The Java programming language is a high-level language that can be characterized by all of the following buzzwords:

Each of the preceding buzzwords is explained in The Java Language Environment,^[1] a white paper written by James Gosling and Henry McGilton.

In the Java programming language, all source code is first written in plain text files ending with the .java extension. Those source files are then compiled into .class files by the javac compiler. A .class file does not contain code that is native to your processor; it instead contains bytecodes—the machine language of the Java Virtual Machine (Java VM).^[2] The java launcher tool then runs your application with an instance of the Java Virtual Machine (Figure 1.1).

Figure 1.1. Compiling and running an application.

Because the Java VM is available on many different operating systems, the same .class files are capable of running on Microsoft Windows, the Solaris Operating System (Solaris OS), Linux, or Mac OS. Some virtual machines, such as the Java HotSpot virtual machine,^[3] perform additional steps at runtime to give your application a performance boost. This includes various tasks such as finding performance bottlenecks and recompiling (to native code) frequently used sections of code (Figure 1.2).

Figure 1.2. Through the Java VM, the same application is capable of running on multiple platforms.

A platform is the hardware or software environment in which a program runs. We’ve already mentioned some of the most popular platforms like Microsoft Windows, Linux, Solaris OS, and Mac OS. Most platforms can be described as a combination of the operating system and underlying hardware. The Java platform differs from most other platforms in that it’s a software-only platform that runs on top of other hardware-based platforms.

The Java platform has two components:

You’ve already been introduced to the Java Virtual Machine; it’s the base for the Java platform and is ported onto various hardware-based platforms (Figure 1.3).

Figure 1.3. The API and Java Virtual Machine insulate the program from the underlying hardware.

The API is a large collection of ready-made software components that provide many useful capabilities. It is grouped into libraries of related classes and interfaces; these libraries are known as packages. The next section highlights some of the functionality provided by the API.

As a platform-independent environment, the Java platform can be a bit slower than native code. However, advances in compiler and virtual machine technologies are bringing performance close to that of native code without threatening portability.

To write your first program, you’ll need:

Your first application, HelloWorldApp, will simply display the greeting “Hello World!”. To create this program, you will:

Create an IDE Project

To create an IDE project:

1. Launch the NetBeans IDE.

   • On Microsoft Windows systems, you can use the NetBeans IDE item in the Start menu.

   • On Solaris OS and Linux systems, you execute the IDE launcher script by navigating to the IDE’s bin directory and typing ./netbeans.

   • On Mac OS X systems, click the NetBeans IDE application icon.

2. In the NetBeans IDE, choose File | New Project (Figure 1.4).

   

   Figure 1.4. NetBeans IDE with the File | New Project menu item selected.

3. In the New Project wizard, expand the General category and select Java Application (Figure 1.5).

   

   Figure 1.5. NetBeans IDE, New Project wizard, Choose Project page.

4. In the Name and Location page of the wizard, do the following (Figure 1.6):

   • In the Project Name field, type Hello World App.

   • In the Create Main Class field, type helloworldapp.HelloWorldApp.

   • Leave the Set as Main Project checkbox selected.

   

   Figure 1.6. NetBeans IDE, New Project wizard, Name and Location page.

5. Click Finish.

The project is created and opened in the IDE. You should see the following components:

• The Projects window, which contains a tree view of the components of the project, including source files, libraries that your code depends on, and so on.

• The Source Editor window with a file called HelloWorldApp open.

• The Navigator window, which you can use to quickly navigate between elements within the selected class (Figure 1.7).

Figure 1.7. NetBeans IDE with the HelloWorldApp project open.

// TODO code application logic here

System.out.println("Hello World!"); // Display the string.

/**
 *
 * @author
 */

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to standard output.
 */

/*
 * HelloWorldApp.java
 *
 * Created on February 5, 2006, 6:43 PM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */

package helloworldapp;

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to standard output.
 */
public class HelloWorldApp {

  /** Creates a new instance of HelloWorldApp */
  public HelloWorldApp() {
  }

  /**
   * @param args the command line arguments
   */
  public static void main(String[] args) {
    System.out.println("Hello World!"); // Display the string.
  }
}

Compile the Source File into a .class File

To compile your source file, choose Build | Build Main Project from the IDE’s main menu.

The Output window opens and displays output similar to what you see in Figure 1.8.

Figure 1.8. Output window showing results of building the HelloWorld project.

If the build output concludes with the statement BUILD SUCCESSFUL, congratulations! You have successfully compiled your program!

If the build output concludes with the statement BUILD FAILED, you probably have a syntax error in your code. Errors are reported in the Output window as hyper-linked text. You double-click such a hyper-link to navigate to the source of an error. You can then fix the error and once again choose Build | Build Main Project.

