How to stop reading multiple lines from stdin usin

问题:

I'm working on an JAVA assignment should process multiple lines of input. The instructions read "Input is read from stdin."

An example of sample input is given:

one 1
two 2
three 3

I don't understand what the above sample input "read from stdin" means.

Here's a test program I wrote that isolates my confusion:

import java.io.*;
import java.util.Scanner;

class Test
{
public static void main(String[] args)
{   
    Scanner stdin = new Scanner(System.in);
    while(stdin.hasNextLine())
    {
        String line = stdin.nextLine();
        String[] tokens = line.split(" ");
        System.out.println(Integer.parseInt(tokens[1]));
    }
}

When I run this program in the console, it waits for my input and each time I input a line it echos it back as I would expect. So I thought perhaps the sample input above would be achieved by entering each of the 3 lines in this fashion. However, there seems to be no way to end the process. After I enter the 3 lines, how do I terminate the input? I tried just pressing enter twice, but that seems to read as a line consisting of only the newline character, which causes an error because the line doesn't fit the 2 token format it expects.

Here's what the console interaction looks like:

javac Test.java
java Test
one 1
1
two 2
2
three 3
3

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1
    at Test.main(Test.java:13)

I'd appreciate any help in pointing out the gap in my understanding.

回答1:

You could try asking for empty inputs

import java.util.Scanner;

public class Test
{
    public static void main(String[] args)
    {   
        String line;
        Scanner stdin = new Scanner(System.in);
        while(stdin.hasNextLine() && !( line = stdin.nextLine() ).equals( "" ))
        {
            String[] tokens = line.split(" ");
            System.out.println(Integer.parseInt(tokens[1]));
        }
        stdin.close();
    }
}

• Your code is almost completed. All that you have to do is to exit the while loop. In this code sample I added a condition to it that first sets the read input value to line and secondly checks the returned String if it is empty; if so the second condition of the while loop returns false and let it stop.
• The array index out of bounds exception you will only get when you're not entering a minimum of two values, delimitted by whitespace. If you wouldn't try to get the second value >token[1]< by a static index you could avoid this error.
• When you're using readers, keep in mind to close after using them.
• Last but not least - have you tried the usual Ctrl+C hotkey to terminate processes in consoles?

Good luck!

回答2:

You could also put your values in a file e.g. input.txt and do:

java Test < input.txt

回答3:

From the shell, hit Ctrl-D and it will close stdin. Alternatively, pipe input in

cat your-input-file | java Test

回答4:

To stop the input, you could either prompt the user to enter quit to exit, and then test for the presence of that String in the input, exiting the loop when found, or you could use a counter in the loop, exiting the loop when the maximum iterations have been reached. The break statement will get you out of the loop.