CLOSE
• JPanel board new MyPanel()
• frame.setContentPane(board)
• frame.show()

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MyPanel - Polymorphism again

• class MyPanel extends JPanel
• public void paintComponent(Graphics g)
• super.paintComponent(g)

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Combining with Mouse Events

• Class MyPanel extends Jpanel
• private int rx 0 private int ry 0
• public void paintComponent(Graphics g)
• super.paintComponent(g)
• g.drawString("hello world", rx, ry)

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Paint and mouse, cont.

• Class MyPanel extends Jpanel
• private int rx 0 private int ry 0
• class MouseSpy extends MouseAdapter
• public void mousePressed(MouseEvent event)
  
• rx event.getX()
• ry event.getY()
• repaint() // VERY IMPORTANT

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Repainting vs painting

• Calling repaint tells the windowing system you
  want to repaint the window.
• At some later point, the windowing system calls
  paint, then paintComponent.
• You never call paint or paintComponent directly.

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Connecting the two

• Class MyPanel extends Jpanel
• public MyPanel()
• addMouseListener (new MouseSpy())

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Sequence of Events

• Creation of Panel causes creation of listener
• Listener is added to set of listeners for the
  Panel
• When mouse goes down, Listener is invoked, sets
  the x and y locations, calls repaint
• Repaint tells run-time system to invoke paint,
  which calls paintComponent
• Invokes Panel.paintComponent, and window is
  painted.

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Event-Driven Programming

• This style of programming is called event-driven
  programming
• Puts the user in charge, the overall flow of
  control is directed by events, not by the program
  logic.
• Hallmark of modern style graphical applications.

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Event Driven vs non Event Driven Execution

• Old style - program controls sequence of events,
  from time to time asks user for input or displays
  output
• New Event Driven Style - program sits and waits
  for events. User controls flow of execution. When
  an event is detected, program reacts

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Questions?

• You will be doing GUI style programming in
  programming assignment 2.
• If no other questions, then we will have the Anti
  Quiz.