Java: Check if command line arguments are null

Question

I am looking to do some error checking for my command line arguments

public static void main(String[] args)
{
    if(args[0] == null)
    {
        System.out.println("Proper Usage is: java program filename");
        System.exit(0);
    }
}

However, this returns an array out of bounds exception, which makes sense. I am just looking for the proper usage.

Answer 1

The arguments can never be null. They just wont exist.

In other words, what you need to do is check the length of your arguments.

public static void main(String[] args)
{
    // Check how many arguments were passed in
    if(args.length == 0)
    {
        System.out.println("Proper Usage is: java program filename");
        System.exit(0);
    }
}

Answer 2

If you don't pass any argument then even in that case args gets initialized but without any item/element. Try the following one, you will get the same effect:

 
public static void main(String[] args) throws InterruptedException {
        String [] dummy= new String [] {};
        if(dummy[0] == null)
        {
            System.out.println("Proper Usage is: java program filename");
            System.exit(0);
        }

    }

Answer 3

@jinguy's answer is correct in most circumstances. You won't ever see a null String in the argument array (or a null array) if main is called by running the application is run from the command line in the normal way.

However, if some other part of the application calls a main method, it is conceivable that it might pass a null argument or null argument array. This is clearly a highly unusual use-case, and an egregious violation of the implied contract for a main entrypoint method. Therefore, I don't think you should bother checking for null values. In the unlikely event that they do occur, I'd say that it is acceptable for the calling code to get a NullPointerException.

Answer 4

You should check for (args == null || args.length == 0). Although the null check isn't really needed, it is a good practice.