1. Introduction to Java 2 Programming
Lecture 9
Java Swing API, Part 2

2. Overview Event Handling Basics Swing Events Examples
Mouse Events
Actions
Window Events
Developing the Calculator

3. Event Handling Basics Event handling always has the following pattern
Involves 3 distinct roles
Source object which is object of interest
Generates events under certain conditions
In Swing these are user activities
Event object that describes what happened
Encapsulates the event context
Who did what, on which object, etc
Listeners which receive events
Receiving an event is a method call on the listener
Different categories of event listeners

4. Event Handling Basics Listeners are registered with source objects
Ties them together
Informs the source object about who to deliver events to
This is a many-many relationship
A source object may have multiple listeners
A listener may listen to multiple source objects

5. Event Handling in Robocode
Think back to the Robocode examples…
The Arena was the source of interesting events
Has a bullet hit, have I seen another robot?
There were different event objects
BulletHitEvent, RobotScannedEvent
The Robot could listen for events by implementing specific methods
onBulletHit, onRobotScanned
Robot base class handled registration for you
And provided “do nothing” versions of the above methods

6. Event Handling Conventions
Listeners are described using interfaces
Naming convention: XXXListener
Where XXX denotes the category of listener E.g. WindowListener
Creating a listener means implementing that interface
Register a listener with the addXXXListener method on the source object
Generally a removeXXXListener also
Source object will be a Swing component, i.e. a sub-class of java.awt.Component
The Component class provides the add/remove methods

7. Event Handling Conventions
Event object typically extend java.awt.Event
Some of the ‘newer’ ones don’t
Events share some common attributes
a timestamp, source object, etc

8. Event Handling Conventions

9. Mouse Events Natural for Swing to expose mouse-related events
It’s how the user interacts with the GUI
MouseListener interface describes the basic events
Each method accepts a MouseEvent object parameter
java.awt.Component has add/remove listener methods

10. Mouse Events

11. Mouse Events So, capturing basic mouse events involves: Example code…
Creating an implementation of MouseListener
Calling addMouseListener on one or more Components to register it
Example code…

12. Action Events
Very tedious implementation if all activities were dealt with as individual clicks
Swing provides higher level ‘action’ event
Meaning of event depends on component
E.g. button click, menu selection, etc
Basic classes:
ActionEvent – identifies action, key modifiers, etc
ActionListener – single actionPerformed method
addActionListener, removeActionListener methods on Component

13. Window Events Swing allows the capturing of window related events
Activate, deactivate, minimise, open, close etc
setDefaultCloseOperation is only useful if you don’t want to do anything complex
Basic classes/methods
WindowEvent – identifies Window
WindowListener – range of methods
addWindowListener, removeWindowListener methods on JFrame, JWindow, JDialog

14. Other Swing Events More Mouse events Item events Key Events
Mouse motion, mouse wheels
Item events
Selecting/deselecting items in lists, checkboxes, etc
Key Events
Raw keyboard input
Tree Events
Opening/closing nodes in a tree component
Drag and drop
…and many more. See javax.swing.event and java.awt.event packages.

15. Developing the Calculator
To add functionality to the Calculator GUI we need to:
Implement the ActionListener interface to respond to button clicks
Can do this on the ButtonPanel class
Associate this with each button on the calculator

16. The Calculator Logic Check which button has been clicked
Use ActionEvent.getObject to get the button
Then ask the button for its action command, getActionCommand
Clicked a number button?
Then update the JTextField to add digits
Use getText and setText methods
Clicked the point button?
Then add a decimal point, but only once!

17. The Calculator Logic Clicked an operator (+, -, etc)?
Then store current value of JTextField (first number)
And, remember which operator
And, reset the field (for the second number)
Finally set a flag to indicate we’re in a calculation
Clicked the equals button?
Then take the first number and the current value of the JTextField, and ask a Calculator object to do the math
Display the result
And reset the calculation flag

18. The Calculator Logic The Calculator object can be very simple
Adapt our earlier implementation…
Use double for extra precision