Java: different output when scanning input with spaces or enters as delimiters

Question

I'm trying to get input from the user and print it to the console using Java 8 (IDE BlueJ, Windows 10). When printing the output there is a bug: the program prints Equation 2 twice instead of printing Equation 1 and Equation 2.

This is the code:

import java.util.Scanner;

public class Equations
{
        public static void main (String[] args)

    {
        Scanner scan = new Scanner (System.in); 

        System.out.println("This program solves a system of 2 linear equations");
        System.out.println("Enter the coefficients a11 a12 a21 a22 b1 b2");

        int a11 = scan.nextInt();
        int a12 = scan.nextInt();
        int a21 = scan.nextInt();
        int a22 = scan.nextInt();
        int b1 = scan.nextInt();
        int b2 = scan.nextInt();

        System.out.println("Eq1: "+ a11 +"*x1+"+ a12 +"*x2="+ b1);
        System.out.println("Eq2: "+ a21 +"*x1+"+ a22 +"*x2="+ b2);

    }
}

this is the expected output:

This program solves a system of 2 linear equations Enter the coefficients a11 a12 a21 a22 b1 b2
1 2 3 4 5 6
Eq1: 1*x1+2*x2=5
Eq2: 3*x1+4*x2=6

And this is the output:

This program solves a system of 2 linear equations Enter the coefficients a11 a12 a21 a22 b1 b2
1 2 3 4 5 6
Eq2: 3*x1+4*x2=6
Eq2: 3*x1+4*x2=6

Note that the bug exists only when typing the input on a single line with white spaces between the numbers, and does not exist when pressing the Enter key after each number.

Meaning, if the input comes one number at a time, the expected output is recieved correctly:

This program solves a system of 2 linear equations Enter the coefficients a11 a12 a21 a22 b1 b2

1

2

3

4

5

6

Eq1: 1*x1+2*x2=5
Eq2: 3*x1+4*x2=6

Since it's hard to believe and hardly reproducible here is a screenshot:

What causes the difference when the input comes in a single line, separated by spaces, vs when it comes in separate lines, separated by enters?

How to have the desired output when the input comes in a single line format?

I can't reproduce it. Are you able to reproduce it on `ideone.com` ? — Yassin Hajaj, Mar 10 '19 at 22:05
The only thing I can imagine is the class you run was not compiled from the code your posted. — PM 77-1, Mar 10 '19 at 22:08
@User8461 It works only if you press the Enter key after each number, if you're typing all of them on the same line with white spaces there's a bug. — noam steiner, Mar 10 '19 at 22:27
unable to reproduce using eclipse, dos/command line or [ideone](https://ideone.com/HTILOC) (I don't use BlueJ) — user85421, Mar 10 '19 at 22:47
Unable to reproduce in IntelliJ IDEA. Typing the input `1 2 3 4 5 6` on one line returns the expected result — isapir, Mar 10 '19 at 23:14
@noamsteiner We believe you. We just also believe that there is a bug in the IDE that you're using that no one has ever heard of before ;) — isapir, Mar 10 '19 at 23:25
@isapir XD I added this to make it clearer. Unfortunately this is the IDE that the university requires to use at the course I'm taking... — noam steiner, Mar 10 '19 at 23:33
@noamsteiner see update in my answer. try to set the delimiter explicitly. — isapir, Mar 10 '19 at 23:40
I could not reproduce with BlueJ 4.2.0. Which version are you using? — Thomas Kläger, Mar 11 '19 at 05:27
Works with OpenJDK 11.0.2, BlueJ 4.2.0 and OpenJFX 11.0.2 on Ubuntu 18.04: https://imgur.com/a/XRmpU2E — User8461, Mar 12 '19 at 12:20
and OpenJDK 1.8.0_181 with BlueJ 4.1.4 on Ubuntu works as well. — User8461, Mar 12 '19 at 12:24

isapir · Accepted Answer · 2019-03-10T23:39:27.653

Looks like a bug with your IDE. Consider the following:

import java.util.Scanner;

public class Equations
{
    public static void main (String[] args) {

        Scanner scan = new Scanner("1 2 3 4 5 6");

        System.out.println("This program solves a system of 2 linear equations");
        System.out.println("Enter the coefficients a11 a12 a21 a22 b1 b2");

        int a11 = scan.nextInt();
        int a12 = scan.nextInt();
        int a21 = scan.nextInt();
        int a22 = scan.nextInt();
        int b1 = scan.nextInt();
        int b2 = scan.nextInt();

        System.out.println("Eq1: "+ a11 +"*x1+"+ a12 +"*x2="+ b1);
        System.out.println("Eq2: "+ a21 +"*x1+"+ a22 +"*x2="+ b2);

    }
}

It's exactly the same code, with the exception that it does not rely on user input. The input is separated by spaces, and the output is the expected:

This program solves a system of 2 linear equations
Enter the coefficients a11 a12 a21 a22 b1 b2
Eq1: 1*x1+2*x2=5
Eq2: 3*x1+4*x2=6

See online Java compiler

Try to set the delimiter explicitly:

scan = new Scanner(System.in).useDelimiter(" |\n");

Java: different output when scanning input with spaces or enters as delimiters

1 Answers1