Mari Göransson - KaU - Datavetenskap - DAVD11 1 Java Event Handling --

Mari Göransson - KaU - Datavetenskap - DAVD11

1

Java Event HandlingJava Event HandlingJava Event HandlingJava Event Handling

----


2

Event Handling• Happens every time a user

interacts with a user interface. For example, when a user pushes a button, or types a character.


3

A Typical Situation: Scrollbar

adjustmentListeners

*

<<create>>

source

<<use>>

InputScrollbarAdapter

+adjustmentValueChanged:

interfaceAdjustable

ComponentScrollBar

+addAdjustmentListener:

AWTEventAdjustmentEvent

EventListenerinterface

AdjustmentListener


4

Event Representation• Each event is represented by an

object that gives information about the event and identifies the event source.

• In Java, every event is a subclass of EventObject.


5

Abstract class EventObject

• getSource returns the object on which the event occured.

• Each subclass adds its own specialized methods.


6

Event Sources• Event sources are usually

components, but they can be other kind of objects too.

• An event source can have multiple event listeners registered on it.

• Examples of event sources are e.g. buttons and windows.


7

Event Listeners• An event listener is an object that

wants to be notified when an event has occured on a component.

• The common interface that all event listener interfaces must extend is EventListener.


8

Event Listeners• For example, to listen for

ActionEvents the class must implement the interface ActionListener, that in turn is an implementation of EventListener.

• An example is shown on the next slide


9

An ActionListener#source

<<use>> <<create>>

* TextField

+addActionListener:

ActionEvent


ActionListener

ItemListenerAdjustmentListener

ConverterModel

+itemStateChanged:+actionPerformed:

InputTextAdapter

+actionPerformed:

TextComponent

Component

Object

AWTEvent

ObjectEventObject

By the classesimplementingthe interface


10

Event Adapters• To highten the ease of use, most

eventlistener interfaces have corresponding adapter classes that have all of the interface methods implemented.

• Must be subclassed.


11

A Complex Listener with Adapter

-listeners

*

source

<<create>>

<<use>>

ManickAdapter

+treatment1Occured:+treatment2Occured:+treatmentnOccured:


TreatmentListener

+treatment1Occured:+treatment2Occured:+treatmentnOccured:

AWTEventTreatmentEvent

interfaceTreatable

MySimpleListener

+treatmentxOccured:

Manick

-listeners:TreatmentListeners[*]

+addTreatmentListener:


12

Three Required Parts• public class MyClass implements

ActionListener• someComponent.addActionListene

r(aMyClass)• public void

actionPerformed(ActionEvent e){}


13

An Example


14

Threads and Event Handling

• All event handling code executes in a single thread, the event-dispatching thread.

• Ensures that one event handler finishes before the next one can start.


15

Components and Events

• Every component support component, focus, key, mouse and mouse-motion listeners.

• Fires only events for which listeners has registered an interest in.


16

Java Event Handling and the Observer

Pattern• Java’s current event handling was

introduced in JDK 1.1.• It is a specialized version of the

Observer pattern.


17


Pattern• The observer pattern is used when

an object wants to be informed of a state change in another object.

• In java, the observer object listens for events to happen.


18


Pattern


19


Pattern


20

Java and the Observer Pattern

• Java has included the Observer-Observable pair of objects that concurs to the Observer pattern, it is therefore easy to implement the pattern behaviour on your own.


21

Events in the Converter• ActionEvent

– Listener must implement the interface ActionListener.

• ChangeEvent (Swing) or AdjustmentEvent– Listener must implement the interface

ChangeListener or AdjustmentListener.

• ItemEvent– Listener must implement the interface

ItemListener.


22

ActionEvent• Generated when the user presses

return in the textfield.• EventSource calls the method

actionPerformed() in the listeners and sends the event as a parameter.


23

ChangeEvent• Generated when the user changes

the slider.• Listeners must implement the

method stateChanged(), that is invoked when the target of the listener has changed its state.


24

AdjustmentEvent• Generated when the user changes

the value of the scrollbar (AWT).• Has several specialized methods:

– getAdjustable()– getAdjustmentType()– getValue()


25

AdjustmentEvent• Listeners must implement the

method adjustmentValueChanged() that is invoked when the srollbar changes its value.


26

ItemEvent• Generated when the user selects

another unit in the choicebox.• Has some specialized methods:

– getItem()– getItemSelectable()– getStateChanged()


27

The ItemEvent Structure

*

<<create>>

source

<<use>>

ComponentChoice

+addItermListener:

ActionListenerAdjustmentListener

ConverterModel

+itemStateChanged:+actionPerformed:


ItemListener

AWTEventItemEvent

interfaceItemSelectable


28

ItemEvent• The event is passed to every

ItemListener object which registered to receive such events using the component's addItemListener() method.

• When an item-selection event occurs, the listener object's itemStateChanged() method is invoked.

Documents

Mari Göransson - KaU - Datavetenskap - DAVD11 1 Java Event Handling --