When you build the project, the bytecode file HelloWorldApp.class is generated. You can see where the new file is generated by opening the Files window and expanding the Hello World App/build/classes/helloworldapp node as shown in Figure 1.9.

Figure 1.9. Files window, showing the generated .class file.

Now that you have built the project, you can run your program.

Run the Program

From the IDE’s menu bar, choose Run | Run Main Project.

Figure 1.10 shows what you should now see.

Figure 1.10. The program prints “Hello World!” to the Output window (along with other output from the build script).

Congratulations! Your program works!

The section “A Closer Look at the “Hello World!” Application” on page 24 will explain the code in this simple application. Although the rest of the tutorial does not give specific instructions about using the NetBeans IDE, you can easily use the IDE to write and run the sample code. The following are some tips on using the IDE and explanations of some IDE behavior that you are likely to see:

To write your first program, you’ll need:

These two items are all you’ll need to write your first application.

Your first application, HelloWorldApp, will simply display the greeting “Hello World!”. To create this program, you will:

Create a Source File

To create a source file, you have two options:

1. You can save the file HelloWorldApp.java^[9] on your computer and avoid a lot of typing. Then, you can go straight to the Compile the Source File into a .class File section (page 16).

2. Or, you can use the following (longer) instructions.

First, start your editor. You can launch the Notepad editor from the Start menu by selecting Programs > Accessories > Notepad. In a new document, type in the following code:

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to 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. Both the compiler (javac) and launcher (java) are case-sensitive, so you must capitalize consistently. In other words, HelloWorldApp is not equivalent to helloworldapp.

Save the code in a file with the name HelloWorldApp.java. To do this in Notepad, first choose the File > Save As menu item. Then, in the Save As dialog box:

1. Using the Save in combo box, specify the folder (directory) where you’ll save your file. In this example, the directory is java on the C drive.

2. In the File name text field, type "HelloWorldApp.java", including the quotation marks.

3. From the Save as type combo box, choose Text Documents (*.txt).

4. In the Encoding combo box, leave the encoding as ANSI.

When you’re finished, the dialog box should look like Figure 1.11.

Figure 1.11. The Save As dialog just before you click Save.

Now click Save, and exit Notepad.

Compile the Source File into a .class File

Bring up a shell, or “command,” window. You can do this from the Start menu by choosing Command Prompt (Windows XP), or by choosing Run . . . and then entering cmd. The shell window should look similar to Figure 1.12.

Figure 1.12. A shell window.

The prompt shows your current directory. When you bring up the prompt, your current directory is usually your home directory for Windows XP (as shown in Figure 1.12).

To compile your source file, change your current directory to the directory where your file is located. For example, if your source directory is java on the C drive, type the following command at the prompt and press Enter:

cd C:java

Now the prompt should change to C:java>.

Note

To change to a directory on a different drive, you must type an extra command: the name of the drive. For example, to change to the java directory on the D drive, you must enter D:, as shown in Figure 1.13.

Figure 1.13. Changing directory on an alternate drive.

If you enter dir at the prompt, you should see your source file, as Figure 1.14 shows.

Figure 1.14. Directory listing showing the .java source file.

Now you are ready to compile. At the prompt, type the following command and press Enter.

javac HelloWorldApp.java

The compiler has generated a bytecode file, HelloWorldApp.class. At the prompt, type dir to see the new file that was generated, as shown in Figure 1.15.

Figure 1.15. Directory listing showing the generated .class file.

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

Run the Program

In the same directory, enter the following command at the prompt:

java HelloWorldApp

Figure 1.16 shows what you should now see.

Figure 1.16. The program prints “Hello World!” to the screen.

Congratulations! Your program works!

To write your first program, you’ll need:

These two items are all you’ll need to write your first application.

Your first application, HelloWorldApp, will simply display the greeting “Hello world!”. To create this program, you will:

Create a Source File

To create a source file, you have two options:

1. You can save the file HelloWorldApp.java^[12] on your computer and avoid a lot of typing. Then, you can go straight to the Compile the Source File into a .class File section (page 22).

2. Or, you can use the following (longer) instructions.

First, open a shell, or “terminal,” window (Figure 1.17).

Figure 1.17. A new terminal window.

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 source 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, use the following commands:

cd
mkdir java

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

cd java

Now you can start creating your source file.

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.

Type the following code into the new buffer:

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to 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. Both the compiler (javac) and launcher (java) are case-sensitive, so you must capitalize consistently. In other words, HelloWorldApp is not equivalent to helloworldapp.

Save the code in a file with the name HelloWorldApp.java. In the Pico editor, you do this by typing Ctrl-O and then, at the bottom where you see the prompt File Name to write:, entering the directory in which you wish to create the file, followed by HelloWorldApp.java. For example, if you wish to save HelloWorldApp.java in the directory /home/jdoe/java, then you type /home/jdoe/java/HelloWorldApp.java and press Return.

