• CS620: First Java Laboratory
• : Check out New things on the Course Web Page
• : Goals
• : Hints
• : Demonstration 1
• : Demonstration 2
• : Your First Java Program
• : Your Second Java Application
• : A Third Silly Application
• : A Modern Version
• : Your First Applet
• : A Second Applet
• : An Application that runs an Applet!
• : Publishing Your Applet
• : Tell Me
• : Glossary
• Glossary
• Index of defined terms etc.

CS620: First Java Laboratory

Check out New things on the Course Web Page

[ http://www.csci.csusb.edu/dick/cs620/ ]

Goals

Your task is to try some the experiments on this page.

Hints

---

1. Keep your Java handout handy.
2. Put an applet called AaAa in file AaAa.java not in a file called aaaa.java.
3. See [ java.html ] and [ http://csci.csusb.edu/public/faculty/dick/ ]

---

Demonstration 1

Here [ test.Goodbye.html ] is a simple Java applet in an HTML page. The page also has links to the Code I wrote, the documentation, and bytecode that I prepared from it. Follow the links to the Source code, Documentation and Byte Code.

You can use this page as a model for your own examples of Java Applets.

Demonstration 2

Here [ test.Henrici.html ] is a demonstration of something that was easy to program in Java. Have a look at it.... the graphic is produced by the Java applet in the HTML page. You can also follow the two links to the Java source code and to the documentation generated by the source code.

Note. You don't have to understand why this picture is called an Henrici spiral or what the mathematics means. Its just a formula that generates rather nice looking graphics -- enjoy it.

Your First Java Program

Here you will need to have two windows working at one time: One running a browser (say Netscape on your workstation) and the other to edit, compile, and run commands in the Java Development Kit(JDK). The various versions can be listed by inputing the UNIX command into a command/terminal window:

 		ls -ld /share/j*

and

          /share/java

is linked to the last of the reliable ones!

In your browser window look at this:

 	import java.lang.*;

 	public class Hello {

 	    public static void main(String argv[]){

 	        System.out.println("Hello, World!");

 	    }

 	}

It is a traditional first C program written in Java.

Use your favorite editor to create a copy of the program above in a file called.

 		Hello.java

Notice: not hello.java, not h.java, but Hello.java. This is important!

Compile it like this:

 		/share/java1.4.1/bin/javac Hello.java

Note: Do not hold your breath waiting for this to compile, the compiler was very slow when I test it.

---

I theory you should be able to write

 		javac Hello.java

but this did not work for me recently.
1. Make sure your PATH variable has /share/java1.4.1/bin added to it when you next plan to use Java.
2. To add /share/j2sdk1.4.1/bin to your UNIX shell PATH
   Net
   edit your ~/.bash_profile file so that the PATH is defined like this

    			PATH="$PATH:/share/j2sdk1.4.1/bin"

   
   (End of Net)
   This is read whenever you login or open a terminal window.

(End of Net)

1. If the above worked you now have a file called 'Hello.class'. It is a binary file. Do not try editing or listing it!
2. If the 'javac...' doesn't work you may have a typographical error... First check that you did type '.../javac Hello.java' as shown. Then try downloading [ Hello.java ] as Source into a file called "Hello.java" and try again.

---

Run the compiled class like this:

 		/share/java1.4.1/bin/java Hello

---

Notice
1. 'java Hello.java' and 'java Hello.class' do not work!
2. The source is compiled, but a class is interpreted.
3. Java code was designed to be interpreted by a machine independent virtual machine that runs bytecode.
4. bytecode::= See http://www.csci.csusb.edu/dick/samples/java.html#bytecode.

---

Your Second Java Application

Modify your Hello.java program -- using your favorite editor -- to output a message with your name on it. (use the same name for the file and the class) Then try out my quickie or Q program to compile, document and run your program.

 		Q Hello.java

---

I named the program for a character played by John De Lancey in StarTrek: The Next Generation, my quickie program is quirky and powerful. It has information about compiling and interpreting many languages and codes on our servers and workstations.

If your shell reports that 'Q can not be found', then you can


1. Use

    		~dick/bin/Q Hello.java

   instead.
2. Add /share/bin to your UNIX shell PATH inside your .bash_profile

    			PATH="$PATH:/share/bin"




---

A Third Silly Application

This is an application that was used to teach me what a loop was, back in the 1960s. It also shows how Java Input/Output works. The documentation for the "Luv" example is at [ Luv.html ]

Study the documentation a bit before you get your own copy of the source code [ Luv.java ] Compile and test the application.

