1. CSE 470 - Software Engineering Fall 1999 Updated by J. Brown
Events in Java
CSE Software Engineering
Fall 1999
Updated by J. Brown
What is the largest Integer that can be represented?
What if we want to represent a larger number?
What if we want to represent a fraction?
Use Scientific Notation, and Significant digits.
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2. Example 1-- Simple Action Event
import java.awt.*;
Import java.awt.event.*;
import java.applet.*;
public class SimpleEvent extends Applet
implements ActionListener {
Button button1 = new Button("Press me");
public void init() {
this.add(button1);
button1.addActionListener(this); }
public void actionPerformed(ActionEvent e) {
if(e.getSource() == button1) {
button1.setLabel("Again");
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3. Handling Events Events are first-class objects.
There is a class Event.
Subclasses identify different kinds of events.
Components fire events.
Other objects can listen for/act upon these events.
Interested objects register themselves as a Listener with the component that fires the event.
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4. Event Routing Lots of different kinds of events
Different events carry different types of information
E.g., ActionEvent carries a command, TextEvent carries a string being edited.
Components fire these events.
E.g., Button fires an ActionEvent.
E.g., TextArea fires a TextEvent.
All components fire WindowEvents.
When fired, events are routed to special Listener objects.
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5. The AWTEvent Hierarchy
ActionEvent
ComponentEvent
TextEvent
InputEvent
WindowEvent
MouseEvent
KeyEvent
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6. Listeners
A listener is an object to which AWTEvents can be routed by components.
Different types of listeners.
One for each sub-class of AWTEvent.
E.g., ActionListener, ComponentListener, WindowListener.
A listener class is abstract:
Operations only; no attributes or methods.
Declared as an interface in Java.
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7. Example: An ActionEvent Listener
import java.awt.*; import java.applet.*;
import java.awt.event.*;
class ButtonListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.out.println(“Button pressed!!”); }
public class ActionExample extends Applet {
public void init() {
Button button = new Button(“Push me”);
button.addActionListener(new ButtonListener());
button.addMouseListener(new ButtonMouseListener());
add(button);
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8. Example: A MouseEvent Listener
class ButtonMouseListener implements MouseListener {
public void mouseEntered(MouseEvent e) {
System.out.println(“Mouse entered button!”); };
public void mouseExited(MouseEvent e) {
System.out.println(“Mouse exited button!”);
public void mousePressed(MouseEvent e) {}
public void mouseClicked(MouseEvent e) {}
public void mouseReleased(MouseEvent e) {} }
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9. Mouse Events mouseClicked(MouseEvent evt);
Called when the mouse button is clicked
mousePressed(MouseEvent evt);
Called when the mouse button is depressed
mouseReleased(MouseEvent evt);
Called when the mouse button is released
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10. More Mouse Events mouseEntered(MouseEvent evt);
Called when the mouse enters the component
mouseExited(MouseEvent evt);
Called when the mouse leaves the component
mouseMoved(MouseEvent evt);
Called when the mouse moves with no mouse button depressed
mouseDragged(MouseEvent evt);
Called when the mouse moves with the mouse button depressed
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11. References
G. Cornell and C. Horstmann, “Core Java”, The Sunsoft Press Java Series, Second Edition, 1997
D. Joshi, L. Lemay, and C. Perkins, “Teach yourself Java in Café in 21 days”, Sams.net publishing, 1996
The Java Tutorial
D. Geary and A. McClellan, “Graphic Java”, The Sunsoft Press Java Series, 1997
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