while loop does not work properly java

Question

In the following code for continuing the program user should press y character but when I press y the loop terminate

public class JavaFile
{

    int i = 0;
    public void systemIO()throws java.io.IOException {
        System.out.println("Enter the character");
        i = System.in.read();
        System.out.println("the character Enter by the user : "+(char)i);
        System.out.println("the assci vlue "+i);
    }
    public void cntApp()throws java.io.IOException{
        char cnt = 'y';
        while(cnt =='y'){
            systemIO();
            System.out.println("press 'y'  if you want continue");
            cnt = (char)System.in.read(); 
            System.out.println("The Entery value   "+cnt);
        }

    }
    public static void main(String args[]) {
        try{
        new JavaFile().cntApp();
    }catch(java.io.IOException ioExe){
        System.out.println(ioExe);
    }
    }

Minor point: consider a `do {} while ();` loop instead - you're guaranteed one iteration that way. Also, adding `System.out.println(Character.getName(cnt));` to your loop should show you exactly why it doesn't work. — JonK, Mar 04 '15 at 09:24

score 1 · Answer 1 · answered Mar 04 '15 at 09:24

If you run it in debug :

while(cnt =='y'){
        systemIO();
        System.out.println("press 'y'  if you want continue");
        cnt = (char)System.in.read(); 
        System.out.println("The Entery value   "+cnt);
}

You'll see that

cnt = (char)System.in.read();

return '\r', correspond to the validation by enter.

So, i will use a Scanner instead and it will be something like that :

public class JavaFile {
    int i = 0;
    Scanner reader = new Scanner(System.in);
    public void systemIO()throws java.io.IOException {
        System.out.println("Enter the character");
        i = reader.next().charAt(0);
        System.out.println("the character Enter by the user : "+(char)i);
        System.out.println("the assci vlue "+i);
    }
    public void cntApp()throws java.io.IOException{
        char cnt = 'y';
        while(cnt =='y'){
            systemIO();
            System.out.println("press 'y'  if you want continue");
            cnt = reader.next().charAt(0);
            System.out.println("The Entery value   "+cnt);
        }

    }
    public static void main(String args[]) {
        try{
            new JavaFile().cntApp();
        }catch(java.io.IOException ioExe){
            System.out.println(ioExe);
        }
    }
}

In systematIO method, after pressing the enter, character works fine And show the value, but in cntApp method it's not we have two same methods whic act different in System.in — mohsen.nour, Mar 04 '15 at 09:43
with System.in i think the only way is to read until is to read it until it's null. That's the reason why i use Scanner object. — vincent, Mar 04 '15 at 10:28

score 1 · Answer 2 · answered Mar 04 '15 at 09:52

It looks like the keyboard input is buffered and the following code is getting line-terminator/null value that causes the while-loop to terminate.

cnt = (char)System.in.read();

Try refactoring the code using Scanner class instead of System.in.read()

import java.util.Scanner;    

public class JavaFile {
        private Scanner scanner = new Scanner(System.in);
...

    public void cntApp() throws java.io.IOException {

        String line = "y";
        while ("y".equals(line)) {
            systemIO();
            System.out.println("press 'y'  if you want continue");
            line = scanner.nextLine();
            System.out.println("The Entery value   " + line);
        }

        scanner.close();
    }
...

Consider the combination of skip() and available() methods as suggested by @Naman Gala — owenrb, Mar 04 '15 at 10:53

score 1 · Accepted Answer · edited May 23 '17 at 12:12

With reference to this answer.

You can add below line add the end of systemIO() and cntApp() method i.e. after the System.in.read() method call.

System.in.skip(System.in.available());

So your methods will look like this.

public void systemIO() throws java.io.IOException {
    System.out.println("Enter the character");
    i = System.in.read();
    System.out.println("the character Enter by the user : " + (char) i);
    System.out.println("the assci vlue " + i);
    System.in.skip(System.in.available());
}

public void cntApp() throws java.io.IOException {
    char cnt = 'y';
    while (cnt == 'y') {
        systemIO();
        System.out.println("press 'y'  if you want continue");
        cnt = (char) System.in.read();
        System.out.println("The Entery value   " + cnt);
        System.in.skip(System.in.available());
    }
}

Note: But using Scanner(System.in) is better approach then using System.in.read() directly.

score 0 · Answer 4 · answered Mar 04 '15 at 09:24

0

You could rewrite your cntApp method to use a Scanner and take the first character entered

public void cntApp() throws java.io.IOException {
    Scanner in = new Scanner(System.in);
    char cnt = 'y';
    while (cnt == 'y') {
        systemIO();
        System.out.println("press 'y'  if you want continue");
        cnt = in.next().charAt(0);
        System.out.println("The Enter value   " + cnt);
    }
}

answered Mar 04 '15 at 09:24

nomis

2,045
11
15

They could, but *why* does that solve their issue? It would be helpful to explain what the problem actually is before offering a solution to it. – JonK Mar 04 '15 at 09:27

while loop does not work properly java

4 Answers4