The document discusses GUI event handling in Java. It explains that window-based Java programs are event-driven, meaning they wait for and respond to user-initiated events like button clicks or key presses. When an event occurs, an event object is passed to a listener object that handles the event. Listeners implement interfaces that correspond to different event types, like ActionListener for button clicks. The delegation event model in Java handles event passing from components to listeners.

1. » GUI
˃ Events
˃ Event Handling
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JAVA
GUI
PART III
Milad Kawesh

2. Agenda
» Events
» Java Delegation Model
» Event Handling
» Practical Examples
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3. Windows based Java Programs
» Console-Based Programming
˃ Every thing is predetermined
˃ The program code determines the sequence of events
» Window-Based Programming
˃ The operation is driven by what you do with the GUI
˃ Selecting menu items, buttons, or keyboard causes particular
actions within a program
˃ The specific program that is executed next is not known
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4. Event driven Programming
» The signals that a program receives from the OS as a
result of your actions are called events
» A window based program is called event driven
program
» Unlike the old rigid old sequential programs,
» it puts user in charge, user control the sequence of
program
» Application waits for the user action
» This approach is called event driven programming
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5. The Event Handling Process
» Suppose a user clicks a button in the GUI
˃ Button is the source of the event
» When a button is clicked, it will create a new
object that have information about event and
its source (in this case ActionListener)
» This object is passed to a method that handles
the event in its listener
» A listener is called Target of an event
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6. Delegation Event Model
» The way in which events are handled in Java, using listener
objects, is called delegation event model
» We can make objects of any class listener objects by making
the class implement a listener interface
» In case of the button, the ActionListener interface needs to
be implemented to receive events from button
» actionPerformed(ActionEvent e) is called when the event
occurs and the event object is passed as an argument
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7. Java Events
» Events are objects
˃ Objects that represent user initiated actions
˃ Examples:
+ button clicked -> ActionEvent
+ mouse dragged -> MouseEvent
+ Enter key pressed -> KeyEvent
˃ EventObject; root event class for all event objects
˃ AWTEvent; root event class for all AWT events
˃ Package java.awt.event
+ Provides interfaces and classes for dealing with different
types of events fired by AWT components.
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9. Event Handling
» Programmer choice to decide how to handle the
generated event
˃ Ignore the Event
˃ Have the Event handled by the component where the
event was generated (Self Contained Event handling)
˃ Delegate event handling to some other object called
Listeners (Event Delegation)
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10. Event Delegation
» Some time component on which event was
generated is not best suited to handle its own
event
» The process of assigning an object to handle a
component’s events is called delegation.
» The event handling objects are called Listeners
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11. Key Methods of Event
» Object getSource()
In ObjectEvent, return the component in
which event took place.
» int getID()
In AwtEvent, return int that describes the
nature of the event e.g. on MouseEvent it will
give MOUSE_PRESSED, MOUSE_DRAGGED.
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12. Event Listeners
» Interfaces to support dispatching of events
» Each Event class has a corresponding Listener interface
» Multiple listeners for the same event type
» Each interface will have one or more method
corresponding to types of events
» ActionEvent -> ActionListener
» MouseEvent -> MouseListener and MouseMotionListener
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13. Registering Listeners
» Listeners register themselves with component
˃ public void addXXXListener(XXXListener)
˃ addActionListener, addItemListener, etc.
» Multiple listeners can be registered for the same
event on a component
˃ One event can trigger numerous responses
˃ Events are broadcast to all listeners
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14. Wiring a Listener
» Define a class to implement the Listener Interface
Public class Applet extends Applet implements ActionListener
public class MyClass implements ActionListener {
» Add the implementation of the Interface
…
public void actionPerformed(ActionEvent e) {
// here’s where I do stuff when the action happens
…
» Add class as a Listener to Component
…
Button ok = new Button(“OK”)
ok.addActionListener(this);
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15. ActionListener Example
import
import
import
import
import
javax.swing.*;
java.awt.FlowLayout;
java.awt.Dimension;
java.awt.event.ActionListener;
java.awt.event.ActionEvent;
public class ActionListenerTest extends JFrame implements
ActionListener {
JTextArea topTextArea;
JTextArea bottomTextArea;
JButton button1, button2;
final static String newline = "n";
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16. ActionListener Example (cont.)
public ActionListenerTest() {
setLayout(new FlowLayout());
topTextArea = new JTextArea();
topTextArea.setEditable(false);
JScrollPane topScrollPane = new JScrollPane(topTextArea);
topScrollPane.setPreferredSize(new Dimension(200, 75));
add(topScrollPane);
bottomTextArea = new JTextArea();
bottomTextArea.setEditable(false);
JScrollPane bottomScrollPane = new JScrollPane(bottomTextArea);
bottomScrollPane.setPreferredSize(new Dimension(200, 75));
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17. ActionListener Example (cont.)
add(bottomScrollPane);
button1 = new JButton("top Text area");
add(button1);
button2 = new JButton("down text area");
add(button2);
button1.addActionListener(this);
button2.addActionListener(this);
setSize(300, 222);
setResizable(false);
setLocationRelativeTo(null);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
}
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18. ActionListener Example(cont.)
public void actionPerformed(ActionEvent e) {
if (e.getSource() == button1) {
topTextArea.append(e.getActionCommand() + newline);
}
if (e.getSource() == button2) {
bottomTextArea.append(e.getActionCommand() + newline);
}
}
public static void main(String[] args) {
new ActionListenerTest();
}
}
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19. OUTPUT
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20. Lets practice
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21. END
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