Talk about Java technology seems to be everywhere, but what exactly is it? The following sections explain how Java technology is both a programming language and a platform, and provide an overview of what this technology can do for you.
Java technology is both a programming language and a platform.
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).
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).
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:
The Java Virtual Machine
The Java Application Programming Interface (API)
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).
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.
The general-purpose, high-level Java programming language is a powerful software platform. Every full implementation of the Java platform gives you the following features:
Development Tools. The development tools provide everything you’ll need for compiling, running, monitoring, debugging, and documenting your applications. As a new developer, the main tools you’ll be using are the javac
compiler, the java
launcher, and the javadoc
documentation tool.
Application Programming Interface (API). The API provides the core functionality of the Java programming language. It offers a wide array of useful classes ready for use in your own applications. The core API is very large; to get an overview of what it contains, consult the Java SE Development Kit 6 (JDK 6) documentation.[4]
Deployment Technologies. The JDK software provides standard mechanisms such as the Java Web Start software and Java Plug-In software for deploying your applications to end users.
User Interface Toolkits. The Swing and Java 2D toolkits make it possible to create sophisticated Graphical User Interfaces (GUIs).
Integration Libraries. Integration libraries such as the Java IDL API, JDBC API, Java Naming and Directory Interface (“J.N.D.I.”) API, Java RMI, and Java Remote Method Invocation over Internet Inter-ORB Protocol Technology (Java RMI-IIOP Technology) enable database access and manipulation of remote objects.
We can’t promise you fame, fortune, or even a job if you learn the Java programming language. Still, it is likely to make your programs better and requires less effort than other languages. We believe that Java technology will help you do the following:
Get started quickly. Although the Java programming language is a powerful object-oriented language, it’s easy to learn, especially for programmers already familiar with C or C++.
Write less code. Comparisons of program metrics (class counts, method counts, and so on) suggest that a program written in the Java programming language can be four times smaller than the same program written in C++.
Write better code. The Java programming language encourages good coding practices, and automatic garbage collection helps you avoid memory leaks. Its object orientation, its JavaBeans component architecture, and its wide-ranging, easily extendible API let you reuse existing, tested code and introduce fewer bugs.
Develop programs more quickly. The Java programming language is simpler than C++, and as such, your development time could be up to twice as fast when writing in it. Your programs will also require fewer lines of code.
Avoid platform dependencies. You can keep your program portable by avoiding the use of libraries written in other languages.
Write once, run anywhere. Because applications written in the Java programming language are compiled into machine-independent bytecodes, they run consistently on any Java platform.
Distribute software more easily. With Java Web Start software, users will be able to launch your applications with a single click of the mouse. An automatic version check at startup ensures that users are always up to date with the latest version of your software. If an update is available, the Java Web Start software will automatically update their installation.
The following sections provide detailed instructions for compiling and running a simple “Hello World!” application. The first section provides information on getting started with the NetBeans IDE, an integrated development environment that greatly simplifies the software development process. The remaining sections provide platform-specific instructions for getting started without an integrated development environment.
It’s time to write your first application! These detailed instructions are for users of the NetBeans IDE. The NetBeans IDE runs on the Java platform, which means that you can use it with any operating system for which there is a JDK 6 available. These operating systems include Microsoft Windows, Solaris OS, Linux, and Mac OS X.
If you prefer to follow the tutorial without using an IDE, see (depending on your platform) “Hello World!” for Microsoft Windows (page 14) or “Hello World!” for Solaris OS and Linux (page 19) to get started.
To write your first program, you’ll need:
The Java SE Development Kit 6 (JDK 6)
The NetBeans IDE
You can download a bundle of the JDK and NetBeans IDE.[5] Bundle installers are available for the Microsoft Windows, Solaris OS, and Linux operating systems.
You can also download and install the JDK and the IDE separately. Separate downloads of the JDK are available at:
http://java.sun.com/javase/6/download.jsp
(Make sure you download the JDK, not the JRE.)
Separate downloads of the NetBeans IDE are available at:
http://www.netbeans.org/downloads/
Your first application, HelloWorldApp
, will simply display the greeting “Hello World!”. To create this program, you will:
Create an IDE project. When you create an IDE project, you create an environment in which to build and run your applications. Using IDE projects eliminates configuration issues normally associated with the javac
compiler and java
launcher tools. You can build or run your application by choosing a single menu item within the IDE.
