"Tutorial" A Cup of Java -- With Two Lumps of Linux

Questions regarding this article should be directed to the author at jc_cann@ix.netcom.com

This is a tutorial article about locating, installing, and using the Java Development Kit (JDK) on Linux. It is intended for Linux developers; however, many aspects discussed are directly applicable to other Unix operating systems. You should have experience using development tools and some Linux system administra tion.

Java on Linux

Javasoft, the spin-off company of Sun Microsystems, Inc., has produced the JDK to run only on Solaris, Microsoft Windows (32-bit release), and Macintosh operating systems. Javasoft has left the porting work up to licensees who obtain the JDK specifications from Javasoft. The Blackdown Organization is responsible for the JDK port to Linux.

The Blackdown Organization started as an Internet service provider that currently houses the java-linux mailing lists and JDK/Linux archives. They provide the ``official'' port of Javasoft's JDK for Linux. The initial porting work was performed by Karl Asha, founder of the Blackdown organization. He continues to provide support and bug fixes for the JDK, in addition to maintenance of the Blackdown Web site. There have been many contributors to the JDK Linux port, most recently Randy Chapman and now Steve Byrne.

Information about the JDK for Lin ux may be found at Blackdown's Java Linux Page (<URL:http://www.blackdown.org/java-linux.html>). You cannot download the JDK from this server, but you can obtain a list of Ftp mirror sites that contain the JDK. In addition, this site contains recent news on the Linux JDK, including latest release notes and bug fixes.

Java Development Kit

The JDK is a comprehensive suite of tools that one can use to produce Java applications or applets. The JDK includes the following:

Java Tools is an impressive suite of development tools. The extensive HTML documentation is thorough, well-written, and useful. Overall, you have literally everything you need to produce sof tware written in Java.

Like all development tools, the JDK tools are designed to allow the developer to minimize time spent peforming routine tasks. Unlike many development tools, the JDK tools take this time-saving premise one step further. For example, the

javadoc

tool will generate HTML pages of documentation from notes in your Java source code. It will document the public and protected classes, interfaces, constructors, methods, and fields. In addition, you can denote documentation in your Java source by using the

/**

delimiter.

Javadoc

will look for this delimiter and include your comments. In a production environment, this tool alone could save many programmers hours of code research.

The

javap

tool will disassemble class files. Command-line options direct the verbosity of the output. For example, you can choose to output only the public members of the class. Included are command-line options for private, data-type signature, an d even the disassembled Java byte code for the class methods.

Another interesting tool is the

javah

, a C-header and stub-file generator. It will allow C code and Java code to interact directly by building C header files based on Java source. This can be used to implement native methods. The C structure definition in the header (

.h

) file contains the corresponding layout of the Java class. The structure fields correspond to the Java class instance variables.

The following table contains a complete description of each component of the JDK:

JDK Development Component	Description
Java Run Time	Core classes ( `classes.zip`) Java byte code interpreter
Java Source Files	Code for public classes in `classes.zip`. Inc luded for instructional / demonstration purposes.
Java Tools	Java Interpreter -- `java` Java Compiler -- `javac` Java Appletviewer -- `appletviewer` Java Debugger -- `jdb` Class File Disassembler -- `javap` Java Documentation generator -- `javadoc` C File Stub Generator -- `javah` Java Archive Tool -- `jar` Digital Signing Tool -- `javakey` Native to ASCII -- `native2ascii` Java RMI Stub Converter -- `rmic` Java Remote Object Registry -- `rmiregistry` Serial Version Command -- `serialver` AWT 1.1 Conversion Tool -- `updateAWT` Various C libraries
Demonstration Applets	A full set of nifty applets that you can morph for your own Web applications.
Installation notes	Ex tensive notes for Solaris and Microsoft Windows (32-bit). Linux JDK includes brief README to describe installation.
Using Appletviewer	Appletviewer is used to view running Java applets. It removes the problems associated with developing applets using a proprietary Web browser.
Using Java Debugger	Java debugger is a symbolic-line debugger, similar to GDB or DBX.

Installing the JDK

Although the JDK is contained in one archive file, there are two other files that you should consider downloading to round out the development system. Again, refer to the Blackdown Organization Java Linux Page to find the nearest Ftp site for downloading these archives.

[ Editor's note: Of course, newer releases become available as Java evolves. At the time this article is written, Linux 1.1.3 J DK is just becoming available, with 1.1.4 coming soon.]

Running Java Programs

One aspect of the JDK is its simple elegance. After the JDK archive is installed, you need only to set the

CLASSPATH

environmental variable and then you are ready to use the Java tools. One enhancement in the JDK release 1.1 over the JDK release 1.0 is that if you install the JDK into

/usr/local/jdk.1.1.1

, then you do not even need to set the

CLASSPATH

variable.

The

CLASSPATH

environmental variable sets the path to the

classes.zip

file. On other Unix operating systems, such as Solaris, this file is

classes.so

. If you installed t he jdk into


/usr/local/jdk1.1.1

, then the

classes.zip

file resides in the

/usr/local/jdk1.1.1/lib

In addition to the

CLASSPATH

variable, you may wish to place the path to the Java binaries:

/usr/local/jdk1.1.1/bin

in your

PATH

environmental variable.

Note: If you have previously installed the jdk1.0.2, you must unset your

CLASSPATH

variable.

