The "Hello World" Application |
The first bold line in the following listing begins the definition of amain()
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:The method signature for thepublic static void main(String[] args)main()
method contains three modifiers:
public
indicates that themain()
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 themain()
method is a class method. Instance and Class Members talks about class methods and variables.void
indicates that themain()
method doesn't return any value.How the main() Method Gets Called
Themain()
method in the Java language is similar to themain()
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'smain()
method. Themain()
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 definedArguments to the main() Method
As you can see from the following code snippet, themain()
method accepts a single argument: an array of Strings.This array of Strings 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)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.-descending
Note to C and C++ Programmers: The number and type of arguments passed to themain()
method in the Java runtime environment differ from the number and type of arguments passed to C and C++'smain()
function. For further information refer to Command Line Arguments in the Setting Program Attributes lesson.
The "Hello World" Application |