Java and Linux

An Extremely Brief Introduction to Java
JDK Installation Procedure
Sample Code
Resource Pointers
Comments on Future Developments

Comes from Sun Microsystems
Was originally called Oak - meant to be a language that controlled home appliances
It is an object-oriented language that follows the tradition of C++ but without it's idiosyncracies
It is designed to run in two modes - as applets and as applications
All code that is compiled into an architecture neutral code called bytecode.
This bytecode gets transported across to the target system and a Java interpreter on that target will execute the Java bytecode
Not much different from other approaches - you can write VB code and ship it across to someone else to execute - the problem being that the target needs a VB interpreter.
Java was designed and developed with the principle aim to make it lightweight, portable, and above all, secure.
What do you need to know as prerequisites before learning Java?
- Programming in C or Pascal, Visual Basic
- Object programming is useful, but not essential
- Understanding of event-based programming

You can get the Java Development Kit from the Linux Java site - www.blackdown.org. It is also available on lugs.org.sg

The latest version is 1.0.1

ftp.infomagic.com:/pub/mirrors/.mirror2/linux/Java/JDK-1.0.1
-r--r--r--   1 ftp-adm  ftp       1701324 Mar 25 10:28 linux.jdk-1.0.1-try1.common.tar.gz
-r--r--r--   1 ftp-adm  ftp        412210 May 20 13:42 linux.jdk-1.0.1-try3.shared-motif.tar.gz
-r--r--r--   1 ftp-adm  ftp         29243 May 20 14:51 linux.jdk-1.0.1-try3.source-diffs.tar.gz
-r--r--r--   1 ftp-adm  ftp       1430351 May 20 13:43 linux.jdk-1.0.1-try3.static-motif.tar.gz

Install it in /usr/local/java

Edit the file /usr/src/linux/fs/binfmt_java.c to show the following:

#define _PATH_JAVA      "/usr/local/java/bin/java"
#define _PATH_APPLET    "/usr/local/java/bin/appletviewer"

Compile your kernel with Java as a built-in feature or as a module. I recommend it being loaded as a module.

Sample Code

Here is what a very simple Java application looks like:

class HelloWorld {
	public static void main(String args[]) {
		System.out.println("Hello World!");
	}
}

You will compile this using the javac command
Make sure your CLASSPATH environment variable is setup properly. Here's an example:
```
export CLASSPATH=/usr/local/bin/moz3_0.zip:/usr/local/java/lib/classes.zip:.
```
Once you compile your Java program (applet or application), you should chmod the *.class to execute mode - chmod +x *.class. The files *.class are the "compiled" versions of the *.java sources.
Then from the command line, you can execute the Java code by typing the filename.

Resource Pointers

www.blackdown.org - Linux/Java port
www.gamelan.com - great examples of code
www.javaworld.com - a magazine devoted to Java
www.jade.org - Java Developer's Organization
www.javasoft.com - Sun's JavaSoft

Comments on Future Developments

Sun's lawyers tried to clarify the licensing of Java. Looks like it is still going to be free.
Java 1.0.2 has been released but the Linux port is not there yet. There are some changes with the Java API dealing mainly with security.

Some Interesting Applets
- LED Sign - LED Sign - http://www.cs.hope.edu/~dbrown/java/LEDSign/WWW/LED.html
- InvestWiz - InvestWiz - http://draco.iss.nus.sg:1024/InvestWiz/ - A local creation rated amongst the top 1% of applets worldwide! Rough around the edges, but still interesting ...
Some Applications
- Client-Side Image Map Editor - Client-Side Image Map Editor - http://miso.wwa.com/~myc/mapframe.html
- Java Contact Book - Java Contact Book - http://www.cc.gatech.edu/gvu/people/qiang.a.zhao/jcontact/ Your standard address book, mostly.