CODEWITHSUNDEEP
java
Shayan_Aryan
Shayan_Aryan

Reputation: 2042

why testing args.length?

I'm a Java beginner and I'm confused about testing args.length at the begining of many codes, and why it's never gets higher than 0 in any of my codes?

import java.net.Socket;
import java.net.UnknownHostException;
import java.io.IOException;

public class LowPortScanner {
public static void main(String[] args) {
String host = "localhost";
if (args.length > 0) {
host = args[0];
}
for (int i = 1; i < 1024; i++) {
try {
Socket s = new Socket(host, i);
System.out.println("There is a server on port " + i + " of "
+ host);
}
catch (UnknownHostException ex) {
System.err.println(ex);
break;
}
catch (IOException ex) {}
} // end for
} // end main
} // end PortScanner

Upvotes: 0

Views: 3637

Answers (7)

kaysush
kaysush

Reputation: 4836

args in public static void main(String[] args is the String array of arguments passed from command line.

java LowPortScanner argument1 argument2

if you try the above command args.length will return 2.

As far as question of checking the length it is done for java programs which can take command line arguments and if arguments are not passed then they prompt for input.

if(args.length >0 ){
  //proceed using passed arguments
}else{
   //proceed with some default value
}

You are running your program using java LowPortScanner hence no arguments are passed and args.length is always zero.

Moreover if you don't pass any argument and use host=args[0] you will get ArrayIndexOutOfBoundsException.

Upvotes: 0

Krushna
Krushna

Reputation: 6110

The variable String[] args hold all the parameter pass to the program thorough command line if you are not passing any argument then the length of args become 0. Now it's better to check it's length before accessing it other wise there is chance to get ArrayIndexOutOfBoundsException if it's size is 0

Upvotes: 0

Lukas Eichler
Lukas Eichler

Reputation: 5913

If you are calling your program from CMD or bash you can asign ARGuments it like
java LowPortScanner google.com

Then "google.com" is your args[0]. When your program supports commandline attributes it is recommended to test if the given arguments are corret.

Upvotes: 0

NPKR
NPKR

Reputation: 5506

You have to inputs to main mathod from Command prompt.

like below

java LowPortScanner TEST1 TEST2

Upvotes: 1

Aniket Inge
Aniket Inge

Reputation: 25725

It depends on how you invoke the Java class file. In command prompt or bash shell:

java LowPortScanner Argument1

typing the above line in the command prompt/bash will cause the argument count to increase to 1. (because Argument1 is one argument, after the class file LowPortScanner)

java LowPortScanner Argument1 Argument2

the above line will make argument count to increase to 2.

hence args.length will be 2 in the second case and 1 in the first case.

Upvotes: 0

Christophe Roussy
Christophe Roussy

Reputation: 17029

If there are no command arguments the args[0] will fail. This is why it must be protected.

Upvotes: 0

Grant Thomas
Grant Thomas

Reputation: 45058

Because if it is not tested then an exception would be thrown because host = args[0]; would illegal.

However, this doesn't look like it's going to help much, an empty or null host looks like it would cause further problems.

If the length of args is always 0, then be sure you're actually passing in parameter arguments.

Upvotes: 0

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