Universidad Nacional de Universidad Nacional de ColombiaColombia

Facultad de IngenieríaFacultad de Ingeniería

Departamento de SistemasDepartamento de Sistemas

ertificación enertificación en

AVAAVA

10. 10. EVENTSEVENTS

ObjectivesObjectives

Interface EventInterface Event

Event Delegation ModelEvent Delegation Model

The Event Class HierarchyThe Event Class Hierarchy

Event ListenersEvent Listeners

Explicit Event EnabliExplicit Event Enablingng

AdaptersAdapters

ObjectivesObjectives

• Write code to implement listener classes and methods, and in listener methods, extract informationfrom the event to determine the affected component,mouse position, nature, and time of the event. Statethe event classname for any specified event listenerinterface in the java.awt.event package.

Interface Event

When the user clicks a button, types text, uses the mouse, or performs any other interface-related action in your applet, an interface event occurs.

When an event occurs, the applet is notified and takes the appropriate action.

Interface Event

In GUI, the program responds to user events when they happen because the user directs the program flow by manipulating the controls in the applet.

Java declares ActionListener as an interface to let you use the ActionListener methods.

You can extend the Applet class to create your applet, and implement the ActionListener interface to use the Action-Listener methods as well.

Interface Event

Most of the event classes reside in the java.awt.event package.

The class that implements the ActionListener interface does not have to be the applet's main class. In fact, you can create an entirely new class, which helps break up the code.

Event Delegation ModelDelegation Model

Delegation-Based Event Model

Listener ---------------- Component

event delegation model

a component may be told which object or objects should be notified when the component generates a particular kind of event

If a component is not interested in an event type, then events of that type will not be propagated

The delegation model is based on four concepts:

• Event classes• Event listeners• Explicit event enabling• Adapters

Event Delegation ModelDelegation Model

The delegation-based event model passes events from source controls to Listener objects.

That means you will connect a Listener to your component.

When an event occurs, the Listener object will hear it.

Event Delegation ModelDelegation Model

In this case, you will make the Listener object the applet object itself by connecting your applet to your component as a listener, using the button's addActionListener() method.

To indicate that you want the applet itself to be the component's listener, you would need to be able to pass the applet as an argument to addActionListener().

public class clicker extends Applet implements ActionListener

Event Delegation ModelDelegation Model

That is accomplished with the this keyword, which refers to the current object. Then, when there are component events, they will be sent to your applet.

The Event Class Hierarchy

Most of the event classes reside in the java.awt.event package

java.awt.AWTEvent

java.util.eventobject

The Event Class Hierarchy

ActionEvent

AdjustmentEvent ComponentEvent

TextEvent

ItemEvent

ContainerEvent

FocusEvent InputEvent

WindowEvent

PaintEvent

KeyEvent MouseEvent

java.util.EventObject: topmost superclass of all the new event classes is. It is a very general class, with only one method of interest:

 • Object getSource(): returns the object that

originated the event One subclass of EventObject is

java.awt.AWTEvent, which is the superclass of all the delegation model event classes.

Again, there is only one method of interest: • int getlD(): returns the ID of the event

An event's ID is an int that specifies the exact nature of the event. For example, an instance of the MouseEvent class can represent one of seven occurrences: a click, a drag, an entrance, an exit, a move, a press, or a release

Each of these possibilities is represented by an int:

MouseEvent.MOUSE_CLICKED, MouseEvent.MOUSE_DRAGGED, and so on.

Subclasses of java.awt.AWTEvent

event types that can be generated by the various AWT components

• ActionEvent: generated by activation of components • AdjustmentEvent: generated by adjustment of adjustable

component such as scroll bars • ContainerEvent: generated when components are added to or

removed from a container

• FocusEvent: generated when a component receives or

loses input focus

• ItemEvent: generated when an item is selected from a

list, choice, or check box • KeyEvent: generated by keyboard activity • MouseEvent: generated by mouse activity

• PaintEvent:generated when a component is painted • TextEvent: generated when a text component is modified • WindowEvent: generated by window activity (such as

iconifying or de-iconifying)

The InputEvent superclass has a getWhen() method that returns the time when the event took place; the return type is long

The MouseEvent class has getX() and getY() methods that return the position of the mouse within the originating component at the time the event took place; the return types are both int

Event Listeners An event listener is an object to which a

component has delegated the task of handling a particular kind of event

• When the component experiences input, an event of the appropriate type is constructed;

• the event is then passed as the parameter to a method call on the listener

• A listener must implement the interface that contains the event-handling method

Example: a button in an applet

When the button is clicked, an action event is to be sent to an instance of class MyActionListener

1. class MyActionListener implements ActionListener {2. public void actionPerformed( ActionEvent ae ) {3. System.out.println( "Action performed." );4. }5. }

1. public class ListenerTest extends Applet {4. public void init() {5. Button btn = new Button( "OK" );6. MyActionListener listener = new7. MyActionListener();5. btn.addActionListener( listener );6. add( btn );7. } 8. }

import java.applet.Applet;import java.awt.*;import java.awt.event.*;public class click extends Applet implements

ActionListener{ TextField texto; Button boton; public void init() { texto = new TextField(20); add(texto); texto.setText("Bienvenido a Java"); boton = new Button("Haga Click!"); add(boton); boton.addActionListener(this); }

public void actionPerformed(ActionEvent event)

{ String msg = new String("Welcome to

Java"); if(event.getSource() == boton) { texto.setText(msg); } } }

A standard formula for giving an action listener to a component

 1. Create a listener class that implements the

ActionListener interface

2. Construct the component

3. Construct an instance of the listener class

4. Call addActionListener() on the component, passing in the listener object

A component may have multiple listeners for A component may have multiple listeners for any event typeany event type