Add code to the generated source file. A source file contains code, written in the Java programming language, that you and other programmers can understand. As part of creating an IDE project, a skeleton source file will be automatically generated. You will then modify the source file to add the “Hello World!” message.
Compile the source file into a .class
file. The IDE invokes the Java programming language compiler (javac
), which takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this file are known as bytecodes.
Run the program. The IDE invokes the Java application launcher tool (java
), which uses the Java virtual machine to run your application.
To create an IDE project:
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.
In the NetBeans IDE, choose File | New Project (Figure 1.4).
In the New Project wizard, expand the General category and select Java Application (Figure 1.5).
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.
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).
Because you have left the Create Main Class checkbox selected in the New Project wizard, the IDE has created a skeleton class for you. You can add the “Hello World!” message to the skeleton code by replacing the line:
// TODO code application logic here
with the line:
System.out.println("Hello World!"); // Display the string.
Optionally, replace these four lines of generated code
/** * * @author */
with these lines:
/** * The HelloWorldApp class implements an application that * simply prints "Hello World!" to standard output. */
These four lines are a code comment and do not affect how the program runs. Later sections of this tutorial explain the use and format of code comments.
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 change by choosing File | Save.
The file should look something like the following:
/* * 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. } }
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.
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.
Now that you have built the project, you can run your program.
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:
Once you have created a project in the IDE, you can add files to the project using the New File wizard. Choose File | New File, and then select a template in the wizard, such as the Empty Java File template.
You can compile and run an individual file (as opposed to a whole project) using the IDE’s Compile File (F9) and Run File (Shift-F6) commands. If you use the Run Main Project command, the IDE will run the file that the IDE associates as the main class of the main project. Therefore, if you create an additional class in your HelloWorldApp
project and then try to run that file with the Run Main Project command, the IDE will run the HelloWorldApp
file instead.
You might want to create separate IDE projects for sample applications that include more than one source file.
As you are typing in the IDE, a code completion box might periodically appear. You can either ignore the code completion box and keep typing, or you can select one of the suggested expressions. If you would prefer not to have the code completion box automatically appear, you can turn off the feature. Choose Tools | Options, click the Editor tab, and clear the Auto Popup Completion Window checkbox.
If you try to rename the node for a source file in the Projects window, the IDE prompts you with the Rename dialog box to lead you through the options of renaming the class and the updating of code that refers to that class. Click Next to display the Refactoring window, which contains a tree view of changes to be made. Then click Do Refactoring to apply the changes. This sequence of clicks might seem unnecessary if you have just a single class in your project, but it is very useful when your changes affect other parts of your code in larger projects.
For a more thorough guide to the features of the NetBeans IDE, see the NetBeans IDE Docs and Support page[6] or explore the documentation available from the IDE’s Help menu.
It’s time to write your first application! The following instructions are for users of Windows XP Professional, Windows XP Home, Windows Server 2003, Windows 2000 Professional, and Windows Vista. Instructions for Solaris OS and Linux are in the “Hello World!” for Solaris OS and Linux section (page 19).
To write your first program, you’ll need:
The Java SE Development Kit 6 (JDK 6)
You can download the Windows version now.[7] (Make sure you download the JDK, not the JRE.) Consult the installation instructions.[8]
A text editor
In this example, we’ll use Notepad, a simple editor included with the Windows platforms. You can easily adapt these instructions if you use a different text editor.
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. A source file contains code, 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 .class
file. The Java programming language compiler (javac
) takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this file are known as bytecodes.
Run the program. The Java application launcher tool (java
) uses the Java virtual machine to run your application.
To create a source file, you have two options:
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).
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. } }
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:
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.
In the File name text field, type "HelloWorldApp.java"
, including the quotation marks.
From the Save as type combo box, choose Text Documents (*.txt).
In the Encoding combo box, leave the encoding as ANSI.
When you’re finished, the dialog box should look like Figure 1.11.
Now click Save, and exit Notepad.
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.
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>
.
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.
