IO delay causes 2 iterations in for loop in JAVA

Question

I was testing some piece of code and came across something interesting on which I would need some expert opinion.

A simple program that stops the loop when the input is s but seems the on each input the loop is iterated twice, i guess that's because of the IO delay. Correct me if I am wrong.

public static void main(String[] args) throws java.io.IOException {
        int i;
        System.out.println("Type s to stop.");
        for(i = 0; ; i++) {
            char value = (char) System.in.read();
            if(value=='s'){
                break;
            }
            System.out.println("Pass # " + i);
        }
    }

output:

score 2 · Accepted Answer · answered Jul 04 '18 at 07:11

2

When you enter a char a as an input, there are actually two characters which are entered.

a
\n

But, when you are reading a char, it reads one by one. Hence, it is looped twice.

answered Jul 04 '18 at 07:11

Pankaj Singhal

15,283
9
47
86

Any explanation why it might even loop 3 times? – XtremeBaumer Jul 04 '18 at 07:12
@XtremeBaumer I can't reproduce your problem with iterating 3 times on Windows. Are you using linux? That might append `\r\n` after you type a char. Which explains the 3 times looping i guess – Lino Jul 04 '18 at 07:14
It would loop the length of the `chars` entered +1. If you enter `ab`, it would loop 3 times. – Pankaj Singhal Jul 04 '18 at 07:15
ab are 2 characters. @PankajSinghal – Danyal Sandeelo Jul 04 '18 at 07:15
Yes. That's what I said, IF you enter `ab`, it would loop 3 times for `a`, `b`, `\n` respectively. A situation which @XtremeBaumer might be encountering. – Pankaj Singhal Jul 04 '18 at 07:16
1

I'm on windows and the characters it gets for input `1` are `49`,`13`,`10`. Looks like I get `\r\n` on windows as well – XtremeBaumer Jul 04 '18 at 07:17
1

`13` is `\r` & `10` is `\n`. So, `a\r\n` is what's actually being entered – Pankaj Singhal Jul 04 '18 at 07:18
So maybe only print when `!Character.isWhitespace(value)` – Lino Jul 04 '18 at 07:19
@Lino Windows is the OS that uses \r\n by default, Linux uses \n only – Erwin Bolwidt Jul 04 '18 at 07:20
@ErwinBolwidt then I got mixed something up. I use windows but have never encountered `\r\n` – Lino Jul 04 '18 at 07:22

IO delay causes 2 iterations in for loop in JAVA

1 Answers1