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 "Hello World!" application consists of three primary components: source code comments, the/** * 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. } }HelloWorldApp
class definition, and themain
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.
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*/
. Thejavadoc
tool uses doc comments when preparing automatically generated documentation. For more information onjavadoc
, see the JavadocTM tool documentation .
// text
- The compiler ignores everything from
//
to the end of the line.
HelloWorldApp
Class DefinitionThe 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 namedname
, 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.
main
MethodThe following bold text begins the definition of themain
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:The modifierspublic static void main(String[] args)public
andstatic
can be written in either order (public static
orstatic public
), but the convention is to usepublic 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 themain
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 typeString
.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:public static void main(String[] args)-descendingThe "Hello World!" application ignores its command-line arguments, but you should be aware of the fact that such arguments do exist.
Finally, the line:
uses theSystem.out.println("Hello World!");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.