Running a Java Applet

Applets are Java programs that follow a set of conventions so they can be run by a Web browser.

Applets are distinguished from application programs in the class definition. All applets must be a subclass of the Applet class. This allows custom applets to inherit methods from the Applet superclass. In the ``Hello World!'' applet below, the import of the

java.applet.Applet

class file in line 1 will allow the applet code to inherit and reference the Applet class methods.

HelloWorld Applet

Writing and compiling Java code is discussed in the next section Running a Java Application . The demonstration applets provided with the JDK will be used to illustrate how to run an applet.

When attempting to run the demonstration HTML pages in a Netscape 3.0 browser, Netscape reported an error depicted in this image:

The

java_301

is a zipped file that contains Java classes. After installing it into the correct location, Netscape still showed the same error.

Netscape 3.0 only supports JDK 1.0.2. Thus, if you want to develop, test, and distribute Java applets that will run in Netscape 3.0, you should either compile your Java applets to be compatible with the JDK 1.0.2 (using the JDK 1.1.1) or retrieve the JDK 1.0.2. However, programmers should note the presence of the

@deprecated

comment in classes, fields, or methods. This is designed to signal which JDK 1.1.1 methods have superceded those in JDK 1.0. In the following code fragment the public method

preferredSize

is replaced by

getPreferredSize

Note that the

@deprecated

flag is found in the comment section of a class. In addition, note the

@see

flag. This instructs the programmer to use the noted method as a replacement.

When the

javac

compiler finds the

@deprecated

flag in the comment section of a class, it will place a "Deprecated" flag in the output classfile. When a deprecated feature is used, the compiler will issue a warning.

Netscape's upgrade of Navigator has been rolled into the Communicator application suite. In a press release dated 10 Oct 1997, Communicator beta release 2 supports 95% of the JDK 1.1.1 features. When Communicator is in production, the JDK 1.1.1 is to be fully supported.

Javasoft provides the Appletviewer in lieu of a Web browser. The Appletviewer accepts an HTML page as input and will process the applet denoted by the

<APPLET>

HTML tag:

To use the Appletviewer to view (and play) a game of TicTacToe, you invoke the viewer in an xterm X Window System window.

Note: Be sure that the directory path to the Appletviewer executable is in your shell's

PATH

environment variable.

Interestingly, the TicTacToe game has sound associated with the moves. Luckily, I had my speakers turned on! When I finally beat the applet, and won a game, it said ``Yahoo!''

At the top of the applet, there is one pulldown menu labeled applet, which has several options:

There are loads of interesting applets to demo. The source code for each class inside a demonstration applet is provided to peruse. These examples prove useful and interesting to see how Java is used.

The

awt-1.1

examples are also Included in the

/usr/local/jdk1.1.1/demos

directory. AWT (Abstract Window Toolkit) is part of the Java API that provides methods common to GUI interfaces. For example, the java.awt.Button class creates a labeled button in your applet. T he AWT is documented in the

/usr/local/jdk1.1.1/docs/api/packages.html

file.

Running a Java Application

Java applications differ from Java applets in that they do not require a Web browser to execute. Also, they need not inherit the Applet class. Java applications are more flexible because they are stand alone. All you need to run a Java application are the application class files and the Java Run-time Environment (JRE).

If you have worked with C or C++ tools, compiling a Java application will be familiar. The code listing below is the application equivalent of the "Hello World!" applet. This program will print "Hello World!" to the standard output.

HelloWorld Application

Use your favorite editor to enter the code for the HelloWorldApp class shown above. Save the file as

HelloWorldApp.java

. A public class, here HelloWorldApp, must be defined in a file named after that class, as in

HelloWorldApp.java

. The

.java

file-name extension is similar to a

.c

used in C or

.cpp

in C++.

To compile the

HelloWorldApp.java

file, execute the

javac

compiler at a shell prompt. (Be sure that the location of the

jdk1.1.1/bin

directory is in your shell's command search path.) The out put is an executable file, named

HelloWorldApp.class

To run the the HelloWorldApp, you invoke the

java

command at a command prompt, omitting the

.class

extension.

Resources

The documentation included with the demonstration applets is an excellent source of information. There are HTML manual pages to describe usage of the Java tools included in the JDK. If you installed the doc+demo archive into

/usr/local/jdk1.1.1

, the JDK 1.1.1 documentation HTML index page will be located at:

/usr/local/jdk/docs/index.html

. This is a great resource page that has links to other local documentation. In addition, links to Javasoft and the Java tutorial are included.

Summary

In total, the JDK 1.1.1 is a solid development environment. The example code is well written, documented, and illustrative. The suite of development tools are extensive and sophisticated. To fully appreciate all the efforts of the Javasoft development team, you could spent many hours learning about Java.

Some developers who have used GUI Integrated Development Environments (IDEs), may find the character-based tools primitive. It is interesting that given that most GUI development environments are written as IDE interfac es, Javasoft does not showcase the JDK development environment.

Acronyms

Author Biography

Jeffery Cann is an IVR (Interactive Voice Response) programmer in Denver, Colorado. He works on Unix in C and Oracle PL-SQL. Looking to bolster his knowledge of object-oriented languages, he has turned to Java to determine if it is applicable to asynchronous telephony applications.