The "Hello World" Application

The main Method

The first bold line in the following listing begins the definition of a 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.
    }
}

Every Java application must contain a main method whose signature looks like this:

public static void main(String[] args)

The method signature for the main method contains three modifiers:

• public indicates that the main method can be called by any object. Controlling Access to Members of a Class covers the ins and outs of the access modifiers supported by the Java language.
• static indicates that the main method is a class method. Instance and Class Members talks about class methods and variables.
• void indicates that the main method doesn't return any value.

How the main Method Gets Called

The main method in the Java language is similar to the main function in C and C++. When the Java interpreter executes an application (by being invoked upon the application's controlling class), it starts by calling the class's main method. The main method then calls all the other methods required to run your application.

If you try to invoke the Java interpreter on a class that does not have a main method, the interpreter refuses to compile your program and displays an error message similar to this:

In class NoMain: void main(String argv[]) is not defined

Arguments to the main Method

As you can see from the following code snippet, 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

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Note to Mac OS JDK Programmers only: The JDK compiler for Mac OS allows an application that does not need to accept command-line arguments to declare its main method without any arguments, like this:

public static void main() {
. . . 
}

An application with such a main method will not prompt the user for command-line arguments.

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The "Hello World" application ignores its command-line arguments, so there isn't much more to discuss here. However, you can get more information about command-line arguments, including the framework for a command-line parser that you can modify for your specific needs, in the Setting Program Attributes lesson.

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Note to C and C++ Programmers: The number and type of arguments passed to the main method in the Java runtime environment differ from the number and type of arguments passed to C and C++'s main function. For further information refer to Command Line Arguments in the Setting Program Attributes lesson.

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The "Hello World" Application