pravin yannawar, docs, nmu jalgaon. basic java : event handling in awt and swing 2 objectives of...

Pravin Yannawar, DOCS, NMU Jalgaon

Basic Java : Event handling in AWT and Swing2

Objectives of This Session

• Explain the Event handling mechanism & demonstrate it using example

• List the categories of Events & Listener interfaces

• Demonstrate the above example using an independent event handler class

• Identify the need for adapter classes & demonstrate it using an independent event handler example


Event Handling

• GUI applications are event-driven applications.

• They generate events when the user of the program interacts with the GUI.

• The underlying OS is constantly monitoring these events.

• When a event occurs, the OS reports these events to the programs that are running.

• The application will handle the event using a appropriate “Event Handler”.


Delegation Event Model

• Source generates an event & sends it to one or more listeners.

• Listener simply waits until it receives an event.

• Once received, listener processes the event & returns.

Advantage : application logic that processes event is clearly separated from the UI logic that generates those events.

An UI element is able to delegate the processing of an event to a separate piece of code ( Event handler)


Delegation Event Model

• Event: is an object that describes a state change in a source.

• Event Source: is an object that generates an event.

A source must register listeners for listeners to receive notifications about a specific type of event.

• Event Listener : is an object that is notified when an event occurs.

It must be registered with a source. It must implement methods to receive & process

these notifications.


Event Handling

import java.awt.*;import java.awt.event.*;class MyFrame extends Frame implements ActionListener{

TextField t1; Button b1; MyFrame() {

t1 = new TextField(20);b1 = new Button(“Click”);add(t1,”North”); b1.addActionListener(this);add(b1,”South”);

}


Event Handling

public void actionPerformed( ActionEvent e ) {

t1.setText(“Click pressed”);}public static void main(String args[])

{Frame f = new MyFrame();

f.setSize(100,100); f.show();}

}


Inheritance diagram of the AWT Event Classes

EventObject

AWT Event

ComponentEvent

ActionEvent

AdjustmentEvent

ItemEvent

TextEvent

InputEvent

ContainerEvent

FocusEvent

PaintEvent

WindowEvent

MouseEvent

KeyEvent


Event Listener Interfaces

ActionListener

AdjustmentListener

ComponentListener

FocusListener

ItemListener

MouseListener

MouseMotionListener

KeyListener

TextListener

WindowListener

ContainerListener

java.util.EventListener


Event Handling Summary

Interface Methods Parameter EventsGenerated by

ActionListner actionPerformed ActionEventgetActionCommandgetmodifiers

ButtonListMenuItemTextField

AdjustmentListner

adjustmentValueChanged

AdjustmentEventgetAdjustablegetAdjustmentTypegetvalue

Scrollbar

ItemListner itemStateChanged ItemEventgetItemgetItemSelectablegetstateChange

CheckboxCheckboxMenuItemChoiceList

TextListner textValue Changed TextEvent TextComponent




ComponentListener

componentMovedcomponentHiddencomponentResized

ComponentEventgetComponent

Component

ContainerListener

componentAddedcomponentRemoved

ContainerEventgetChildgetContainer

Container

FocusListener focusGainedfocusLost

focusEventIsTemporary

Component

KeyListener keyPressedkeyRealsedkeyTyped

KeyEventgetKeyChargetKeyCodegetkeyModifiersTextisActionKey

Component




MouseListener

mousePressedmouseRealesedmouseEnteredmouseExitedmouseClicked

MouseEventgetClickCountgetXgetYgetPointtranslatePointisPopupTrigger

Component

MouseMotionListener

mouseDraggedmouseMoved

Component

WindowListener

windowClosingwindowOpenedwindowIconedwindowDeiconedwindowClosedwindowActivatedwindowDeactivated

WindowEventgetWindow

Window


Delegation Event Model with Another Class

import java.awt.*;class MyFrame extends Frame{

TextField t1; Button b1;

MyFrame() {t1 = new TextField(20);b1 = new Button(“Click”);add(t1,”North”); b1.addActionListener(new

ButtonHandler(this));add(b1,”South”);

}


Delegation Event Model with Another Class

class ButtonHandler implements ActionListener { MyFrame f ; ButtonHandler(MyFrame mf)

{f=mf;

}public void actionPerformed( ActionEvent e ) {

// code for processing button press.}

}


Adapter Classes

• Many of the listener interfaces have more than one method.

• Thus, if a particular interface is implemented, all the methods of that interface should also be implemented.

• To simplify this task, listener interfaces with more than one method come with adapter classes.

• Adapter classes implement all the methods of an interface.

• You can extend the adapter class to specify the desired reaction to some methods.


Without Adapter Classes

class MyFrame extends Frame implements WindowListener

{public void windowActivated(WindowEvent e){}public void windowDeactivated(WindowEvent e){}public void windowIconified(WindowEvent e){}public void windowDeiconified(WindowEvent e){}public void windowOpened(WindowEvent e){}public void windowClosed(WindowEvent e){}public void windowClosing(WindowEvent e){

System.exit(0);}


Without Adapter Class

public static void main(String args[]) {

Frame f = new MyFrame(); f.setSize(100,100); f.show();

f.addWindowListener(f);}

}


Using Adapter Class

import java.awt.*;import java.awt.event.*;

class MyFrame extends Frame{

MyFrame() {

addWindowListener(new WindowHandler()); }

}


Adapter Classes

class WindowHandler extends WindowAdapter {{

public void windowClosing(WindowEvent e){

System.exit(0);}

}

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