Lecture 1
First
Steps
Writing your first program
There
are two types of Java programs, applets and applications. Applications
are stand-alone programs that operate like any other applications, for
instance Notepad. Applets are programs that run inside a Java enabled web browser,
such as Internet Explorer or Netscape Navigator. Below is a clock applet that is embedded in
the HTML code for this page.
The
following notes walk you through:-
Creating
Your First Application
Creating
Your First Applet
Creating
Your First Application
-
Create
a Directory for your Program Files
It
is good practice to create a new folder for every Java program that you write. In this example I would create a
folder called "HelloWorld".
-
Write
the Source Code
The next thing to do is to write the
source code for your application. This simple example prints out the phrase "Hello
World"
at the command prompt.
Using
your favorite text editor such as Notepad or WordPad type in the following
exactly. Double-check your capitalization - Java is case-sensitive.
/* The HelloWorld class implements an application that simply displays "Hello
World!" to the standard output.
*/
public
class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello
World!");
}
}
|
The name of our Java source
file must be the same as the name of the class followed by the ".java"
extension. Therefore the file above must be named
HelloWorld.java
Save the source file
you have created in your "HelloWorld"
directory with the name HelloWorld.java.

Note:
-- When you save your file, make sure you save it as a text file. Do not save it
as a WordPad document or any other format.
Also make sure that the file you create
does not have a ".txt"
extension. For example, Notepad may have saved the file as HelloWorld.java.txt.
You need to make sure it is correctly called HelloWorld.java.
In some versions of Notepad, to avoid the ".txt"
extension, you can simply surround the name of the file in quotation marks
when you save the file.
|
-
Compile
the Source Code
So far you
have created a .java source code file. Your computer does not
understand your source code so it has to be translated into code that your
computer understands. This translation process is called
compiling.
When you
compile your source code it is changed into
bytecode and stored in a .class file. Every time you run
your compiled code, the JVM (Java Virtual Machine) reads your bytecode from
your .class file and translates this code into machine code that your
computer can understand and run the code.
Now lets compile your source code.
To compile your HelloWorld.java
source code, open the command
prompt window and change to your "HelloWorld"
directory.

Note:
-- If you are unsure about DOS commands then see the DOS
page
There
are also details on the cmdPromptHere utility on the Utilities
page.

|
At the command prompt type;-
javac HelloWorld.java
This will produce a ".class"
file named "HelloWorld.class".
If all goes well the system should
pause for a minute and then return you to the prompt. Now type the
following;-
dir
You should get the following listing
>HelloWorld.java
HelloWorld.class
You are now ready to run your first
application!!!!

Note:
-- If you get
any errors check your HelloWorld.java
source code to make sure it looks exactly as written above.
If
you get the error that javac was not found then your PATH
variable has not been set
To
fix this check out how to modify
your PATH.

|
-
Run
the HelloWorld Application
At the command prompt window type:-
java HelloWorld
You should see the following
Hello World!
If this is the case, congratulations!
- You have just written your first Java
application
Now follow the link for an explanation
of the Hello
World Application.
Creating
Your First Applet
As
mentioned earlier, applets are designed to run in a web browser, rarely can they
be run as stand-alone applications. We will be using both the Java AppletViewer
tool and a browser to run an applet that you create.
In
this exercise, we are going to create a HelloWorld
applet. This will be similar to the HelloWorld
application you wrote but will run
in a browser or AppletViewer
-
Create
a Directory for your Program Files
Create a
directory called "HelloWorldApplet".
-
Write
the Source Code
The next thing to do is to write the
source code for your applet.
The procedure for compiling and running
applets is very similar to that for applications.
The first thing you must do is write
your ".java"
source file.
Using
your favorite text editor such as Notepad or WordPad type in the following
exactly. Double-check your capitalization - Java is case-sensitive.
/*
The HelloWorldApplet class implements an applet that
simply displays "Hello World!".
*/
import java.applet.Applet;
import java.awt.Graphics;
public class HelloWorldApplet extends Applet {
public void paint(Graphics g) {
g.drawString("Hello World!",0,50);
}
}
|
Since
the name of our Java source
file must be the same as the name of the class followed by the ".java"
extension the file must be saved as;-
HelloWorldApplet.java
Save the source file
you have created in your
"HelloWorldApplet"
directory with this name.
-
Compile
the Source Code
Open the command prompt window and
change to your "HelloWorldApplet"
directory.
The compilation
step is identical to compiling a Java
application source file. So at the DOS command prompt
you must give the following command:
javac HelloWorldApplet.java
This will produce a ".class"
file named "HelloWorldApplet.class".
This contains the byte code that your browser or
AppletViewer
will use to execute your applet.
If all goes well the system should
pause for a minute and then return you to the prompt. Now type the
following;-
dir
You should get the following listing
>HelloWorldApplet.java
HelloWorldApplet.class
You are now ready to run your first
applet!
-
Create
a HTML File
Applets
are meant to be included in HTML pages. this means you must create a HTML file
that contains a reference to your applet. The reference to your applet
is created using an <Applet>tag which will run the applet you have
just compiled.
Using
the <APPLET> tag, you specify (at a minimum) the location
of the applet and the dimensions of the applet's onscreen display
area. When a Java-capable browser encounters an <APPLET>
tag, it reserves onscreen space for the applet, loads the Applet into
the browser.
Thus the tag must look something like this:
<applet code="HelloWorldApplet.class"
width=100 height=100>
</applet>
|

Note:
-- The applet tag is case sensitive. So if you have used
capital letters in the name of your class, you must use capital letters in the
code tag.

|
Using
your favorite text editor such as Notepad or WordPad type in the following
exactly.
<HTML>
<HEAD>
<TITLE> My First Applet </TITLE>
</HEAD>
<BODY>
Here is the output of my program:
<APPLET CODE="HelloWorldApplet.class" WIDTH=250 HEIGHT=125>
</APPLET>
</BODY>
</HTML> |
Save
your HTML file into the same directory as your applet files. That is to
your "HelloWorldApplet"
directory.
You
can name it anything you like but "HelloWorldApplet.html"
seems appropriate.
Now
you have should have three files in this directory.
HelloWorldApplet.java
Your source code
HelloWorldApplet.class Your
compiled code
HelloWorldApplet.html
Your HTML code
|
Finally, to run your applet you
can open the HTML file in
a browser or run the AppletViewer from the DOS command line (preferred):
-
Run
the Applet using a Browser
Using
Windows "My Computer" navigate to your HelloWorldApplet
directory and double-click your HTML file. The browser should load and display
your applet
-
Run
the Applet using a AppletViewer
You can use the
AppletViewer tool that comes with the JDK software to view your applets.
One reason for using this tool is that there is less overhead than using a
browser. The applet is loaded more quickly and so is useful in the code
development stage.
Below is a screenshot of
my HelloWorld
applet running.

At the command prompt type
appletviewer
HelloWorldApplet.html
You should see Hello
World! displayed in the
AppletViewer.
If this is the case, congratulations!
- You have just written your first Java
applet!
Now follow the link for an explanation
of the Hello
World Applet.
|