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Event Listeners
Some Events and Their Associated Event Listeners
Act that Results in the Event
Listener Type
User clicks a button, presses Enter while typing
in a text field, or chooses a menu item
ActionListener
User closes a frame (main window)
WindowListener
User presses a mouse button while the cursor is
over a component
MouseListener
User moves the mouse over a component
MouseMotionListener
Component becomes visible
ComponentListener
Component gets the keyboard focus
FocusListener
Table or list selection changes
ListSelectionListener
Any property in a component changes such as
the text on a label
PropertyChangeListener
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ActionListener
public interface ActionListener extends EventListener
The listener interface for receiving action events. The
class that is interested in processing an action event
implements this interface, and the object created with
that class is registered with a component, using the
component's addActionListener method.
void actionPerformed(ActionEvent e)
Invoked when an action occurs
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ActionEvent
ActionEvent extends AWTEvent
A semantic event which indicates that a componentdefined action occured. This high-level event is
generated by a component (such as a Button) when
the component-specific action occurs (such as being
pressed). The event is passed to every every
ActionListener object that registered to receive such
events using the component's addActionListener
method.
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ActionEvent
Method Summary
String getActionCommand()
Returns the command string associated with this action.
int getModifiers()
Returns the modifier keys held down during this action event.
long getWhen()
Returns the timestamp of when this event occurred.
String paramString()
Returns a parameter string identifying this action event.
Methods Inherited
…
public Object getSource()
Returns The object on which the Event initially occurred.
Seconda applicazione
JFrame
JPanel
ActionListener
ButtonPanel
Painter
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Button
addActionListener
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Esempio
package actionlistener;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.JFrame;
public class ButtonPanel extends JPanel {
public ButtonPanel() {
super();
JButton button1 = new JButton("Giallo");
JButton button2 = new JButton("Verde");
JButton button3 = new JButton("Rosso");
this.add(button1);
this.add(button2);
this.add(button3);
Painter azioneGiallo = new Painter(Color.YELLOW,this);
Painter azioneVerde = new Painter( Color.GREEN,this);
Painter azioneRosso = new Painter(Color.RED,this);
button1.addActionListener(azioneGiallo);
button2.addActionListener(azioneVerde);
button3.addActionListener(azioneRosso);
}
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Esempio - continua
public static void main(String a[]) {
JFrame f=new JFrame();
f.setContentPane(new ButtonPanel());
f.setSize(300,300);
f.setVisible(true);
}
}
class Painter implements ActionListener {
private Color colore;
private JPanel contenitore;
public Painter(Color colore, JPanel contenitore) {
this.colore = colore;
this.contenitore=contenitore;
}
public void actionPerformed(ActionEvent actionEvent) {
contenitore.setBackground(colore);
}
}
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Esempio 2
public class ButtonPanel extends JPanel {
public ButtonPanel() {
super();
Painter p=new Painter(this);
String c[]={"Giallo","Verde","Rosso"};
for (int i=0;i<c.length;i++) {
JButton b=new JButton(c[i]);
this.add(b);
b.addActionListener(p);
// b.setActionCommand(c[i]);
}
}
public static void main(String a[]) {
JFrame f=new JFrame();
f.setContentPane(new ButtonPanel());
f.setSize(300,300);
f.setVisible(true);
}
}
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Esempio 2 - continua
class Painter implements ActionListener {
private JPanel contenitore;
private Color colore;
public Painter(JPanel contenitore) {
this.contenitore=contenitore;
}
public void actionPerformed(ActionEvent actionEvent) {
String s=((JButton)actionEvent.getSource()).getText();
//String s=actionEvent.getActionCommand();
if (s.equals("Giallo")) colore=Color.YELLOW;
else if (s.equals("Rosso")) colore=Color.RED;
else if (s.equals("Verde")) colore=Color.GREEN;
contenitore.setBackground(colore);
}
}
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Esempio 3
public class ButtonPanel extends JPanel {
public ButtonPanel() {
super();
Painter p=new Painter(this);
this.setLayout(null);
String c[]={"Giallo","Verde","Rosso"};
for (int i=0;i<c.length;i++) {
JButton b=new JButton(c[i]);
b.setSize(100,50);
b.setLocation(i*100,i*50);
this.add(b);
b.addActionListener(p);
b.setActionCommand(c[i]);
}
}
NON CONSIGLIATO – LAYOUT NON LIQUIDO!
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Compito
Scrivere un applicazione contenente un bottone
che quando viene premuto si sposta altrove
nella finestra.
Scrivere una applicazione contenente due
bottoni: uno ingrandisce la dimensione della
finestra, l’altro la rimpicciolisce
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Mouse events
This low-level event is generated by a component object for:
Mouse Events
 a mouse button is pressed
 a mouse button is released
 a mouse button is clicked (pressed and released)
 the mouse cursor enters the unobscured part of component's
geometry
 the mouse cursor exits the unobscured part of component's
geometry
Mouse Motion Events
 the mouse is moved
 the mouse is dragged
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Compito
Scrivere un applicazione contenente un TextField
il cui valore inizialmente è zero, e che viene
incrementato di uno ogni volta che il bottone
del mouse viene cliccato.
Scrivere un’applicazione contenente un bottone
che si posiziona dove viene cliccato il bottone
del mouse. Se invece viene cliccato il bottone
grafico, questo si riposiziona nella sua sede
iniziale.
