如何从键盘和文件中读取？

Question

I have a Scanner that could be reading from either keyboard or from a file (via pipes), and apparently there's no way to tell which.

I have te following code:

Scanner scan = new Scanner(System.in);
while (scan.hasNextLine()) {
    String line = scan.nextLine();
    doStuff();
}

That works wonderfully when redirecting input to a file. But if I try to run the program by itself and read from keyboard, it enters an infinite loop. Is there a way to differentiate between reading from keyboard and from a file? Thanks in advance!

Edit 1: As requested by @Abra, this is what my code looks like with your suggestion:

Scanner scan = new Scanner(System.in);
do {
    String linea = scan.nextLine();
    doStuff();
} while (System.in.available() != 0 && scan.hasNextLine());

And here's the command I'm running:

java -jar Class.jar < File.txt

Edit 2: Solved it, turns out I should only evaluate System.in.available():= 0 once:

Scanner scan = new Scanner(System.in);
boolean file = System.in.available() != 0;
do {
    String linea = scan.nextLine();
    doStuff();
} while (file && scan.hasNextLine());

Answer 1

The classic practice used commonly in Linux and Unix is to read input from standard input, as you are already doing. In Java, standard input is called System.in .

The program reads from standard input and processes what it reads in a loop until it detects end-of-file, which you are already doing.

So your program is not stuck - it is merely waiting for more input or for the end-of-file signal to come from the outside.

If you want to use this program with input from a file, you run it like this:

myprogram < input_file.txt

And if you want your program to get its input from terminal (where you type it), you run it just like

myprogram

In this case, and after typing your input, you are also responsible to send a special signal from your terminal that will act as a "end-of-file" and will be picked by the program, causing the while -loop to exit. Typically, you do this by pressing Control-D.

Keep in mind that reading from standard input is not strictly the same as reading from keyboard. Standard input only knows about text and end-of-file; it has no concept of line editing, testing for when shift key is pressed/released etc.

Answer 2

based on the answer of @Abra you can break the loop if System.in is keyboard so:

import java.util.Scanner;
import java.io.IOException;

public class Main {
    public static void main(String[] argv) throws IOException {
        Scanner scan = new Scanner(System.in);
        boolean isKeyboard = System.in.available() == 0;
        while (scan.hasNextLine()) {
            String line = scan.nextLine();
            doStuff();
            
            if (isKeyboard) {
                break;
            }
        }
    }
}

Answer 3

The class of System.in is java.io.InputStream . That class has method available() . If you redirect System.in to a file, as in

myprogram < input_file.txt

Then method available() returns a number greater than zero (assuming that input_file.txt has non-zero size) but when System.in refers to the standard input stream, ie when you run your program without redirecting standard input, as in

myprogram

Then method available() returns zero.