Post on 26-Mar-2015
transcript
Ancora sugli Eventi
Gestione deglieventi
Modello 1.1
Event isgenerated for a
component
Listener executessuitable method
Component getsthe event
Event isconsumed
Componentpasses the Event
to Listener
Component has notregistered for that
event classEvent is discarded
Component has registered for that event class
Component
Listener interface
AWTEvent
x
3
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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multiListenerDemoJScrollPane 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().getLength());
}}
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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().getLength());
}}
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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 event-listening methods are executed in the same thread, a slow event-listener 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)
Fondamenti di Java
Introduzione alla costruzione di GUI Pluggable Look & Feel
Pluggable Look&Feel
Scelta del Look&Feelpublic static void main(String[] args) { try { UIManager.setLookAndFeel( UIManager.getCrossPlatformLookAndFeelClassName()); } catch (Exception e) { } new SwingApplication(); //Create and show the GUI. }
UIManager.getCrossPlatformLookAndFeelClassName() Returns the Java look and feel.
UIManager.getSystemLookAndFeelClassName() Specifies the look and feel for the current platform.
Scelta del Look&Feelpublic static void main(String[] args) { try { UIManager.setLookAndFeel( "com.sun.java.swing.plaf.gtk.GTKLookAndFeel"); } catch (Exception e) { } new SwingApplication(); //Create and show the GUI. }
UIManager.getSystemLookAndFeelClassName(String s) Specifies the look and feel for the platform described by “s”.
Available Look&Feel"com.sun.java.swing.plaf.gtk.GTKLookAndFeel"
Specifies the GTK+ look and feel. Introduced in release 1.4.2.
"javax.swing.plaf.metal.MetalLookAndFeel" Specifies the Java look and feel.
"com.sun.java.swing.plaf.windows.WindowsLookAndFeel" Specifies the Windows look and feel. Currently, you can use this look and feel only on Microsoft Windows systems.
"com.sun.java.swing.plaf.motif.MotifLookAndFeel" Specifies the CDE/Motif look and feel. This look and feel can be used on any platform.
Sezione: Costruttori
Costruttori
Definizione dei costruttori
Se per una classe A non scrivo nessun costruttore, il sistema automaticamente crea il costruttore A();
Se invece definisco almeno un costruttore non void, ad es. A(int s), il sistema non crea il costruttore A();
Definizione dei costruttori
Se B è figlia di A, il costruttore di B come prima cosa invoca A(), a meno che la prima istruzione non sia una super.
B(int k) { super(k)...}
A(int k) { ...}
A() { ...}
B(int k) { ...}
Invocazione dei costruttoripublic class A { public A() { System.out.println("Creo A"); }}public class B extends A { public B() { System.out.println("Creo B"); } public B(int k) { System.out.println("Creo B_int”); }}
Output:Creo ACreo B_int
public static void main(String [] a) { B b=new B(1); }
Invocazione dei costruttoripublic class A { public A(int k) { System.out.println("Creo A"); }}public class B extends A { public B() { System.out.println("Creo B"); } public B(int k) { System.out.println("Creo B_int”); }}
Output:ERRORE !
Perchè ?
public static void main(String [] a) { B b=new B(1); }
Sezione: Modificatori
Abstract e Controllo di accesso
Modificatori: abstract
Classi dichiarate abstract non possono essere istanziate, e devono essere subclassate.
Metodi dichiarati abstract devono essere sovrascritti
Una class non abstract non può contenere abstract metods
Modificatori: visibilità
public visibile da tutti(non def.)visibile da tutti nello stesso packageprotected visibile dalle sottoclassiprivate nascosta da tutti
public class ACorrectClass {
private String aUsefulString;
public String getAUsefulString() {
return aUsefulString; // "get" the value
}
private void setAUsefulString(String s) {
//protected void setAUsefulString(String s) {
aUsefulString = s; // "set" the value
}
}
Uso di metodi “di accesso”:
Matrice degli accessi
Access LevelsSpecifier Class Package Subclass World
private Y N N N
no specifier Y Y N N
protected Y Y Y N
public Y Y Y Y
Vedi anche http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html
Sezione: Upcast - downcast
Upcast & downcast
Coercion
Una funzione può essere polimorfa senza essere stata disegnata tale intenzionalmente.
Sia f una funzione che prende un argomento di tipo T, e S sia un tipo che può essere automaticamente convertito in T. Allora f può essere detta polimorfa respetto a S e T.
float somma(float x, float y) accetta anche somma (3, 3.14)somma(2,3)(coercion di int a float)
public class Test {
public static void main(String a[]) {
new Test();
} cast Test() {
A a;
B b = new B();
a=b;
a.f1();
a.f2();
}
}
OK: upcast implicito
class A { void f1()
{System.out.println("f1");} }
class B extends A { void f2()
{System.out.println("f2");} }
class C extends B { void f3()
{System.out.println("f3");} }
NO: "method f2 not found in class A"
(compiler)
public class Test {
public static void main(String a[]) {
new Test();
} cast Test() {
A a;
B b = new B();
a=b;
a.f1();
((B)a).f2();
}
}
class A { void f1()
{System.out.println("f1");} }
class B extends A { void f2()
{System.out.println("f2");} }
class C extends B { void f3()
{System.out.println("f3");} }
OK: downcast corretto
OK: upcast implicito
public class Test {
public static void main(String a[]) {
new Test();
} cast Test() {
A a;
B b = new B();
a=b;
a.f1();
((C)a).f3();
}
}
class A { void f1()
{System.out.println("f1");} }
class B extends A { void f2()
{System.out.println("f2");} }
class C extends B { void f3()
{System.out.println("f3");} }
NO: downcast illecito (runtime)java.lang.ClassCastException
OK: upcast implicito
Type conversion - cast
Si può applicare cast SOLO all’interno di unagerarchia di ereditarietà
È consigliabile usare l'operatore instanceof per verificare prima effettuare un downcast
if (staff[1] instanceof Manager) { Manager n = (Manager)staff[1];...
}