Transcript Introduction to Java 2 Programming

Introduction to Java 2
Programming
Lecture 9
Java Swing API, Part 2
Overview
• Event Handling Basics
• Swing Events Examples
– Mouse Events
– Actions
– Window Events
• Developing the Calculator
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
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
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
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 subclass of java.awt.Component
– The Component class provides the add/remove
methods
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
Event Handling Conventions
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
Mouse Events
Mouse Events
• So, capturing basic mouse events involves:
– Creating an implementation of
MouseListener
– Calling addMouseListener on one or more
Components to register it
• Example code…
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
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
Other Swing Events
• More Mouse 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.
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
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!
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
The Calculator Logic
• The Calculator object can be very simple
– Adapt our earlier implementation…
– Use double for extra precision