Monday, May 3, 2010

Language Fundamentals : The main() Method and command line arguments

The main() method is used as the entry point for a Java application program. All programs must have
a main() method or they cannot be run. The main() method is a method of the class that is executed
to run the program.
Note Importing java.lang The java.lang package is always imported by default and does not need to be imported by an import statement.
For example, if your program's name is MyProgram, then the MyProgram class must be defined in a
file named The MyProgram class must have a correctly defined main() method.
A correctly defined main() method has the following form:
public static void main(String[] args) {
// Statements go here
The main() method must be declared as public, static, and void.
The void keyword must appear immediately before main().
The public and static keywords may be interchanged.
The main() method has one argument—an array of String arguments. This argument may be defined as
String[] args or String []args or String args[]. The args argument may use any valid
identifier. For example, you can use arg, myArgs, or parms. However, args is standard, and you
should probably stick with it. As a convention, when I refer to args, I'm referring to the argument to a
program's main() method.
The args array is used to access a program's command-line arguments. These arguments are passed
to a program when it is invoked. They are passed as part of the command that is used to invoke the

Note Applets Applets are not required to have a main() method.

For example, to run the MyProgram program, you would enter
java MyProgram
Suppose that you wanted to pass the arguments 2 and 3 to MyProgram. You would invoke it as follows:
java MyProgram 2 3
The String object "2" would be accessed as args[0], and the String object "3" would be accessed as args[1]. If you are a C or C++ programmer—pay attention. Java accesses commandline
arguments using different indices than do C and C++ programs.
The ArgsTest program of Listing 2.1 shows how command-line arguments are accessed using the
args array. When you run the program using the following command line
java ArgsTest this is a test
it displays the following results
args[0] = this
args[1] = is
args[2] = a
args[3] = test
Listing 2.1: The Argstest Program
class ArgsTest {
public static void main(String[] args) {
for(int i=0;i
System.out.println("args["+i+"] = "+args[i]);

No comments:

Post a Comment