Your First Cup of Java (for UNIX)

Detailed Instructions
for Your First Program

The following instructions will help you write your first Java program. These instructions are for users of UNIX-based platforms, including Linux and Solaris.

1. A Checklist 2. Creating Your First Application
     a. Create a Java Source File
     b. Compile the Source File
     c. Run the Program
3. Creating Your First Applet


1. A Checklist  

To write your first program, you will need:
  1. The JavaTM 2 Platform, Standard Edition.
  2. A text editor. In this example, we'll use Pico, an editor available for many UNIX-based platforms. You can easily adapt these instructions if you use a different text editor, such as VI or Emacs.
These two items are all you need to write your first Java program.

2. Creating Your First Application
 
Why Bytecodes are Cool

So, you've heard that with the Java programming language, you can "write once, run anywhere." This means that when you compile your program, you don't generate instructions for one specific platform. Instead, you generate Java bytecodes, which are instructions for the Java Virtual Machine (Java VM). If your platform--whether it's Windows, UNIX, MacOS, or an Internet browser--has the Java VM, it can understand those bytecodes.

 
Your first application, HelloWorldApp, will simply display the greeting "Hello world!". To create this program, you will: 
  • Create a Java source file. A source file contains text, written in the Java programming language, that you and other programmers can understand. You can use any text editor to create and edit source files.


  • Compile the source file into a bytecode file. The Java compiler, javac, takes your source file and translates its text into instructions that the Java Virtual Machine (Java VM) can understand. The compiler puts these instructions into a bytecode file.


  • Run the program contained in the bytecode file. The Java VM is implemented by a Java interpreter, java. This interpreter takes your bytecode file and carries out the instructions by translating them into instructions that your computer can understand. 
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a. Create a Java Source File.

You have two options:

  • You can save the file HelloWorldApp.java(in a .java source file) on your computer and avoid a lot of typing. Then, you can go straight to step b.

  • Or, you can follow these (longer) instructions:

1. Bring up a shell (sometimes called terminal) window. When the window comes up, it should look like this:

When you first bring up the prompt, your current directory will usually be your home directory. You can change your current directory to your home directory at any time by typing cd at the prompt and then pressing Return.

The Java files you create should be kept in a separate directory. You can create a directory by using the command mkdir. For example, to create the directory java in your home directory, you would first change your current directory to your home directory by entering the following command:

 cd 
Then, you would enter the following command:
 mkdir java 

To change your current directory to this new directory, you would then enter:

 cd java 

Now you can start creating your source file.

2. Start the Pico editor by typing pico at the prompt and pressing Return. If the system responds with the message pico: command not found, then Pico is most likely unavailable. Consult your system administrator for more information, or use another editor.

When you start Pico, it'll display a new, blank buffer. This is the area in which you will type your code.

 
Pico? VI? Emacs?

Pico is probably the easiest of the three editors to use. If you're curious about how to use the other editors, however, check out these handy reference cards for VI(outside of the tutorial) and Emacs(outside of the tutorial) .

3. Type the following code into the new buffer: 
/** 
 * The HelloWorldApp class implements an application that
 * simply displays "Hello World!" to the standard output.
 */
class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); //Display the string.
    }
}
 Be Careful When You Type

Type all code, commands, and file names exactly as shown. The Java compiler and interpreter are case-sensitive, so you must capitalize consistently.

HelloWorldApp helloworldapp

4. Save the code by typing Ctrl-O. At the bottom, you will see the prompt File Name to write:. Enter HelloWorldApp.java, preceded by the directory in which you wish to the create the file. For example, if you wish to save HelloWorldApp.java in the directory /home/rortigas/java, then you would type /home/rortigas/java/HelloWorldApp.java and press Return.

You can type Ctrl-X to exit Pico.

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b. Compile the Source File.

Bring up another shell window. To compile your source file, change your current directory to the directory where your file is located. For example, if your source directory is /home/rortigas/java, you would type the following command at the prompt and press Return:

 cd /home/rortigas/java 

If you enter pwd at the prompt, you should see the current directory, which in this example has been changed to /home/rortigas/java.

If you enter ls at the prompt, you should see your file.

Now you can compile. At the prompt, type the following command and press Return:
 javac HelloWorldApp.java 
If your prompt reappears without error messages, congratulations. You have successfully compiled your program.
Error Explanation javac: Command not found If you receive this error, UNIX cannot find the Java compiler, javac. Here's one way to tell UNIX where to find javac. Suppose you installed the Java 2 Software Development Kit in /usr/local/jdk1.2.2. At the prompt you would type the following command and press Return:  
/usr/local/jdk1.2.2/javac HelloWorldApp.java
Note: If you choose this option, each time you compile or run a program, you'll have to precede your javac and java commands with /usr/local/jdk1.2.2/.
The compiler has generated a Java bytecode file, HelloWorldApp.class. At the prompt, type ls to see the new file that was generated: 

Now that you have a .class file, you can run your program.

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c. Run the Program.

In the same directory, enter at the prompt:
 java HelloWorldApp 

Now you should see:

Congratulations! Your program works. 

Error Explanation Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp If you receive this error, java cannot find your bytecode file, HelloWorldApp.class. One of the places java tries to find your bytecode file is your current directory. So, for example, if your bytecode file is in /home/rortigas/java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Return: 
cd /home/rortigas/java

If you enter pwd at the prompt, you should see /home/rortigas/java. If you enter ls at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH variable. To see if this is necessary, try "unsetting" the classpath with the following command: 
unset CLASSPATH
Now enter java HelloWorldApp again. If the program works now, you'll have to change your CLASSPATH variable.
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