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java redirection

Timothy Leong
Ranch Hand

Joined: May 25, 2005
Posts: 55
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
High Plains Drifter
Sheriff

Joined: Oct 25, 2000
Posts: 7292

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


Make visible what, without you, might perhaps never have been seen.
- Robert Bresson
Timothy Leong
Ranch Hand

Joined: May 25, 2005
Posts: 55
how should i detect there is no file?
Michael Ernest
High Plains Drifter
Sheriff

Joined: Oct 25, 2000
Posts: 7292

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
Ranch Hand

Joined: May 25, 2005
Posts: 55
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
author and iconoclast
Marshal

Joined: Jul 08, 2003
Posts: 24183
    
  34

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.


[Jess in Action][AskingGoodQuestions]
Timothy Leong
Ranch Hand

Joined: May 25, 2005
Posts: 55
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.

 
It is sorta covered in the JavaRanch Style Guide.
 
subject: java redirection
 
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