The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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.
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 */
/*
to */
./** documentation */
/*
and */
. The javadoc
tool uses doc comments when preparing automatically generated documentation. For more information on javadoc
, see the
Javadoc™ tool documentation .// text
//
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 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.
main
MethodThe 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.