A problem with System.in.read()

[Merkey]

Hi, I am trying to make a simple code, which would read characters from console and write them back:

package com.company;

import java.io.IOException;

public class Main {

    public static void main(String[] args)throws IOException {
        // write your code here
        byte[] byteArray = null;
        System.out.println("Write some text:");
        System.in.mark(100);
        int textLenght = 0;
        while (System.in.read() != 10) {
            textLenght++;
        }
        byteArray = new byte[textLenght];
        System.in.reset();
        System.in.read(byteArray);
        String text = new String(byteArray);
        System.out.println("You have written " + text);


    }
}

 

The code above works normally, program ends and doesn't throw any exceptions. When I put this code in while(true) block to reuse the code though, I get this result:

Write some text:
helloworld
You have written helloworld
Write some text:
You have written 
Write some text:
You have written 
Write some text:
You have written 
Write some text:
You have written 
Write some text:
You have written 
Write some text:
You have written

I have no idea why the Thread doesn't just wait for input after one loop. Any ideas why?

[Guest]

I am not quite sure what you want your code to do?

The code you posted does not output what you wrote.

 

What did you change?

You could look into using the Scanner instead?

[Merkey]

4 minutes ago, Lanng said:

I am not quite sure what you want your code to do?

The code you posted does not output what you wrote.

 

What did you change?

I just put it inside while brackets, as I said:

public class Main {

    public static void main(String[] args)throws IOException {
        // write your code here
        while(true){
            byte[] byteArray = null;
            System.out.println("Write some text:");
            System.in.mark(100);
            int textLenght = 0;
            while (System.in.read() != 10) {
                textLenght++;
            }
            byteArray = new byte[textLenght];
            System.in.reset();
            System.in.read(byteArray);
            String text = new String(byteArray);
            System.out.println("You have written " + text);
        }
    }
}

I just want the code to ask for an input again and wait.

[Guest]

The code fails at your while(System.in.read() != 10).

What that code checks for, is if System.in.read() returns a new line (in ASCII code http://www.asciitable.com/).

What System.in is reading is controlled by the system and therefore you can't just call reset, as far as I understand.

 

I would suggest another implementation that I find more readable.

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		while(true) {
			System.out.println("What is your name?");
			String name = sc.nextLine();
			System.out.println("Your name is " + name);
		}
	}
}

 

[Merkey]

11 minutes ago, Lanng said:

The code fails at your while(System.in.read() != 10).

What that code checks for, is if System.in.read() returns a new line (in ASCII code http://www.asciitable.com/).

What System.in is reading is controlled by the system and therefore you can't just call reset, as far as I understand.

 

I would suggest another implementation that I find more readable.

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		while(true) {
			System.out.println("What is your name?");
			String name = sc.nextLine();
			System.out.println("Your name is " + name);
		}
	}
}

 

I know the Scanner, but wanted to try the System.in stream. What I don't understand, is why does it run normally for the first time (waits until I write something and after pressing enter starts reading it) but does not wait again?

[Guest]

The problem with your while(System.in.read() != 10). You are checking if System.in.read() is not a new line.

I guess that when you enter your first input and hit enter, you create a new line and therefore your while-loop will not be true.

To solve this, you would have to find another statement that holds true after your first input.

 

The Scanner is using the System.in (new Scanner(System.in);).

If you want to try other methods, you could try to use a buffered reader and an input stream reader?

[Merkey]

11 minutes ago, Lanng said:

The problem with your while(System.in.read() != 10). You are checking if System.in.read() is not a new line.

I guess that when you enter your first input and hit enter, you create a new line and therefore your while-loop will not be true.

To solve this, you would have to find another statement that holds true after your first input.

 

The Scanner is using the System.in (new Scanner(System.in);).

If you want to try other methods, you could try to use a buffered reader and an input stream reader?

Ah I see. Thanks, I'll try other methods as well