Core Java:"Hello World!" Application

Lesson: The "Hello World!" Application

The sections listed below 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 NetBeans IDE runs on all of the platforms listed below. The remaining sections provide platform-specific instructions for getting started without an integrated development environment. If you run into problems, be sure to consult the common problems section; it provides solutions for many issues encountered by new users.

"Hello World!" for the NetBeans IDE

These 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. We recommend using the NetBeans IDE instead of the command line whenever possible.

"Hello World!" for Microsoft Windows

These command-line instructions are for users of Windows XP Professional, Windows XP Home, Windows Server 2003, Windows 2000 Professional, and Windows Vista.

"Hello World!" for Solaris OS and Linux

These command-line instructions are for users of Solaris OS and Linux.

Common Problems (and Their Solutions)

Consult this page if you have problems compiling or running your application. 

A Closer Look at the "Hello World!" Application

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:

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 the code, but the deeper implications will only become apparent after you've finished reading the rest of the tutorial.

Source Code Comments

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^TM tool documentation .

// text

The compiler ignores everything from // to the end of the line.

The HelloWorldApp Class Definition

The following bold text begins the class definition block for the "Hello World!" application:

/** 
 * 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.
    }
}

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 main Method

The following bold text begins the definition of the main method:

/** 
 * 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.
    }
}

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. For example:

java MyApp arg1 arg2

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 core library to print the "Hello World!" message to standard output. Portions of this library (also known as the "Application Programming Interface", or "API") will be discussed throughout the remainder of the tutorial.