Handling Arguments

Remember that an array is an object that must be constructed before use.

• A method that takes an array as an argument must first check that the array itself is not null before trying to access its length or its elements.

  ...
  int[] nums = null;
  methodThatTakesAnArray(nums);
}
public static void methodThatTakesAnArray( int[] numbers ) {
  System.out.println(numbers.length); // NullPointerException
}

  ...
  int[] nums = null;
  betterMethodThatTakesAnArray(nums);
}
public static void betterMethodThatTakesAnArray(int[] numbers) {
  if (numbers != null) {
    System.out.println(numbers.length);
  }
  else {
    System.out.println("numbers is null");
  }
}

The args array is always created and passed to main, even if no arguments were passed on the command line.

• So in main (and only in main) it's safe to just start using args.

public static void main(String[] args) {
  if (args.length < 2) {
    System.err.println("I expect to be passed two filenames on the command line.");
    System.exit(1);
  }
  String fileName1 = args[0];
  String fileName2 = args[1];
  //...
}

Practice Exercise

All programs, regardless of the language they're written in, have access to the array of command-line arguments. Some languages include the program name itself at the beginning of the array. For example, if our program were written in the C language and we ran it as ShowMe cat dog frog, args[0] would always be ShowMe.

Java does not include either the class name or the java command itself in args - only the words after our class name.

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