The Linux Development Platform Configuring, Using, and ... - Classes

CH09.fm Page 270 Monday, October 7, 2002 9:18 PM
270 Chapter 9 • Platform Independent Development with Java
9.5 Building Java Applications
A Java application is a compiled Java program that can be executed using a Java virtual machine.
There are three basic steps to create a Java application. These steps are:
1. Create source code.
2. Compile it.
3. Run it using the virtual machine.
Let’s see how you can go through these steps.
9.5.1 Creating Source Code File
Like any other programming language, Java source code files are created using an editor.
These source code files contain Java instructions. The simplest source code file contains an
instruction to print the message “Hello World”, similar to your first C program. Please note that
Java examples in this chapter are just to demonstrate how to use development tools.
The program that prints the “Hello World” message is shown below:
class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
The program must have a function main(), like C programs. The program may be saved
as HelloWorld.java text file.
Note that you can use any editor to create the source code file. I would recommend using
Emacs which you have already learned in Chapter 2. Emacs understands Java programming language
syntax. There are other GUI editors and integrated development environment (IDE) packages
that you can use to create Java source code.
9.5.2 Compiling Java Code
The Java compiler will compile the Java code into an executable. The following command
creates an output file HelloWorld.class from source code file HelloWorld.java.
/usr/java/j2sdk1.4.0/bin/javac HelloWorld.java
Note that the output file name is the same as the name of the class that contains function
main().
You can check the type of the output file type by using the file command. The following
command shows type of the output file as compiled class data.
[root@desktop /root]# file HelloWorld.class
HelloWorld.class: compiled Java class data, version 46.0
[root@desktop /root]#