Introduction to Java 2 Programming

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 • Eventobject 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: • 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 • 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