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 sub-class 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