Ancora sugli Eventi
Gestione
degli
eventi
Modello 1.1
Listener interface
Component
x
AWTEvent
Event is
generated for a
component
Component has not
registered for that
event class
Event is discarded
Component has registered
for that event class
Component gets
the event
Event is
consumed
Component
passes the Event
to Listener
Listener executes
suitable method
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multiListenerDemo
package listenersdemo;
import javax.swing.*; import java.awt.*; import java.awt.event.*;
public class MultiListener
extends JPanel implements ActionListener {
JTextArea topTextArea;
JTextArea bottomTextArea;
JButton button1, button2;
JLabel l = null;
final static String newline = "\n";
public static void main(String[] args) {
createAndShowGUI();
}
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multiListenerDemo
private static void createAndShowGUI() {
//Create and set up the window.
JFrame frame = new JFrame("MultiListener");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//Create and set up the content pane.
JComponent newContentPane = new MultiListener();
frame.setContentPane(newContentPane);
//Display the window.
frame.pack();
frame.setVisible(true);
}
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multiListenerDemo
public MultiListener() {
super(new FlowLayout());
l = new JLabel("Cosa sento io:");
add(l);
topTextArea = new JTextArea();
topTextArea.setEditable(false);
JScrollPane topScrollPane = new JScrollPane(topTextArea);
Dimension preferredSize = new Dimension(200, 75);
topScrollPane.setPreferredSize(preferredSize);
add(topScrollPane);
l = new JLabel("Cosa sente la spia:");
add(l);
bottomTextArea = new JTextArea();
bottomTextArea.setEditable(false);
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multiListenerDemo
JScrollPane bottomScrollPane = new JScrollPane(bottomTextArea);
bottomScrollPane.setPreferredSize(preferredSize);
add(bottomScrollPane);
button1 = new JButton("Fra Martino campanaro");
add(button1);
button2 = new JButton("Dormi tu?");
add(button2);
button1.addActionListener(this);
button2.addActionListener(this);
button2.addActionListener(new Spia(bottomTextArea));
setPreferredSize(new Dimension(400, 300));
}
public void actionPerformed(ActionEvent e) {
topTextArea.append(e.getActionCommand() + newline);
}
}
topTextArea.setCaretPosition(topTextArea.getDocument().getLe
ngth());
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multiListenerDemo
class Spia
implements ActionListener {
JTextArea myTextArea;
public Spia(JTextArea ta) {
myTextArea = ta;
}
public void actionPerformed(ActionEvent e) {
myTextArea.append(e.getActionCommand()
+ MultiListener.newline);
}
}
myTextArea.setCaretPosition(myTextArea.getDocument().getLe
ngth());
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Design considerations
The most important rule to keep in mind about event listeners that
they should execute very quickly. Because all drawing and eventlistening methods are executed in the same thread, a slow eventlistener method can make the program seem unresponsive and
slow to repaint itself.
You might choose to implement separate classes for different kinds of
event listeners. This can be an easy architecture to maintain, but
many classes can also mean reduced performance.
When designing your program, you might want to implement your
event listeners in a class that is not public, but somewhere more
hidden. A private implementation is a more secure
implementation.
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Low-Level Events and Semantic Events
Events can be divided into two groups: low-level events and semantic
events. Low-level events represent window-system occurrences or
low-level input. Everything else is a semantic event.
Examples of low-level events include mouse and key events — both
of which result directly from user input.
Examples of semantic events include action and item events.
Whenever possible, you should listen for semantic events rather than
low-level events. That way, you can make your code as robust and
portable as possible. For example, listening for action events on
buttons, rather than mouse events, means that the button will
react appropriately when the user tries to activate the button
using a keyboard alternative or a look-and-feel-specific gesture.
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Listeners/Adapters
public class MyClass implements MouseListener {
...
someObject.addMouseListener(this);
...
/* Empty method definition. */
public void mousePressed(MouseEvent e) { }
/* Empty method definition. */
public void mouseReleased(MouseEvent e) { }
/* Empty method definition. */
public void mouseEntered(MouseEvent e) { }
/* Empty method definition. */
public void mouseExited(MouseEvent e) { }
public void mouseClicked(MouseEvent e) { ...//Event listener
implementation goes here... } }
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Listeners/Adapters
/* * An example of extending an adapter class instead of
* directly implementing a listener interface. */
public class MyClass extends MouseAdapter {
...
someObject.addMouseListener(this);
...
public void mouseClicked(MouseEvent e) {
//Event
... }
}
listener implementation goes here
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Inner classes
//An example of using an inner class.
public class MyClass extends JFrame {
...
someObject.addMouseListener(
new MyAdapter());
...
class MyAdapter extends MouseAdapter {
public void mouseClicked(MouseEvent e){
...//Event listener implementation goes here... }
}
}
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Anonymous Inner classes
//An example of using an inner class.
public class MyClass extends JFrame {
...
someObject.addMouseListener(
new MouseAdapter () {
public void mouseClicked(MouseEvent e){
...//Event listener implementation goes here...
}
}
);
…
}
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Inner classes
An instance of InnerClass can exist only within an
instance of EnclosingClass and it has direct
access to the instance variables and methods
of its enclosing instance.
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Anonymous Inner classes
//An example of using an inner class.
public class MyClass extends JFrame {
...
someObject.addMouseListener(
new MouseAdapter() {
public void mouseClicked(MouseEvent e){
...//Event listener implementation goes here... }
}
});
...
}
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Listeners supported by all Swing components
component listener
 Listens for changes in the component's size, position, or
visibility.
focus listener
 Listens for whether the component gained or lost the ability to
receive keyboard input.
key listener
 Listens for key presses; key events are fired only by the
component that has the current keyboard focus.
mouse listener
 Listens for mouse clicks and mouse movement into or out of
the component's drawing area.
mouse-motion listener
 Listens for changes in the cursor's position over the
component.
mouse-wheel listener (introduced in 1.4)
 Listens for mouse wheel movement over the component.
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Altri listeners
action
caret
change
document
undoable edit
item
list
selection
window
+ Listeners speciali per componenti specifiche
(treeNodeExpansion, ecc)