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• Event Handling Overview• Listeners, Adapters and Event Sources• Inner classes• Event Handling Details• Applets and GUI Applications

Event Handling

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Event Handling

• Used by the Abstract Window Toolkit (AWT) – for basic GUI programming Java 1.0• Used by Swing -- Better components than AWT Java 2• Used by JavaBeans -- reusable software components, like Visual Basic, that can be manipulated in a builder tool

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Babysitting

A baby in the home needs attention.Many events surrounding the baby are not easily ignored.Events can be of different types.

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One possible source of events

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Babysitting

Before responding to an event, a babysitter typically takes noteof the source and the type of event.

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Babysitting

The sitter needs to know both the event type and the event source.

Event source

Event type

Handler

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Event Handling

The window manager software may generate hundreds of differentevents.

Examples include mouse clicks, mouse movements, key strokes,and timer ticks.

A programmer is typically interested in a small subset of all of the possible events that may occur.

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Events and Event Objects

• Since events may be of different types but still exhibit some shared traits (inheritance) Java represents the event classes in a hierarchy.

• The root class is called java.util.EventObject. The only common feature shared by all events is a source object. So we find the following method in the EventObject class :

public Object getSource();

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EventObject

AWTEvent

ActionEvent ComponentEvent

InputEvent WindowEvent

MouseEvent KeyEvent

Some classes andmethods in the eventhierarchy.

Object

String getActionCommand()

int getX()char getKeyChar()

boolean isAltDown() Window getWindow()

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Event handling usually involves three types of objects

• Objects are used to hold and report on information about the event.• Objects are typically the source of events.• Objects, called listeners, are used to handle events.

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ExampleA mouseobject

A mouseobject

An event object that describes, say, the x and y coordinate of where the mouse was clicked

An event object that describes, say, the x and y coordinate of where the mouse was clicked

The listener object has methods that are called for particular events

The listener object has methods that are called for particular events

Event Source

Event data and methods

Listener

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ExampleA mouseobject

A mouseobject

An event object that describes, say, the x and y coordinate of where the mouse was clicked

An event object that describes, say, the x and y coordinate of where the mouse was clicked

The listener object has methods that are called for particular events

The listener object has methods that are called for particular events

ImplementsMouseListener

The event object is sentto a listener method

A mouse object mustbe told who its listener is.

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The Listener

There are different types of Listeners.

MouseListenersWindowListenersScrollBarListenersEtc..

These Listeners are interfaces. Remember what an interfaceprovides? If class X implements an interface then class X promisesto provide (at least) the methods declared in the interface.

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Some of the Listener Hierarchy

EventListener

ActionListener

abstract void actionPerformed(ActionEvent e);

MouseListener

abstract void mouseClicked(MouseEvent e)

KeyListener

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A Listener Interface

public interface MouseListener {

void mouseClicked(MouseEvent e); void mouseEntered(MouseEvent e); void mouseExited(MouseEvent e); void mouseReleased(MouseEvent e);

}

//What does it mean if I claim that I implement this interface?

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An Example

// MouseSpyApplet.java

import java.applet.Applet;import java.awt.event.MouseEvent;import java.awt.event.MouseListener;

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class MouseSpy implements MouseListener{ public void mouseClicked(MouseEvent event) { System.out.println("Mouse clicked. x = " + event.getX() + " y = " + event.getY()); } public void mouseEntered(MouseEvent event) { System.out.println("Mouse entered. x = " + event.getX() + " y = " + event.getY()); } public void mouseExited(MouseEvent event) { System.out.println("Mouse exited. x = " + event.getX() + " y = " + event.getY()); } public void mousePressed(MouseEvent event) { System.out.println("Mouse pressed. x = " + event.getX() + " y = " + event.getY()); } public void mouseReleased(MouseEvent event) { System.out.println("Mouse released. x = " + event.getX() + " y = " + event.getY()); }}

I have to provideALL of these methods!!

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public class MouseSpyApplet extends Applet{ public MouseSpyApplet() { MouseSpy listener = new MouseSpy(); addMouseListener(listener); }}

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<html><head></body></head><B>Spying on the mouse</b><p><applet code="MouseSpyApplet.class" width=400 height=200></applet>

</html>

Java Applets need an HTML file.

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Another approachSuppose a friendly sole created this class:

public class MouseAdapter implements MouseListener { void mouseClicked(MouseEvent e){} void mouseEntered(MouseEvent e){} void mouseExited(MouseEvent e){} void mousePressed(MouseEvent event) {} void mouseReleased(MouseEvent e){}}

Now, suppose I extend this class. What must I provide?

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Only those methods that I am interested in.

The other methods, when called, do nothing. We’ll visit this issue again later.

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Inner Classes

• Nested Top Level Classes (not inner)

• Member Classes

• Local Classes

• Anonymous Classes

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Nested Top Level Class

• Nested top-level classes are not inner classes.• Use as a convenient way to group related classes• Since the class must be static and has no 'this' pointer, it has no access to the instance data of objects for its enclosing class.• It behaves just like a 'normal' class or interface.

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//NestedTopLevelExample.java

class Top { int i,j; static class SomeClass { // static makes it top-level nested int k; SomeClass() { System.out.println("Constructing SomeClass"); } void foo() { System.out.println("Hello"); } } Top() { System.out.println("Constructing a Top object"); }

}

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public class NestedTopLevelExample {

public static void main(String args[]) {

Top myTop = new Top();

Top.SomeClass myObject = new Top.SomeClass(); myObject.foo(); }}

Output

Constructing a Top objectConstructing SomeClassHello

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Member Classes (1)

• Member classes (there is no such thing as a 'member‘ interface)• This inner class (it's not a top-level class) has no static keyword and can access the members of each object of its outer class.• The class 'appears in every instance'.