A Modern Version

These days users expect to point and click rather than input text to answer questions. To see how the previous program looks as an event-driven applet follow this link: [ test.Luv5.html ] While there follow the links to the Code and Documentation.

Your First Applet

Your task is to construct a public WWW page that has an applet that displays some text. The name of the page is:

 		test.HelloWorld.html

and it should contain the following HTML:

 		<head><title>Test Hello World</title></head><body>

          <APPLET CODE="HelloWorld.class" HEIGHT=150 WIDTH=150>

 		You can not see this brilliant Java Applet.

		</APPLET>

 		</body>

You now need a file called:

 		HelloWorld.java

containing

 	import java.applet.*;

 	import java.awt.*;

	public class HelloWorld extends Applet {

	      public void init() {

	          setSize(150,25);

	      }//init

	      public void paint(Graphics g) {

	          g.drawString("Hello world!", 50, 25);

	      }//paint

	  }//HelloWorld

The Applet is compiled just like any other program:

 		 /share/java1.4.1/bin/javac HelloWorld.java

This will generate a file with "HelloWorld.class". Notice that the compiler forces you to name the file

 		HelloWorld.java

and the Class

 		HelloWorld

must be public for it to be used over the World Wide Web.

Alternately you can copy all the *HelloWorld* files to your public directory and view them with Netscape..... BUT make sure it is Netscape 6.* not 4.* on your workstation. Or else try Konqueror or Mozilla (and turn on the Java security and console).

Note
Net

1. Java is supposed to offer: Write Once, Run Anywhere.
2. What you get is: Write once, Debug Everywhere

(End of Net)


A Second Applet

Your next task is to change your HelloWorld.java program to output the result in color by adding

 		g.setColor(Color.blue);

to HelloWorld.java. Compile and then publish to check the result. Make sure you shut netscape down and restart it.... otherwise you'll probably get the OLD cached version:-(

Next change the font in your graphic:

 		g.setFont(new Font("Helvetica", Font.BOLD, 24));

---

1. If Netscape doesn't show your Applet... Look under Netscape's Option menu and set the "Show Java Console" option. This will show any errors and also any System.out messages.

2. Netscape does not make a useful tool when developing Java applets for one simple reason. Netscape does not want to reload your Java Applet's compiled code. The reload button makes no difference! Sometimes a SHift or Control click on the reload button may work. Personally I include a test harness in the class itself, as follows.

---

An Application that runs an Applet!

An application is a class with a 'main' function. An Applet needs a graphic window to run in..... so if we write a main function for HelloWorld that creates a graphic window then it can run the Applet. Get a copy of this: [ HelloWorld.java ] and compile and test it like this:

 		javac HelloWorld.java

 		java HelloWorld

Once this runs ok, change the color and form of the graphic string. Make this run.

---

1. Adding a suitable main function is a viable development technique. However Java does not make it easy to add a function to a class.
2. It is worth keeping 'main' in a special file and learning how to read files into Java code using your favorite editor.

---

Publishing Your Applet

Don't forget that you must always publish the *.class files for all Applets on your web pages... and all the classes that they call. You should always make the documentation available on any class you publish so that others can use it. In this class you also need to publish the source code so that It can be graded! You should have noticed that my favorite technique for Applets is to add links to a published test page that links to the source code.

Edit test.HelloWorld.html so that is has a links to the Source code and documentation after the end of the applet:

 		<a href="HelloWorld.java">Source Code</A>

and publish all the files by copying them all to your public directory:

 		cp *HelloWorld* /www/public/..../

or

 		cp *HelloWorld* /web/public/..../

(depending where your public WWW/Web directory is).

Now use Netscape to look at the page... Follow the links to the code and then to the documentation, and from there to the documentation on String.

Tell Me

Glossary

1. Application::java=a normal program that is executed on its owner's machine.
2. Applet::java=a program that can be sent accross the web and executed inside a web page on a remote user's computer.

. . . . . . . . . ( end of section CS620: First Java Laboratory) <<Contents | Index>>

Glossary