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1.4 Using Command-Line Arguments
As we've seen, every
standalone Java program must declare a method with exactly the
following signature:
public static void main(String[ ] args)
This signature says that an array of
strings is passed to the main( ) method. What are
these strings, and where do they come from? The
args array contains any arguments passed to the
Java interpreter on the command line, following the name of the class
to be run. Example 1-4 shows a program,
Echo, that reads these arguments and prints them
back out. For example, you can invoke the program this way:
% java je3.basics.Echo this is a test
The program responds:
this is a test
In this case, the args
array has a length of four. The first element in the array,
args[0], is the string
"this", and the last element of the
array, args[3], is
"test". As you can see, Java arrays
begin with element 0. If you are coming from a
language that uses one-based arrays, this can take quite a bit of
getting used to. In particular, you must remember that if the length
of an array a is n, the last
element in the array is a[n-1]. You can determine
the length of an array by appending .length to its
name, as shown in Example 1-4.This example also demonstrates the use
of a while loop. A while loop
is a simpler form of the for loop; it requires you
to do your own initialization and update of the loop counter
variable. Most for loops can be rewritten as a
while loop, but the compact syntax of the
for loop makes it the more commonly used
statement. A for loop would have been perfectly
acceptable, and even preferable, in this example.
Example 1-4. Echo.java
package je3.basics;
/**
* This program prints out all its command-line arguments.
**/
public class Echo {
public static void main(String[ ] args) {
int i = 0; // Initialize the loop variable
while(i < args.length) { // Loop until the end of array
System.out.print(args[i] + " "); // Print each argument out
i++; // Increment the loop variable
}
System.out.println( ); // Terminate the line
}
}
