Java gui event

11
Events and Applet
JAVA / Adv. OOP

22
Contents
 What is an Event?
 Events and Delegation Event Model
 Event Class, and Listener Interfaces
 Discussion on Adapter Classes
 What, Why, Purpose and benefit
 GUI Discussion

33
Events & Event Handling
 Components (AWT and Swing) generate events in response
to user actions
 Each time a user interacts with a component an event is
generated, e.g.:
 A button is pressed
 A menu item is selected
 A window is resized
 A key is pressed
 An event is an object that describes some state change in a
source
 An event informs the program about the action that must be
performed.
 The program must provide event handlers to catch and
process events. Unprocessed events are passed up
through the event hierarchy and handled by a default (do
nothing) handler

16-16-44
Listener
 Events are captured and processed by “listeners” —
objects equipped to handle a particular type of events.
 Different types of events are processed by different
types of listeners.
 Different types of “listeners” are described as interfaces:
ActionListener
ChangeListener
ItemListener
etc.

16-16-66
Listeners (cont’d)
 Event objects have useful methods. For example,
getSource returns the object that produced this event.
 A MouseEvent has methods getX, getY.
 When implementing an event listener, programmers
often use a private inner class that has access to all the
fields of the surrounding public class.
 An inner class can have constructors and private fields,
like any other class.
 A private inner class is accessible only in its outer class.

88
The Delegation Event Model
 The model provides a standard
mechanism for a source to
generate an event and send it to
a set of listeners
 A source generates events.
 Three responsibilities of a
source:
 To provide methods that allow
listeners to register and unregister
for notifications about a specific type
of event
 To generate the event
 To send the event to all registered
listeners.
Source
Listener
Listener
Listener
events
container

99
The Delegation Event Model
 A listener receives event notifications.
 Three responsibilities of a listener:
 To register to receive notifications about specific events by
calling appropriate registration method of the source.
 To implement an interface to receive events of that type.
 To unregister if it no longer wants to receive those notifications.
 The delegation event model:
 A source multicasts an event to a set of listeners.
 The listeners implement an interface to receive notifications
about that type of event.
 In effect, the source delegates the processing of the event to
one or more listeners.

1010
Event Classes Hierarchy
Object
EventObject
AWTEvent
ActionEvent ComponentEvent ItemEvent TextEvent
FocusEvent InputEvent WindowEvent
KeyEvent MouseEvent

1212
AWT Event Classes and Listener
Interfaces
Event Class Listener Interface
ActionEvent ActionListener
AdjustmentEvent AdjustmentListener
ComponentEvent ComponentListener
ContainerEvent ContainerListener
FocusEvent FocusListener
ItemEvent ItemListener
KeyEvent KeyListener
MouseEvent MouseListener, MouseMotionListener
TextEvent TextListener
WindowEvent WindowListener

1313
Semantic Event Listener
 The semantic events relate to operations on the components in the
GUI. There are 3semantic event classes.
 An ActionEvent is generated when there was an action performed on a component
such as clicking on a menu item or a button.
― Produced by Objects of Type: Buttons, Menus, Text
 An ItemEvent occurs when a component is selected or deselected.
― Produced by Objects of Type: Buttons, Menus
 An AdjustmentEvent is produced when an adjustable object, such as a scrollbar, is
adjusted.
― Produced by Objects of Type: Scrollbar
 Semantic Event Listeners
 Listener Interface: ActionListener, Method: void actionPerformed(ActionEvent e)
 Listener Interface: ItemListener, Method: void itemStateChanged (ItemEvent e)
 Listener Interface: AdjustmentListener, Method: void adjustmentValueChanged
(AdjustmentEvent e)

1414
Using the ActionListener
 Stages for Event Handling by ActionListener
 First, import event class
import java.awt.event.*;
 Define an overriding class of event type (implements
ActionListener)
 Create an event listener object
ButtonListener bt = new ButtonListener();
 Register the event listener object
b1 = new Button(“Show”);
b1.addActionListener(bt);
class ButtonListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
/ / Write what to be done. . .
label.setText(“Hello World!”);
}
} addActionListener
ButtonListener
action
Button Click
Event
①
②

two ways to do the same thing.
 Inner class method
 Using an anonymous inner class, you will typically use
an ActionListener class in the following manner:
ActionListener listener = new ActionListener({
public void actionPerformed(ActionEvent actionEvent) {
System.out.println("Event happened");
}
1616

OR
 Interface Method
 Implement the Listener interface in a high-level
class and use the actionPerformed method.
public class MyClass extends JFrame implements ActionListener {
...
public void actionPerformed(ActionEvent actionEvent) {
System.out.println("Event happened");
}….
}
1717

