java redirection

[Timothy Leong]

hi guys,

my program require user to read in file using the following command

1) java test < input.txt

or using this

2)
java test
Enter your filename: input.txt


I only able to do the first 1. I am using
Scanner input = new Scanner(System.in);

how do the program detect there is not file redirection and display the second option instead?
i tried input.hasNext() however it cannot work. Thank you

[Michael Ernest]

What does input.hasNext() return when no file has been redirected to the program?

[Timothy Leong]

how should i detect there is no file?

[Michael Ernest]

You don't need to. If a file has been redirected to your program and there's stuff in it, then System.in will have a handle it. If there's nothing in the System.in buffer, then you can assume no file has been passed to you.

[Timothy Leong]

I wan to design my program in the way that
user can use redirection to read in file

java program < input.txt

if the user enter command java program,
the program will display "Enter file name:"
from there, the user will key in the file name.

how do System.in detect there's no file redirection instead?

Thank you

[Ernest Friedman-Hill]

It doesn't matter. Just write the program as if there's someone there typing. If input is coming from a pipe instead, then the prompts will be printed, but no-one will read them. They'll just be ignored.

[Timothy Leong]

Hi, I tried just the following code but do not work.
I would like the program to skip the User input if file redirection is used. or skip file redirection and request for user input if command "java program" is used.

            Scanner input = new Scanner(System.in);
            Scanner txt = null;
            if(/* what should be the condition */)
            {
                System.out.println("Enter file Name:");
                String fileName = input.nextLine().trim();
                System.out.println(fileName);
                txt = new Scanner(new File(fileName));
            }
            else
                txt = input;