Java event processing model in c# and java

Agenda
 Introduction
 C# Event Processing Macro View
 Required Components
 Role of Each Component
 How To Create Each Type Of Component

Introduction
 C# is a modern programming language supported
by an extensive set of API structures and classes.
 i.e., The .Net Framework
 C# supports event-driven programming normally
associated with Microsoft Windows applications.
 One normally thinks of events as being generated
by GUI components
 …but any object can generate an “event” if it’s
programmed to do so…

C# Event Processing Macro
View
 Generally speaking, two logical components are
required to implement the event processing model:
 1) An event producer (or publisher)
 2) An event consumer (or subscriber)
 Each logical components has assigned
responsibilities
 Consider the following diagram

View
When an Event occurs Object B processes the
notification is sent to all event notification in its
the subscribers on the list event handler code
for that particular event…

 Object A Object B
 (Event Publisher) (Event Subscriber)

Subscriber List Event Handler Code
(Object B)

Object A maintains a Object B subscribes to event
list of subscribers for (or events) generated by Object A.
each publishable event

View
When a Click event occurs
notification is sent to all
the subscribers on the
list…

 Button MainApp
 (Event Publisher)
(Event Subscriber)

Subscriber List
(MainApp.onButtonClick) onButtonClick

Button maintains a list
of subscribers of its
Click event

View
 These two diagrams hide a lot of details
 How is the subscriber list maintained?
 How is the event generated?
 How is notification sent to each subscriber?
 What is an event – really?
 How can you add custom event processing to
your programs?
 This presentation attempts to answer these
questions in a clear manner…

View
 The .Net API contains lots of classes that
generate different types of events…
 Most are GUI related
○ Can you think of a few?
 It also contains lots of Delegates
 Can you name at least one?
 Let’s take a closer look at the C# event
processing model

View
 To implement custom event processing in your
programs you need to understand how to create
the following component types:
 Delegates
 Event Generating Objects (publishers)
○ Events
○ Event Notification Methods
 Event Handling Objects (subscribers)
○ Event Handler Methods

Required Components
 To implement custom event processing in your
programs you need to understand how to create
the following component types:
 Delegates
 Event Generating Objects (publishers)
○ Events
○ Event Notification Methods
 Event Handling Objects (subscribers)
○ Event Handler Methods

Role of Each Component
- Delegate -

 Delegate
 Delegate types represent references to methods
with a particular parameter list and return type
○ Example
 EventHandler(Object sender, EventArgs e)
 Represents a method that has two parameters, the
first one being of type Object and the second being
of type EventArgs. Its return type is void.
 Any method, so long as its signature matches that
expected by the delegate, can be handled by the
delegate.

- Delegate -

 But just what is a delegate?
 A delegate is a reference type object.
 A delegate extends either the System.Delegate
or MulticastDelegate class
○ Depends on whether one (Delegate) or more
(MulticaseDelegate) subscribers are involved
 You do not extend Delegate or
MulticastDelegate
○ The C# compiler does it for you

- Delegate -

 The delegate object contains the
subscriber list.
 It is actually implemented as a linked list where
each node of the list contains a pointer to a
subscriber’s event handler method
 Delegates are types – like classes
 Except – you declare them with the delegate
keyword and specify the types of methods they
can reference

- Publisher -

 A publisher is any class that can fire an event and
send event notifications to interested subscribers
 A publisher class contains the following critical
elements:
 An event field
○ This is what subscribers subscribe to…
 An event notification method
○ This activates the subscriber notification
process when the event occurs

- Subscriber -

 A subscriber is a class that registers its interest in
a publisher’s events
 A subscriber class contains one or more event
handler methods
- Event Handler Method –
 An event handler methods is an ordinary method
that is registered with a publisher’s event.
 The event handler method’s signature must match
the signature required by the publisher’s event
delegate.

- Event -

 An event is a field in a class
 Events are declared with the event keyword
 Events must be a delegate type
 Delegates, remember, are objects that contain a list of
pointers to subscriber methods that delegate can process
 An event field will be null until the first subscriber
subscribes to that event

- Event Notification Method -
 In addition to an event field a publisher will have a
method whose job is to start the subscriber
notification process when the event occurs
 An event notification method is just a normal method
 It usually has a parameter of EventArgs or a user-defined
subtype of EventArgs.
○ But it can have any number and type of parameters as
required.

