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Thread: Help understanding Command-Line Arguments

  1. #1
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    Red face Help understanding Command-Line Arguments

    I am having trouble understanding what exactly Command-Line Arguments are. To my understanding they are arrays of the type String that is passed to the main method. In the program below, command-line information is displayed. When the program is executed, it displays contents in the “arg” array. Indexes 0, 1, and 2 are displayed with the elements “one”, “two”, and “three” also displayed in them respectively. Why is this? Also, the length instance variable is used determine the size of the “arg” array. Why is it three? Could someone answer these questions or also just explain to me how command-line arguments are used and what are the benefits for using them.

    Thanks,
    Truck35

    // Display all command-line information. 
    class CLDemo {  
      public static void main(String args[]) { 
        System.out.println("There are " + args.length + 
                           " command-line arguments."); 
     
        System.out.println("They are: "); 
        for(int i=0; i<args.length; i++)  
          System.out.println("arg[" + i + "]: " + args[i]);  
      }  
    }


    --- Update ---

    Sorry, I read my text book wrong and I now understand what my text book was getting at. There is no need to respond to this post.

    Thanks,
    Truck35


  2. #2
    Super Moderator pbrockway2's Avatar
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    Default Re: Help understanding Command-Line Arguments

    Yes, the args[] parameter is an array of strings that come from the command line. When you execute the java executable there will be

    (1) Some switches like -cp etc
    (2) Then the class name: in this case CLDemo
    (3) Then some other strings

    It's the ones in the last category that will be delivered to the main() method as the args parameter.

    The usefulness lies in the fact that the arguments can tell the CLDemo program what you want it to do. To take a non Java example you could issue the command:

    type CLDemo.java

    which will display the contents of the CLDemo.java file. It is necessary for the type command to be told the name of the file you wish to display! The way it finds out is by having access to the command line arguments. In this case there is one of them: the file to display.

    The reason why your program prints "arg[0]: foo" etc is that you have written a for loop that goes through the args array and prints the contents.

    ---

    I was too slow I'm glad you've got it sorted out!

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