Compiling Java Programs

We'll assume you've written a program in Java and you're ready to compile it. The name you've given the saved file is myprogram.java. The following will compile your program:

javac myprogram.java

The output will be myprogram.class

Executing Java Programs

If you want to execute the program you just compiled, type:

java myprogram

Notice that when you told it to run the program you left the .class off of the filename.

Creating JAR Files

A JAR (Java ARchive) file is a collection of Java files that has been combined into a single file (it also compresses it). This makes it easy to store and redestribute. To create a JAR, go into the directory where all your .class files are and type:

jar cf myjarfile.jar *.class

This is saying to take all my .class files and insert them into a new JAR:

jar cf myjarfile.jar file1.class file2.class file3.class

This is saying to take those three class files and insert them into a new JAR.

Extracting files from a JAR

The following will "uncompress" the JAR file:

jar xf myjarfile.jar

Configuring your CLASSPATH

The CLASSPATH is an environmental variable that tells the Java compiler where to look for class files to import or where to find class files to interpret. If you do medium to large Java programming projects you'll probably want to edit this variable. You can set this just like you do your PATH variable. Here is what my CLASSPATH variable is set to:

export CLASSPATH="/opt/jakarta/tomcat/common/lib/servlet.jar:/opt/sun-j2ee-1.3.1/lib/j2ee.jar:/home/emcnabb/postgresql.jar:."

For more information on how to set these variables permanently in your shell, read up on the bash document.

If you want to include class files without actually changing the environmental variables you can do the following:

java --classpath .:class1:class2 myprogram

See the java man page for more details.

Using Java with Makefiles

If you've used Makefiles in C/C++, you might be wishing you could do the same thing with Java. The Makefile HOWTO explains how to do this.

Using JDB, the Java Debugger

You'll run into bugs that will be hard to find with just println's; that's when debuggers come in handy. The debugger packaged with Sun's Java is JDB. We won't go into detail here about using it, but there are many HOWTOs that will help out.

• Debug with jdb (Javaworld)
• jdb: The Java Debugger (Sun)
• Java Developers Reference

Also read the JDB manpage.

Man Pages

For specific information on the java, javac, jar, and jdb commands use the man (manual) pages. Just type the following on the command line:

man java

or:

man javac

or:

man jar

or:

man jdb