Transcript Lecture-24-Applets

Lecture 24
Applets
Introduction to Applets
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Applets should NOT have main method but
rather init, stop, paint etc
They should be run through javac compiler
getting a .class file as before
Create an HTML file (say HelloWorld.HTML in
same directory as .class file) and include in
this
<applet code="Example.class" width=500
height=200 > </applet> in simplest case with
no parameters where applet will run in window
of given width and height (in pixels)
More on Applets
Given the following HTML
<APPLET CODE="StockGraph"
CODEBASE="http://www.javasoft.com/applets/
" WIDTH=200 HEIGHT=200>
</APPLET>
 Runs the "StockGraph.class" executable as an
applet
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Java applets
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Java application executes from a command window
an applet is a Java program that runs in the
appletviewer or in a Web browser
The appletviewer (or browser) executes an applet
when a HTML document containing the applet is
opened in the appletviewer (or browser).
Applets in Code Warrior
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Start a new project in CW
choose java stationary and applet (in a window
jdk1.3)
Project with three source files: TrivialApplet.java,
TrivialApplet.html and TrivialAppletDebug.html
Open the java file.
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import java.awt.*;
import java.applet.Applet;
We import objects from two packages which supports graphical
user interface: AWT (abstract windowing tool). There is a
newest version of class Applet called JApplet, from the new
package java.swing (SWING is built on awt and exdends its
capabilities).
Applets in Code Warrior
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We import objects from two packages which supports graphical
user interface:
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AWT (abstract windowing tool).
the new package java.swing (SWING is built on awt and exdends
its capabilities)
public class TrivialApplet extends Applet
The extends keyword followed by a class name indicates the class (in this case Applet) from which
our new class inherits existing pieces. In this inheritance relationship, Applet is called the superclass
or base class and TrivialApplet is called the subclass or derived class. We will discuss inheritance in
detail later. Using inheritance a new class TrivialApplet has the data and methods of the Applet class
as well as the new features, like a method paint(), which overrides Applet's method paint().
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three methods init, start and paint that are guaranteed to be called
automatically for you when any applet begins execution.
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called exactly in that order
init, start methods
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init
public void init()
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Called by the browser or applet viewer to inform this applet
that it has been loaded into the system. It is always called
before the first time that the start method is called. A
subclass of Applet should override this method if it has
initialization to perform.
start
public void start()
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Called by the browser or applet viewer to inform this applet
that it should start its execution. It is called after the init
method and each time the applet is revisited in a Web page. A
subclass of Applet should override this method if it has any
operation that it wants to perform each time the Web page
containing it is visited. For example, an applet with animation
might want to use the start method to resume animation, and
the stop method to suspend the animation.
init, start and paint methods
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The appletviewer or browser expects each of these methods to
be defined so it can provide a consistent start-up sequence for
an applet.
Some sample code
import java.awt.*;
import java.applet.Applet;
public class test extends Applet
{
// Initialize the applet
public void init()
{
}
// Paint graphics on the screen
public void paint(Graphics g)
{
g.drawString( "Hello World!!!", 30, 30 );
}
}
drawString method
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The first argument to drawString is the string to
draw
The last two arguments in the list, 30 and 30, are
the coordinates at which the bottom-left corner of
the string should be drawn in the applet's area on
the screen
Coordinates are measured from the upper-left
corner of the applet in pixels: first 30 (x-coordinate)
is a number of pixel to the right, and the second
parameter (y-coordinate) is number of pixel down.
Executing a Java Applet
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provide an HTML text file that indicates which applet
the appletviewer (or browser) should load and
execute
<applet codebase="Java Classes"
code="TrivialApplet.class" width=200
height=200></applet>
Java Resources
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There is a large number of Java applet resources
available to you. The best place to start is
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http://java.sun.com/applets/index.html
Event Listeners
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If a class want to respond to a user event, it must
implement the interface (we will discuss interfaces
in a few lectures) that deals with the event
These interfaces are called event listeners.
Each listener handles a specific kind of event
A sample of event listeners:
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ActionListener - action event that generated by a user
taking an action such as a click on a button.
AdjustmentListener - an event that generated when the
componenet such as a scrollbar is moved.
ItemListener - item event that generated when an item such
as a check box has been changed.
Event Listeners
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more sample event listeners:
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KeyListener - keyboard event that occurs when a user types
on a keyboard.
MouseListener - mouse event that generated by a mouse
click.
MouseMotionListener - mouse event that generated by a
mouse movement.
WindowListener - window event that generated by window
move or adjustment.
an example of a proper class
declaration:
public class Foo extends Applet implements
ActionListener, MouseListener
{
...
}
 By making a class event listener you have to set up
a specific type of event
 You have to add a matching listener.
 Once a component (a button, for example) is
created you can add one of following methods
associated with it:
Listener Methods
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addActionListener() - Button, CheckBox, ComboBox,
TextField, RadioButton.
addAdjustmentListener() - ScrollBar.
addItemListener() - Button, CheckBox, ComboBox,
RadioButton.
addKeyListener() - all components.
addMouseListener() - all components.
addMouseMotionListener() - all components.
Names
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names of the component may be different
depending on which package you use
All above components are in AWT (import
java.applet.Applet; )
In the new package SWING (import
javax.swing.JApplet; ), all above components have
the capital letter J in front: JButton, JCheckBox and
so on.
An Example
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create a button and associate an action event
listener
JButton b = new JButton("give him a name");
b.addActionListener(this);
The ActionListener interface contains only one
method actionPerformed()
public void actionPerformed(ActionEvent e)
{
//what to do when a user
//clicks the button?
}
Example
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public void actionPerformed(ActionEvent e)
{ if (e.getSource() == buttonOne)
...
else
...
}
If you have more than one componenet has an event listener,
you have to figure out which one does what.
The class ActionEvent (a parameter in actionPerformed()) has a
method getSource() which can help to determine which
component generated the event.