You can type Ctrl-X to exit Pico.

Compile the Source File into a .class 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/jdoe/java, type the following command at the prompt and press Return:

cd /home/jdoe/java

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

If you enter ls at the prompt, you should see your file (Figure 1.18).

Figure 1.18. Results of the ls command, showing the .java source file.

Now are ready to compile the source file. At the prompt, type the following command and press Return.

javac HelloWorldApp.java

The compiler has generated a bytecode file, HelloWorldApp.class. At the prompt, type ls to see the new file that was generated: Figure 1.19.

Figure 1.19. Results of the ls command, showing the generated .class file.

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

Run the Program

In the same directory, enter at the prompt:

java HelloWorldApp

Figure 1.20 shows what you should now see.

Figure 1.20. The output prints “Hello World!” to the screen.

Congratulations! Your program works!

'javac' is not recognized as an internal or external command,
operable program or batch file

If you receive this error, Windows cannot find the compiler (javac).

Here’s one way to tell Windows where to find javac. Suppose you installed the JDK in C:jdk6. At the prompt you would type the following command and press Enter:

C:jdk6injavac HelloWorldApp.java

If you choose this option, you’ll have to precede your javac and java commands with C:jdk6in each time you compile or run a program. To avoid this extra typing, consult the Update the PATH variable section^[14] in the JDK 6 installation instructions.

javac: Command not found

If you receive this error, UNIX cannot find the compiler, javac.

Here’s one way to tell UNIX where to find javac. Suppose you installed the JDK in /usr/local/jdk6. At the prompt you would type the following command and press Return:

/usr/local/jdk6/javac HelloWorldApp.java

If you mistype part of a program, the compiler may issue a syntax error. The message usually displays the type of the error, the line number where the error was detected, the code on that line, and the position of the error within the code. Here’s an error caused by omitting a semicolon (;) at the end of a statement:

testing.java:14: ';' expected.
System.out.println("Input has " + count + " chars.")
                                                      ^
1 error

Sometimes the compiler can’t guess your intent and prints a confusing error message or multiple error messages if the error cascades over several lines. For example, the following code snippet omits a semicolon (;) from the bold line:

while (System.in.read() != -1)
    count++
System.out.println("Input has " + count + " chars.");

When processing this code, the compiler issues two error messages:

testing.java:13: Invalid type expression.
        count++
                 ^
testing.java:14: Invalid declaration.
    System.out.println("Input has " + count + " chars.");
                      ^
2 errors

The compiler issues two error messages because after it processes count++, the compiler’s state indicates that it’s in the middle of an expression. Without the semicolon, the compiler has no way of knowing that the statement is complete.

If you see any compiler errors, then your program did not successfully compile and the compiler did not create a .class file. Carefully verify the program, fix any errors that you detect, and try again.

In addition to verifying that your program is syntactically correct, the compiler checks for other basic correctness. For example, the compiler warns you each time you use a variable that has not been initialized:

testing.java:13: Variable count may not have been initialized.
        count++
        ^
testing.java:14: Variable count may not have been initialized.
    System.out.println("Input has " + count + " chars.");
                                       ^
2 errors

Again, your program did not successfully compile, and the compiler did not create a .class file. Fix the error and try again.

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 .class file is your current directory. So if your .class file is in C:java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Enter:

cd c:java

The prompt should change to C:java>. If you enter dir at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

CLASSPATH If you still have problems, you might have to change your CLASSPATH variable. To see if this is necessary, try clobbering the classpath with the following command:

set CLASSPATH=

Now enter java HelloWorldApp again. If the program works now, you’ll have to change your CLASSPATH variable. To set this variable, consult the Update the PATH variable section in the JDK 6 installation instructions. The CLASSPATH variable is set in the same manner.

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/jdoe/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/jdoe/java

If you enter pwd at the prompt, you should see /home/jdoe/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 environment variable. To see if this is necessary, try clobbering 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 in the same manner as the PATH variable.

A common error of beginner programmers is to try and run the java launcher on the .class file that was created by the compiler. For example, if you try to run your program with java HelloWorldApp.class instead of java HelloWorldApp, you’ll see this error message:

Exception in thread "main" java.lang.NoClassDefFoundError:
HelloWorldApp/class

The argument is the name of the class that you want to use, not the filename.

The Java Virtual Machine requires that the class you execute with it have a main method at which to begin execution of your application. The A Closer Look at the “Hello World!” Application section (page 24) discusses the main method in detail. If you are missing this method, you’ll see the following error at runtime:

Exception in thread "main" java.lang.NoSuchMethodError: main