An Custom Event Example
- Elapsed Minute Timer -
 This example includes five separate source files:
 Delegate.cs
 Publisher.cs
 Subscriber.cs
 MinuteEventArgs.cs
 MainApp.cs

using System;

namespace CustomEventExample {
public delegate void ElapsedMinuteEventHandler(Object sender, MinuteEventArgs
e);
} // end CustomEventExample namespace

using System;
public class MinuteEventArgs : EventArgs {
private DateTime date_time;
public MinuteEventArgs(DateTime date_time){
this.date_time = date_time;
}

public int Minute {
get { return date_time.Minute; }

}
}
}

using System;


public class Publisher {

public event
ElapsedMinuteEventHandler MinuteTick;
public Publisher(){

Console.WriteLine("Publisher
Created");
}

public void countMinutes(){
int current_minute = DateTime.Now.Minute;
while(true){
if(current_minute != DateTime.Now.Minute){
Console.WriteLine("Publisher:
{0}", DateTime.Now.Minute);
onMinuteTick(new
MinuteEventArgs(DateTime.Now));
current_minute = DateTime.Now.Minute;
}//end if
} // end while
} // end countMinutes method

public void onMinuteTick(MinuteEventArgs e){
if(MinuteTick != null){
MinuteTick(this, e);
}
}// end onMinuteTick method
} // end Publisher class definition

using System;


public class Subscriber {

private Publisher publisher;

public Subscriber(Publisher publisher){
this.publisher = publisher;
subscribeToPublisher();
Console.WriteLine("Subscriber Created");
}

public void subscribeToPublisher(){
publisher.MinuteTick += new ElapsedMinuteEventHandler(minuteTickHandler);
}

public void minuteTickHandler(Object sender, MinuteEventArgs e){
Console.WriteLine("Subscriber Handler Method: {0}", e.Minute);

}
} // end Subscriber class definition

using System;


public class MainApp {
public static void Main(){
Console.WriteLine("Custom Events are Cool!");

Publisher p = new Publisher();
Subscriber s = new Subscriber(p);
p.countMinutes();

} // end main
} //end MainApp class definition

AGENDA

 The Delegation Event model
 Event Listeners
 Event Classes
 Event Listener Interfaces
 Anonymous Inner Classes
 Examples

The Delegation Event
Model
The concept of Delegation Event model is

 A source generates an event and sends it to one or
more listeners.
 The listener simply waits until it receives an event. Once
an event is received, the listener processes events and
then returns.
 Listeners must register with a source in order to receive
an event notification.

Event Handling Model of
AWT
Event object
• Event handling objects

Event handling
methods

Event source Event listener
• Button • Methods
• Textbox
• Etc.,

Events
o An event is an object that describes a state
change in a source.

o Events are generated by
• Pressing a button
• Entering a character via the keyboard
• Selecting an item in a list
• Clicking the mouse
• And so on…………..

Event Sources

o A source is an object that generates an event.
o A source must register listeners.

General form

public void addTypeListener(TypeListener el)
public void removeTypeListener(TypeListener el)

Type - name of the event.
el - reference to the event listener.

Event Listeners
o A listener is an object that is notified when an event
occurs.

o Two Requirements :
 It must have been registered with one or more sources to
receive notifications.
 It must implement methods to receive and process these
notifications.

EventObject
o The root of the java event class hierarchy is
EventObject, which is in java.util.
o It is the super class of all the events.

 Two methods:
o getsource() - returns the source of the event.
o toString() - returns the string equivalent of
the event.

AWTEvent class

o Defined within the java.awt package.
o Subclass of EventObject
o Superclass of all AWT-based events that are handled by
the delegation event model.

Java.awt.event
Event class Description

Action Event Generated when a button is pressed,a list item is
double-clicked, or a menu item is selected.

AdjustmentEvent Generated when a scroll bar is manipulated.

ComponentEvent Generated when a component is hidden,
moved,resized, or becomes visible.

ContainerEvent Generated when a component is added to or
removed from a container.

FocusEvent Generated when a component gains or loses
keyboard focus.

Event class Description

InputEvent Abstract superclass for all component input event
classes.

ItemEvent Generated when a check box or list item is clicked;
also occurs when a choice selection is made or a
checkable menu item is selected or deselected.

KeyEvent Generated when input is received from the keyboard.

MouseEvent Generated when the mouse is dragged, moved,
clicked, pressed, or released; also generated when
the mouse enters or exits a component.

MousewheelEvent Generated when the mouse wheel is moved.

TextEvent Generated when the value of a text area or text field
is changed.

WindowEvent Generated when a window is activated,closed,
deactivated, deiconified, iconified, opened or quit.

