1. Graphical User Interfaces
User interaction problem:
User can spontaneously change the value (state) of a user interface object x (e.g., a check box or scrollbar), often called the subject
You want a set of observer objects o1, o2, ... — not necessarily GUI objects — to “know about” every change in x’s value
Other (non-GUI) situations are similar
Solution: ???

2. Solution #1: Use Pollingx o1 o1
Each observer continually (periodically, occasionally) polls the subject to ask if its state has changed
Inefficient, cumbersome...

3. Solution #2: Use Events x x
event o1
Each observer goes about its own business, until the subject tells it that its value has changed

4. Implementing Solution #2: The Observer Pattern
Subject expects each observer to register with the subject to indicate its interest in subject’s events
Subject keeps track of this (dynamic) set of interested observers
When its state changes, subject calls a specific callback method of each registered observer, passing as parameter an event object that describes the state change

5. Example: Simple GUI Demo
interface that any observer
of a subject that is a JButton
object must implement
actionPerformed is
the callback method
<<interface>>
ActionListener
+ actionPerformed
(ActionEvent e)
a DemoFrame object in
GUIDemo_1 is the observer
(it implements ActionListener)
a frame is a window
on the screen that acts as
a container for GUI objects
(aka GUI components)
a JButton object in
DemoFrame is the subject
(its events are of interest)
JFrame
GUIDemo_1
DemoFrame …
+ actionPerformed
(ActionEvent e)
DemoFrame
JButton
holds a reference to
a DemoFrame object

6. A Little More Detail call this method from main to
make sure program can be stopped!
JFrame …
+ setDefaultCloseAction
(int operation)
+ show ( )
+ getContentPane ( ) :
Container
+ pack ( )
<<interface>>
ActionListener
+ actionPerformed
(ActionEvent e)
call this method from main to display
the frame (window) on the screen
call this method from DemoFrame to
get the content pane that contains
the frame’s GUI components
call this method from DemoFrame to organize
the GUI components in the frame’s content
pane using the layout manager you’ve selected
call this method from DemoFrame
to add an observer for any
subject that is a JButton object
DemoFrame …
+ actionPerformed
(ActionEvent e)
JButton …
+ addActionListener
(ActionListener o)

7. This Specific Case of the Observer Pattern
A JButton object (the subject) expects each observer to call addActionListener to indicate interest in that JButton’s events
Each JButton object keeps track of its own set of ActionListener objects
When its state changes, a JButton object calls actionPerformed on each registered ActionListener object, passing as a parameter an ActionEvent object

8. Java GUI Fundamentals JFrame JTextArea JPanel JButton
This is a simple sample application that uses most (if not all) the fatures you’ll need for the calculator application
JButton

9. Java GUI Packages java.awt java.awt.event javax.swing ...
These are the main packages for GUI stuff
There are others, but these should be the primary places where you should look in the documentation when you need information

10. Basic Java GUI Widgets (Swing)
All in package javax.swing:
JFrame
JPanel
JButton
JTextArea
JScrollPane
...
Use widgets starting with J from the swing library
They are more portable, nicer looking, and more powerful than AWT widgets

11. Layout Managers
A layout manager allows you to arrange components inside panels without providing specific coordinates of widgets
In packages java.awt, javax.swing:
FlowLayout (default for JPanel)
BorderLayout (default for JFrame content pane)
GridLayout
...
Briefly describe each of those listed drawing pictures to explain the difference in how they arrange components