ArrayIndexOutOfBounds exception

[Fantine Ponter]

Code Magnets in Head First Java, page 351:
The following code (Exercise solution straight from the book) compiles, but when I run it, I get the above exception on main because of the statement " String test = args[0];"

When I comment out that statement and uncomment my own statement just below that (String test = "no";) , I get no runtime error.

Please would someone explain what the statement, String test = args[0];
means, and why I get a runtime error. thanks in advance.

 public class ExTestDrive
{
public static void main(String [] args)
{
    String test = args[0];
//String test = "no";
 
try
{
System.out.print("t");
doRisky(test);
System.out.print("o");
}
 
catch (MyEx e)
{
         System.out.print("a");
}
 
finally
{
System.out.print("w");
}
 
System.out.println("s");
}
static void doRisky(String t) throws MyEx
{
System.out.print("h");
if ("yes".equals (t))
{
throw new MyEx();
}
System.out.print("r");
}
 
}   

[edit]Disable smilies. CR[/edit]
[ December 05, 2008: Message edited by: Campbell Ritchie ]

[Paul Clapham]

It gets the first of the parameters that you typed on the command line after the class name.

If you didn't type any parameters, then you get the error that you observe.

[marc weber]

The String array in main contains the arguments you supply at runtime. If you do not supply any arguments, then this array will have a length of zero, and trying to access its elements will result in an exception.

class ShowMyArgs {
  public static void main(String[] args) {
    if(args.length == 0) {
      System.out.println("You are running this program without any arguments.");
    }
    else {
      //only access args[0] if args.length > 0...
      System.out.println("Your first argument is " + args[0] + ".");
    }
  }
}

To supply arguments at runtime, enter them on the command line after the class name. For example...

java ShowMyArgs here are some args
[ December 05, 2008: Message edited by: marc weber ]

[Fantine Ponter]

Thanks Paul and Marc.

I'll try to be more observant in future!

[fady zohdy]

marc weber wrote:
To supply arguments at runtime, enter them on the command line after the class name. For example...

how can i supply arguments at run time when i am using an IDE like netbeans?!!!
the only way to run a java file in netbeans is through the run button which doesn't give you the chance to insert any arguments.

[fred rosenberger]

fady zohdy wrote:

marc weber wrote:
To supply arguments at runtime, enter them on the command line after the class name. For example...

how can i supply arguments at run time when i am using an IDE like netbeans?!!!
the only way to run a java file in netbeans is through the run button which doesn't give you the chance to insert any arguments.

a quick google search returned this article. It may be out of date, but on the other hand, it may be close enough to what you need.

[Liutauras Vilda]

fady zohdy wrote:how can i supply arguments at run time when i am using an IDE like netbeans?!!!

It is worth to remind you, that HFJ strongly advise you do not use such a clever IDE as NetBean or any other, until book is read and content understood. Instead is better to use simple text editor (vim, emacs, TextWrangler, Notepad++) and command line compiler.
It would help you better understand common use packages, classes, methods, import statements as well as compilation process, which can be a critical part of mastering language.

[fady zohdy]

Liutauras Vilda wrote:

Instead is better to use simple text editor (vim, emacs, TextWrangler, Notepad++) and command line compiler.

first thanks for the reply.
i did use notepad at the beginning of the book but sometimes the code is so complicated and it needs a another java file to compile. i don't seem to be able to get the java compiler to read a whole package of java classes

[fady zohdy]

fred rosenberger wrote:
a quick google search returned this article. It may be out of date, but on the other hand, it may be close enough to what you need.

done it using netbeans. the article was really helpful
thanks

[J. Kevin Robbins]

fady zohdy wrote: i don't seem to be able to get the java compiler to read a whole package of java classes

And this is a classic example of why you should NOT be using an IDE. Until you learn to build, compile, and run you application outside of an IDE, you aren't really learning Java, you are learning to use a crutch. Please read the IDE FAQ, I think you'll find it quite useful.

[fady zohdy]

J. Kevin Robbins wrote: I think you'll find it quite useful.

i did use notepad and the java compiler at the beginning. i used to put all the .java files in a folder and the compiler would compile them in the same folder. i couldn't make a good structure for my directories and head first java didn't give hints about that. the structure netbeans use is very complicated and has files that i don't even know how to make like .xml file and .settings folder

[Campbell Ritchie]

fady zohdy wrote: . . . i did use notepad . . .

You mean Microsoft Notepad? A dreadful program for coding. Have a look at this FAQ.

[J. Kevin Robbins]

I recommend Notepad++ or one of the other editors listed in that FAQ. You'll get things like syntax highlighting and bracket matching, features that are helpful but still force you to understand what you are doing.

Run "javac -help" for a list of all the compiler options. You can use the "-d <directory>" option to specify where you want the compiled class files to go. I know it seems painful to do things "the hard way" instead of using an IDE, but in the end, you'll be glad you did. You'll come out of it with a better understanding of what the IDE is doing for you.

[fady zohdy]

Campbell Ritchie wrote:You mean Microsoft Notepad? A dreadful program for coding.

my problem is not with the editor. my problem is with the directories structure and making the compiler put the compiled .class files in a specific folder automatically like what IDEs do.

[Campbell Ritchie]

Try the -d option to the javac tool. Windows® page and *nix page for javac.

[edit]I see JKR has already suggested the -d option.[/edit]

[fady zohdy]

Campbell Ritchie wrote:Try the -d option to the javac tool.

got it ,thanks.

[Campbell Ritchie]

Well done sorting it out