Event Listener Interfaces
 The ActionListener Interface
void actionPerformed(ActionEvent ae)

 The AdjustmentListener Interface
void adjustmentValueChanged(AdjustmentEvent ae)

 The ComponentListener Interface
void componentResized(ComponentEvent ae)
void componentMoved(ComponentEvent ae)
void componentShown(ComponentEvent ae)
void componentHidden(ComponentEvent ae)

 The ContainerListener Interface
void componentAdded(ContainerEvent ae)
void componentRemoved(ContainerEvent ae)

 The FocusListener Interface
void focusGained(FocusEvent fe)
void focusLost(FocusEvent fe)

 The ItemListener Interface
void itemStateChanged(ItemEvent ie)

 The KeyListener Interface
void KeyPressed(KeyEvent ke)
void KeyReleased(KeyEvent ke)
void KeyTyped(KeyEvent ke)

 The MouseListener Interface
void mouseClicked(MouseEvent me)
void mouseEntered(MouseEvent me)
void mouseExited(MouseEvent me)
void mousePressed(MouseEvent me)
void mouseReleased(MouseEvent me)

 The MouseMotionListener Interface
void mouseDragged(MouseEvent me)
void mouseMoved(mouseEvent me)

 The MouseWheelListener Interface
void mouseWheelMoved(MouseWheelEvent mwe)

 The TextListener Interface
void textChanged(TextEvent te)

 The WindowFocusListener Interface
void windowGainedFocus(WindowEvent we)
void windowLostFocus(WindowEvent we)

 The WindowListener Interface
void windowActivated(WindowEvent we)
void windowClosed(WindowEvent we)
void windowClosing(WindowEvent we)
void windowDeactivated(WindowEvent we)
void windowDeiconified(WindowEvent we)
void windowIconified(WindowEvent we)
void windowOpened(WindowEvent we)

Anonymous Inner Classes
Import java.applet.*;
Import java.awt.event.*;
Public class AnonymousInnerClassDemo extends Applet
{
public void init()
{
addMouseListener(new MouseAdapter()
{
public void mousePressed(MouseEvent me) {
showStatus(“Mouse Pressed”); }
});
} }

Handling Mouse Events
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class MouseEvents extends Applet
implements MouseListener,MouseMotionListener
{
String msg = “ “;
int mouseX=0, mouseY=0;
public void init()
{
addMouseListener(this);
addMouseMotionListener(this);
}

public void mouseClicked(MouseEvent me)
{
mouseX=0;
mouseY=10;
msg = “Mouse clicked”;
repaint();
}
public void mouseEntered(MouseEvent me)
{
mouseX=0;
mouseY=10;
msg = “Mouse Entered”;
repaint();
}

public void mouseDragged(MouseEvent me)
{
mouseX= me.getX();
mouseY=me.getY();
msg=“ * “;
showStatus(“ Dragging mouse at “+ mouseX +
“mouseY” +mouseY);
repaint();
}
public void paint(Graphics g)
{
g.drawString(msg,mouseX,mouseY);
}
}

DIFFERENCES WITH JAVA & C#

1. Differences in terms of syntax of Java
and C#
2. Modification of concepts in C# that
already exist in Java
3. Language features and concepts that
do not exist in Java at all.

Differences in terms of Syntax

JAVA MAIN vs C#
MAIN
Java:
public static void main(String[]
args)

C#:
static void Main(string[] args)
 string is shorthand for the System.String class in C#.
Another interesting point is that in C#, your Main method
can actually be declared to be parameter -less
static void Main()


PRINT STATEMENTS

Java:
System.out.println("Hello world!");
C#:
System.Console.WriteLine("Hello
world!");
or
Console.WriteLine("Hello again!");

DECLARING CONSTANTS

Java:
 In Java, compile-time constant values are declared inside a
class as

static final int K = 100;

C#:
 To declare constants in C# the const keyword is used for
compile time constants while the readonly keyword is used for
runtime constants. The semantics of constant primitives and
object references in C# is the same as in Java.

const int K = 100;

Modified concepts from
Java
IMPORTING LIBRARIES
 Both the langugaes support this functionality
and C# follows Java’s technique for importing
libraries:

 C#: using keyword
using System;
using System.IO;
using System.Reflection ;

 Java: import keyword
import java.util .*;
import java.io.*;

Enumerations
 Java's lack of enumerated types leads to the use of
integers in situations that do not guarantee type
safety.
 C# code:
public enum Direction {North=1, East=2, West=4, South=8};
Usage:
D i r e c t io n w a l l = D i r e c t i o n. No rt h;

 Java equivalent code will be:
public class Direction {
public final static int NORTH = 1;
public final static int EAST = 2;
public final static int WEST = 3;
public final static int SOUTH = 4;
}
Usage:
i n t w a l l = D i r e c t i o n . NO RT H;

Reference
 The complete Reference, Java2, Fifth edition.

Websites:
 Java vs. C#: Code to Code Comparison
http://www.javacamp.org/javavscsharp/
 A Comparative Overview of C#:
http://genamics.com/developer/csharp_comparativ
e.htm

Java event processing model in c# and java

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