If you enter dir
at the prompt, you should see your source file, as Figure 1.14 shows.
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.
Now that you have a .class
file, you can run your program.
In the same directory, enter the following command at the prompt:
java HelloWorldApp
Figure 1.16 shows what you should now see.
Congratulations! Your program works!
It’s time to write your first application! These detailed instructions are for users of Solaris OS and Linux. Instructions for Microsoft Windows are in the “Hello World!” for Microsoft Windows section (page 14).
To write your first program, you’ll need:
The Java SE Development Kit 6 (JDK 6)
You can download the Solaris OS or Linux version now.[10] (Make sure you download the JDK, not the JRE.) Consult the installation instructions.[11]
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’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. A source file contains code, 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 .class
file. The Java programming language compiler (javac
) takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this .class
file are known as bytecodes.
Run the program. The Java application launcher tool (java
) uses the Java virtual machine to run your application.
To create a source file, you have two options:
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).
Or, you can use the following (longer) instructions.
First, open a shell, or “terminal,” window (Figure 1.17).
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. } }
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.
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).
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.
In the same directory, enter at the prompt:
java HelloWorldApp
Figure 1.20 shows what you should now see.
Congratulations! Your program works!
Now that you’ve seen the “Hello World!” application (and perhaps even compiled and run it), you might be wondering how it works. Here again is its 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. } }
The “Hello World!” application consists of three primary components: source code comments, the HelloWorldApp
class definition, and the main
method. The following explanation will provide you with a basic understanding of of the code, but the deeper implications will only become apparent after you’ve finished reading the subsequent chapters.
The following bold text defines the comments of the “Hello World!” application:
/** * 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. } }
Comments are ignored by the compiler but are useful to other programmers. The Java programming language supports three kinds of comments:
/*
text
*/
. The compiler ignores everything from /*
to */
.
/**
documentation
*/
. This indicates a documentation comment (doc comment, for short). The compiler ignores this kind of comment, just like it ignores comments that use /*
and */
. The javadoc
tool uses doc comments when preparing automatically generated documentation. For more information on javadoc
, see the Javadoc tool documentation.[13]
//
text
. The compiler ignores everything from //
to the end of the line.
The following bold text begins the class definition block for the “Hello World!” application:
/** * 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. } }
As shown above, the most basic form of a class definition is:
class name {
...
}
The keyword class
begins the class definition for a class named name
, and the code for each class appears between the opening and closing curly braces marked in bold above. Chapter 2 provides an overview of classes in general, and Chapter 4 discusses classes in detail. For now it is enough to know that every application begins with a class definition.
The following bold text begins the definition of the main
method:
/** * 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. } }
In the Java programming language, every application must contain a main
method whose signature is:
public static void main(String[] args)
The modifiers public
and static
can be written in either order (public static
or static public
), but the convention is to use public static
as shown above. You can name the argument anything you want, but most programmers choose “args” or “argv.”
The main
method is similar to the main
function in C and C++; it’s the entry point for your application and will subsequently invoke all the other methods required by your program.
The main
method accepts a single argument: an array of elements of type String
.
public static void main(String[] args)
This array is the mechanism through which the runtime system passes information to your application. Each string in the array is called a command-line argument. Command-line arguments let users affect the operation of the application without recompiling it. For example, a sorting program might allow the user to specify that the data be sorted in descending order with this command-line argument:
-descending
The “Hello World!” application ignores its command-line arguments, but you should be aware of the fact that such arguments do exist.
Finally, the line:
System.out.println("Hello World!");
uses the System
class from the API to print the “Hello World!” message to standard output.
'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 choose this option, each time you compile or run a program, you’ll have to precede your javac
and java
commands with /usr/local/jdk6/
. To avoid this extra typing, you could add this information to your PATH variable. The steps for doing so will vary depending on which shell you are currently running.
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
[2] The terms “Java Virtual Machine” and “JVM” mean a Virtual Machine for the Java platform.
[3] http://java.sun.com/javase/technology/hotspot.jsp
[4] docs/index.html
[9] tutorial/getStarted/application/examples/HelloWorldApp.java
[12] tutorial/getStarted/application/examples/HelloWorldApp.java
[13] docs/guide/javadoc/index.html
3.144.109.75