Lesson: A Closer Look at the "Hello World!" Application

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Lesson: 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: /** * 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 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 JavadocTM 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. 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.