Transcript Document
CSE 1340
Class 5
Objectives
Differentiate between an application and
an applet
Create an applet from Java source code
Write code to display a graphic, text, color,
and the date in an applet
Create an HTML host document
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Class 3 Objectives
Get an elementary understand objectorientation
Write a JAVA program with a Graphical
User Interface (GUI) using the
JOptionPane class
Differentiate between a Java Applet and a
Java Application
Write a Java Applet
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Import Packages
Use the import statement to access
classes in the SDK
– The java.lang package is
automatically imported
– Place the import statement before
the class header
– Use an asterisk (*) after the
package name and period delimiter
to import all necessary classes in
the package
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Call a System Date Constructor
Use the Date class in the java.util package
to access the system date
Store the Date in an object variable
Declare the object variable by calling the
Date constructor
– The constructor is a method denoted by
the new keyword followed by the object
type and parentheses
– Declaration syntax:
objectType variableName = new
objectType();
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java applet
public class WelcomeApplet2 extends JApplet
{
public void paint( Graphics g)
{
g.drawString( “Welcome to”, 25, 25 )
g.drawString(“Java Programming”, 25, 40);
}
} 1. Name two methods in the above code.
paint drawString
2. What kind of data is being passed to the
paint method?
A Graphics object
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java applet
public class WelcomeApplet2 extends JApplet
{
public void paint( Graphics g)
{
g.drawString( “Welcome to”, 25, 25 )
g.drawString(“Java Programming”, 25, 40);
}
} 3. What is the name of the Graphics object
g
4. What kind of data is being passed to the
drawString method?
A string and two integers
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import javax.swing.JOptionPane;
The import statement includes particular
classes from a particular package (folder)
in the java library of classes
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import javax.swing.JOptionPane;
public class Welcome4
All code in java must be inside a class
definition; the above line begins the
definition of a class called Welcome4.
The rules for class names are the same
as the rules for any user-defined Java
identifier (variables, constants, class
names, etc.)
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import javax.swing.JOptionPane;
public class Welcome4
{ public static void main(String args[ ])
Every Java application (one run on a
desktop, not over the web through an
html file) must have a method called
main; that is beginning/entry point of
execution of your Java application.
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import javax.swing.JOptionPane;
public class Welcome4
{ public static void main(String args[ ])
{ JOptionPane.showMessageDialog
(null,”Welcome\nto\nJava\n” +
“Programming!”);
System.exit(0);
}
}
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import javax.swing.JApplet;
The import statement includes particular
classes from a particular package (folder) in
the java library of classes
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import javax.swing.JApplet;
The import statement includes particular
classes from a particular package (folder) in
the java library of classes
import java.awt.Graphics;
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import javax.swing.JApplet;
import java.awt.Graphics;
public class WelcomeApplet2 extends JApplet
Keyword extends gives us inheritance in Java
Applets (programs that run over the web
through an html file) don’t have a method
main; instead we want our class to be an
applet type so we use the word extends to
make our class a JApplet
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import javax.swing.JApplet;
import java.awt.Graphics;
public class WelcomeApplet2 extends JApplet
{ public void paint(Graphics g)
{ g.drawString(”Welcome to ” ,25,25);
g.drawString(“Java Programming!”, 25,40);
}
}
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Every applet must have an associated html file
that contains the name of the class created
with the .java file; the following code would be
entered into a text editor and saved as
WelcomeApplet2.html
<html>
<applet code=“WelcomeApplet2.class”
width = 300 height =30>
</applet>
</html>
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Every applet must have an associated html file
that contains the name of the class created
with the .java file; the following code would be
entered into a text editor and saved as
WelcomeApplet2.html
<html>
<applet code=“WelcomeApplet2.class”
width = 300 height =30>
</applet>
</html>
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import javax.swing.JOptionPane;
public class Addition
{ public static void main(String argvs[ ])
{String firstNumber, secondNumber;
int number1, number2;
int sum;
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firstNumber =
JOptionPane.showInputDialog(
“Enter first integer”);
secondNumber=
JOptionPane.showInputDialog(
“Enter second integer”);
number1= Integer.parseInt(firstNumber);
number2=Integer.parseInt(secondNumber);
sum = number1 + number2;
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JoptionPane.showMessageDialog(
null, “The sum is “ + sum,”Results”,
JOptionPane.PLAIN_MESSAGE);
System.exit(0);
}
}
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Class Addition
written as an Applet
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import java.awt.Graphics;
import javax.swing.*;
// make available all classes
// (not all packages BUT all classes in the
// swing package (folder)
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import java.awt.Graphics;
import javax.swing.*;
public class AdditionJApplet extends JApplet
{ double sum;
public void init()
{String firstNumber, secondNumber;
double number1, number2;
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firstNumber = JOptionPane.showInputDialog(
“Enter first floating-point value”);
secondNumber=
JOptionPane.showInputDialog(
“Enter second floating-point value”);
number1= Double.parseDouble(firstNumber);
number2=
Double.parseDouble(secondNumber);
sum = number1 + number2;
}
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public void paint(Graphics g)
{ g.drawRect(15, 10, 270, 20);
g.drawString(“The sum is “ + sum,25,25);
}
}
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html file associate with
AdditionApplet.java file
<html>
<applet code = “AdditionApplet.class”
width = 300 height = 50>
</applet>
</html>
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Moving to the Web
Characteristics of an applet
– Applets run within a browser/viewer and
are usually delivered to the client
machine via the Web
– Applets cannot use system resources
or files on the client machine
Convert the application into an applet
– Import two packages
– Change the class name and extend the
Applet class
– Include a paint method to draw text and 28
Import Applet Packages
Applet package (java.applet.*)
– Allows applets to inherit attributes
and methods
AWT package (java.awt.*)
– Provides access to color, draw
methods, and GUI elements
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Change the Class Name and
Extend the Applet Class
Change the class name and file name
to create a new applet file
Edit the comment header in the
applet file
Add “extends Applet” in the class
header to inherit from the
superclass, Applet
– Provides the init() method to load
the applet in the browser window
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The paint() Method
Accepts a Graphics object as a
parameter
The Graphics object is commonly
referred to by the variable name g
– The variable g is created and
initialized in the init() method
– The variable g is a reference
variable, or a specific instance of
an object
The return type is void
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The drawString() Method
Displays text in the applet window
Accepts three arguments
– The String data
• If the data is not a String object,
convert it to a String object using
the toString() method
– The horizontal and vertical
coordinates of the String
• The coordinates are measured in
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Draw an Image
Declare an Image object
Use the getImage() method to load the image
– The getImage() method calls the
getDocumentBase() method to pull the image
from the current folder
Use the drawImage() method to set the
coordinates of the image
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Set the Background Color
Use the setBackground() method to
change the background color of the applet
window
– The setBackground() method does not
need to be called from a reference
variable
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Creating an HTML Host
Document
A host program, such as a Web page
executes the applet
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Creating an HTML Host
Document
The Web page contains HTML tags to
define a section or format
– A tag consists of a start tag,
denoted by <> and an end tag,
denoted by </>
The tag, <APPLET>…</APPLET>,
informs the browser of the applet
– The applet tag encloses the name
of the bytecode applet file and the
width and height of the applet
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Running an Applet
An applet is run by opening the HTML
host document
In TextPad, use the Run Java Applet
command
At the command prompt, type
appletviewer followed by the name of the
host document
Use Applet Viewer to test the applet
– Ignores irrelevant HTML code
– Uses less memory than a browser
– Does not have to be Java-enabled
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