Compiling the Program into a Class File

Before you can try out the program, it must be compiled. The term compile might be unfamiliar to you now, but you will become quite familiar with it in the coming hours. When you compile a program, you take the instructions you have given the computer and convert them into a form the computer can better understand. You also make the program run as efficiently as possible. Java programs must be compiled before you can run them. With the Java Development Kit, programs are compiled using the javac program.

The javac program, like all programs included with the Kit, is a command-line utility. You run the program by using your keyboard to type a command at a command line that can accept the input.

Because most Linux usage is handled at the command line, readers with that operating system will be familiar with how the Kit's programs are used. Anyone who used MS-DOS prior to the introduction of Windows also has used a command line.

Many Windows users might not be aware that their operating system includes a command-line feature of its own: the MS-DOS Command Prompt window.

To get to a command line in Windows:

• On Windows 98 or Me, choose Start, Programs, and then choose MS-DOS Prompt.

• On Windows NT or 2000, choose Start, Programs, Accessories, and then choose Command Prompt.

• On Windows XP or Windows Server 2003, choose Start, All Programs, Accessories, and then choose Command Prompt.

On a Mac, access a command line by choosing Applications, Utilities, Terminal. To compile the Saluton program using the Java Development Kit, go to a command line, open the folder on your system where the Saluton.java file is located, then type the following at the command line:

javac Saluton.java

If the program compiles successfully, a new file called Saluton.class is created in the same folder as Saluton.java. If you see any error messages, refer to the following section, "Fixing Errors." All Java programs are compiled into class files which are given the .class file extension. A Java program can be made up of several classes that work together, but in a simple program such as Saluton only one class is needed.

By the Way Did you ever have a relative, spouse, or other loved one who only says something when things go wrong? (Me neither.) A Java compiler only speaks up when there's an error to complain about. If you compile a program successfully without any errors, nothing happens in response.