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•The parent class must declare an instance of an inner class, before it can invoke the inner class methods, assign to data fields (including private ones), and so on.

•Unlike nested top-level classes, inner classes are not directly part of a package and are not visible outside the class in which they are nested.

• Inner classes are often used for GUI event handlers.

Member Classes (2)

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// MemberClassExample.javaclass Top { int i = 33; public class SomeClass { // access the outer object's state. private int k = i; SomeClass() { System.out.println("Constructing SomeClass"); } void foo() { System.out.println("Hello"); } } Top() { System.out.println("Constructing a Top object"); SomeClass sc = new SomeClass(); System.out.println(sc.k); }}

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public class MemberClassExample {

public static void main(String args[]) {

Top myObject = new Top(); }}

// OUTPUTConstructing a Top objectConstructing SomeClass33

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Local Classes

•A local class is an inner class. Typically, a local class is declared within a method. It is not a member of an enclosing class. It is visible only within a block.

•These classes are used primarily as "adapter classes".

• For example, a block of code that creates a Button object could use a local class to define a simple implementation of the ActionListener Interface. Then it could instantiate this simple implementation and pass the resulting object to the button's ActionListener method, thereby connecting the button to the "callback" code that is executed when the button is pressed.

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// Local Class exampleclass Top { int i = 33; Top() { System.out.println("Constructing a Top object"); // define a class within a method class Wow { int t; Wow() { System.out.println("Building a Wow"); i = 8; t = 9; } } Wow h = new Wow(); System.out.println(" h.t == " + h.t); System.out.println(" i == " + i); }}

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public class LocalExample {

public static void main(String args[]) {

Top myObject = new Top(); }}

// OUTPUT

Constructing a Top objectBuilding a Wow h.t == 9 i == 8

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• An anonymous class is refinement of inner classes.• It allows you to combine the definition of the class with the instance allocation.• Since it is instantiated in the same expression that defines it, it can only be instantiated once. This is very similar to local classes. • When writing a simple adapter class, the choice between a named local class and an unnamed anonymous class typically comes down to a matter of style and code clarity, rather than any difference in functionality.•The new class can't have a constructor.

Anonymous Classes

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// Anonymous.java

interface SmallClass { public void foo();}

class Top { int i = 33; void someMethod(SmallClass s) { s.foo(); } void anotherMethod() { someMethod(new SmallClass() { public void foo() { System.out.println("Really fun"); } }); }

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Top() { System.out.println("Constructing a Top object"); someMethod(new SmallClass() { public void foo() { System.out.println("Strange but fun"); } }); }}

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public class Anonymous {

public static void main(String args[]) {

// We can't create interface objects // error: SmallClass s = new SmallClass();

Top myObject = new Top(); myObject.anotherMethod(); }}// OUTPUTConstructing a Top objectStrange but funReally fun

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Event Handling – Details

register Source Object Listener Object

fire events

After the listener object registers itself with the sourceobject, the source object calls a method found in thelistener object and passes an object that describes the event.

Event object

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Event Handling

Suppose we have a Button object Button b = new Button();

We must determine what events a particular component generates.

A Button component may generate an ActionEvent object.

Button b

ActionEventObject

ActionEventObject

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Implementing a Listener

class BabySitter implements ActionListener { public void actionPerformed(ActionEvent e) { // handle the event object e in some way }}

•We need an object that will listen for the ActionEvent.•We implement the ActionListener class (this class listens for ActionEvents from buttons, menus, etc.) and override its actionPerformed method.

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Create a BabySitter object

BabySitter sitter = new BabySitter();

sitter

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Registering The Listener

• We want to listen for the ActionEvent object coming from the button.

• So, we tell the button what object will listen for the ActionEvent object:

• b.addActionListener(sitter)

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The button and its listener

Button Object Sitter Object

The button calls the actionPerformed()method of the sitter objectand passes an ActionEvent objectas a parameter

addActionListener

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Hit the X and the program exits

Once again but with a window

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Create a WindowCloseSitter Class

import javax.swing.*;import java.awt.event.*;

public class CloseDemo { public static void main(String[] args) { JFrame f = new JFrame("Example"); f.setSize(400,100); f.setVisible(true); WindowCloseSitter h = new WindowCloseSitter(); f.addWindowListener(h); }}

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class WindowCloseSitter implements WindowListener {

public void windowClosing(WindowEvent e) { System.exit(0); } public void windowClosed(WindowEvent e) { } public void windowOpened(WindowEvent e) { } public void windowIconified(WindowEvent e) { } public void windowDeiconified(WindowEvent e) { } public void windowActivated(WindowEvent e) { } public void windowDeactivated(WindowEvent e) { }

}

But we have to implement ALL of the functions !!

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Java Provides Adapter Classes

• ComponentAdapter• MouseMotionAdapter• WidowAdapter• ContainerAdapter• MouseAdapter• FocusAdapter• KeyAdapter

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The WindowAdapter class

public abstract class WindowAdapter implements WindowListener {

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

A built in class -- we already have the empty bodies!!

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The Window again

import javax.swing.*;import java.awt.event.*;public class CloseDemo2 extends WindowAdapter { public static void main(String[] args) { JFrame f = new JFrame("Example"); f.setSize(400,100); f.setVisible(true); f.addWindowListener(new CloseDemo2()); } public void windowClosing(WindowEvent e) { System.exit(0); }}

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Again with anonymous classes

import javax.swing.*;import java.awt.event.*;public class CloseDemo3 { public static void main(String[] args) { JFrame f = new JFrame("Example"); f.setSize(400,100); f.setVisible(true); f.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } } ); } }