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Events in Java. CSE 470 - Software Engineering Fall 1999 Updated by J. Brown. Example 1-- Simple Action Event. import java.awt.*; Import java.awt.event.*; import java.applet.*; public class SimpleEvent extends Applet implements ActionListener {

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events in java

Events in Java

CSE 470 - Software Engineering

Fall 1999

Updated by J. Brown

example 1 simple action event
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");

}

}

}

handling events
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.
event routing
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.
the awtevent hierarchy

AWTEvent

ActionEvent

ComponentEvent

TextEvent

InputEvent

WindowEvent

MouseEvent

KeyEvent

The AWTEvent Hierarchy
listeners
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.
example an actionevent listener
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);

}

}

example a mouseevent listener
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) {}

}

mouse events
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
more mouse events
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
references
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
    • http://java.sun.com/docs/books/tutorial/index.htm
  • D. Geary and A. McClellan, “Graphic Java”, The Sunsoft Press Java Series, 1997
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