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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 Classcovers the ins and outs of the access modifiers supported by the Java language.
static
indicates that themain
method is a class method. Understanding Instance and Class Memberstalks about class methods and variables.
void
indicates that themain
method doesn't return any value.
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 run your program and displays an error message similar to this:In class NoMain: void main(String argv[]) is not defined
main
Method
As you can see from the following code snippet, themain
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, 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.
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 Argumentsin the Setting Program Attributes
lesson.
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