There is no guarantee that listeners will be There is no guarantee that listeners will be notified in the order in which they were notified in the order in which they were addedadded

There is also no guarantee that all listener There is also no guarantee that all listener notification will occur in the same thread; notification will occur in the same thread; thus listeners must take precautions thus listeners must take precautions against corrupting shared dataagainst corrupting shared data

 

Listener InterfacesListener Interfaces

Interface Interface methods Add Method

ActionListener actionPerformed( ActionEvent ) addActionListener()

AdjustmentListener adjustmentValueChanged( AdjustmentEvent )

addAdjustmentListener()

ComponentListener componentHidden( ComponentEvent )componentMoved( ComponentEvent )componentResized( ComponentEvent )componentShown( ComponentEvent )

addComponentlistener()

ContainerListener componentAdded( ContainerEvent )componentRemoved( ContainerEvent )

addContainerListener()

FocusListener focusGained( FocusEvent )focusLost( FocusEvent )

addFocusListener()

Listener Interfaces ...Listener Interfaces ...

Interface Interface methods Add Method

ItemListener itemStateChanged( ItemEvent ) addItemListener()

KeyListener keyPressed( KeyEvent )keyReleased( KeyEvent )keyTyped( KeyEvent )

addKeyListener()

MouseListener mousedClicked( MouseEvent ) mouseEntered( MouseEvent ) mouseExited( MouseEvent ) mousePressed( MouseEvent ) mouseReleased( MouseEvent )

addMouseListener()

MouseMotionListener mouseDragged( MouseEvent )mouseMoved( MouseEvent )

addMouseMotionListener()

Listener Interfaces ...Listener Interfaces ...

Interface Interface methods Add Method

TextListener textValueChanged( TextEvent ) addTextListener()

WindowListener windowActivated( WindowEvent ) windowClosed( WindowEvent ) windowClosing( WindowEvent ) windowDeactivated( WindowEvent ) windowDeiconified( WindowEvent ) windowlconified( WindowEvent ) windowOpened( WindowEvent )

addWindowListener()

An event listener may be removed from a component's list of listeners by calling a

removeXXXListener() method, passing in the listener to be removed

btn.removeActionListener( al );

Explicit Event Enabling

There is an alternative to delegating a component's events

It is possible to subclass the component and override the method that receives events and dispatches them to listeners

Example:components that originate action events have

a method called processActionEvent( ActionEvent ), which dispatches its action event to each action listener

1. class MyBtn extends Button {2. public MyBtn( String label ) {3. super( label );4. enableEvents( AWTEvent.ACTION_EVENT_MASK );5. }6.7. public void processActionEvent( ActionEvent ae ) {8. System.out.println( "Processing an action event."

);9. super.processActionEvent( ae );10. }11. }

1. class MyBtn extends Button implements ActionListener {2. public MyBtn( String label ) {3. super( label );4. addActionListener( this );5. }6. 7. public void actionPerformed( ActionEvent ae

) {8. // Handle the event here9. }10.}

you can always make a component subclass handle its own events by making the subclass an event listener of itself

The only difference between this strategy and the enableEvents() strategy is the order in which event handlers are invoked

When you explicitly call enableEvents(), the component's processActionEvent() method will be called before any action listeners are notified

When the component subclass is its own event listener, there is no guarantee as to order of notification

Event MasksEvent Masks

Each of the 11 listener types has a corresponding

XXX_EVENT_MASK

constant defined in the AWTEvent class, and corresponding

processXXXEvent()

methods

Event MasksEvent Masks

Mask Method

 AWTEvent.ACTION EVENT MASK processActionEvent()

AWTEvent.ADJUSTMENT EVENT MASK processAdjustmentEvent()

AWTEvent.COMPONENT EVENT MASK processComponentEvent()

AWTEvent.CONTAINER EVENT MASK processContainerEvent()

AWTEvent.FOCUS EVENT MASK process FocusEvent()

AWTEvent.ITEM EVENT MASK processItemEvent()

AWTEvent.KEY_EVENT_MASK processKeyEvent()

AWTEvent.MOUSE_EVENT_MASK processMouseEvent()

AWTEvent.MOUSE_MOTION_EVENT_MASK processMouseMotionEvent()

AWTEvent.TEXT_EVENT_MASK AWT processTextEvent()

Event.WINDOW EVENT MASK processWindowEvent()

The strategy of explicitly enabling events for a component: 

1. Create a subclass of the component 2. In the subclass constructor, call

enableEvents( AWTEvent.XXXEVENT_MASK ) 3. Provide the subclass with a

processXXXEvent() method; this method should call the superclass' version before returning 

Example:Catching iconified events on a frame

AdaptersAdapters

1. class MyIkeListener implements WindowListener {2. public void windowIconified( WindowEvent we ) {3. // process the event4. }5. }

1. class MylkeListener extends WindowAdapter {2. public void windowIconified( WindowEvent we ) {3. // process the event4. }5. }

The java.awt.event provides seven adapter classes, one for each listener interface that defines more than just a single method

An adapter is simply a class that implements an interface by providing do-nothing methods

Adapter ClassAdapter Class Listener InterfaceListener Interface______________________________________________________________________________________________________________________

ComponentAdapter ComponentAdapter ComponentListenerComponentListenerContainerAdapter ContainerAdapter ContainerListenerContainerListenerFocusAdapterFocusAdapter FocusListenerFocusListenerKeyAdapter KeyAdapter KeyListenerKeyListenerMouseAdapter MouseAdapter MouseListenerMouseListenerMouseMotionAdapter MouseMotionAdapter MouseMotionListenerMouseMotionListenerWindowAdapterWindowAdapter WindowlistenerWindowlistener

AdaptersAdapters