1818
MouseListener (Low-Level
Listener)
 The MouseListener interface
defines the methods to
receive mouse events.
 mouseClicked(MouseEvent me)
 mouseEntered(MouseEvent me)
 mouseExited(MouseEvent me)
 mousePressed(MouseEvent me)
 A listener must register to
receive mouse events
import javax.swing.*;
class ABC extends JFrame implements MouseListener {
ABC()
{
super("Example: MouseListener");
addMouseListener(this);
setSize(250,100);
}
public void mouseClicked(MouseEvent me) {
}
public void mouseEntered(MouseEvent me) {
}
public void mouseExited(MouseEvent me) {
}
public void mousePressed(MouseEvent me) {
}
public void mouseReleased(MouseEvent me) {
System.out.println(me);
}
public static void main(String[] args) {
new ABC().setVisible(true);
}
}
MouseEvent
Mouse click

Adapter Classes
 Java Language rule: implement all the methods of an interface
 That way someone who wants to implement the interface but does not want or
does not know how to implement all methods, can use the adapter class
instead, and only override the methods he is interested in.
i.e.
 An adapter classes provide empty implementation of all methods
declared in an EventListener interface.
1919

2020
Adapter Classes (Low-Level Event Listener)
 The following classes show examples of listener and adapter pairs:
 package java.awt.event
 ComponentListener / ComponentAdapter
 ContainerListener / ContainerAdapter
 FocusListener / FocusAdapter
 HierarchyBoundsListener /HierarchyBoundsAdapter
 KeyListener /KeyAdapter
 MouseListener /MouseAdapter
 MouseMotionListener /MouseMotionAdapter
 WindowListener /WindowAdapter
 package java.awt.dnd
 DragSourceListener /DragSourceAdapter
 DragTargetListener /DragTargetAdapter
 package javax.swing.event
 InternalFrameListener /InternalFrameAdapter
 MouseInputListener /MouseInputAdapter

Listener Interface Style
2121
public class GoodJButtonSubclass extends JButton implements MouseListener { ...
public void mouseClicked(MouseEvent mouseEvent) { // Do nothing }
public void mouseEntered(MouseEvent mouseEvent) { // Do nothing }
public void mouseExited(MouseEvent mouseEvent) { // Do nothing }
public void mousePressed(MouseEvent mouseEvent) {
System.out.println("I'm pressed: " + mouseEvent);
}
public void mouseReleased(MouseEvent mouseEvent) { // Do nothing } ...
addMouseListener(this); ... }

Listener Inner Class Style
MouseListener mouseListener = new MouseListener() {
public void mouseClicked(MouseEvent mouseEvent) { System.out.println("I'm
clicked: " + mouseEvent); }
public void mouseEntered(MouseEvent mouseEvent) { System.out.println("I'm
entered: " + mouseEvent); }
public void mouseExited(MouseEvent mouseEvent) { System.out.println("I'm
exited: " + mouseEvent); }
public void mousePressed(MouseEvent mouseEvent) { System.out.println("I'm
pressed: " + mouseEvent); }
public void mouseReleased(MouseEvent mouseEvent) { System.out.println("I'm
released: " + mouseEvent); }
};
2222

Using Adapter Inner Class
2323
MouseListener mouseListener = new MouseAdapter() {
public void mousePressed(MouseEvent mouseEvent) {
System.out.println("I'm pressed: " + mouseEvent);
}
public void mouseReleased(MouseEvent mouseEvent)
{ System.out.println("I'm released: " + mouseEvent);
}
};

Extend inner class
from Adapter Class
2424
import java.awt.*;
public class FocusAdapterExample {
Label label;
public FocusAdapterExample() {
Frame frame = new Frame();
Button Okay = new Button("Okay");
Button Cancel = new Button("Cancel");
Okay.addFocusListener(new MyFocusListener());
Cancel.addFocusListener(new MyFocusListener());
frame.add(Okay, BorderLayout.NORTH);
frame.add(Cancel, BorderLayout.SOUTH);
label = new Label();
frame.add(label, BorderLayout.CENTER);
frame.setSize(450, 400);
frame.setVisible(true);
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent we) {
System.exit(0);
}
});
}
public class MyFocusListener extends FocusAdapter {
public void focusGained(FocusEvent fe) {
Button button = (Button) fe.getSource();
label.setText(button.getLabel());
}
}
public static void main(String[] args) {
FocusAdapterExample fc = new FocusAdapterExample();
}
}

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