How to detect EOF in Java?

Question

I've tried some ways to detect EOF in my code, but it still not working. I've tried using BufferedReader, Scanner, and using char u001a to flag the EOF, but still not make any sense to my code. Here is my last code :

    Scanner n=new Scanner(System.in);
    String input;
    int counter=0;

    while(n.hasNextLine())
    {
        input=n.nextLine();
        char[] charInput=input.toCharArray();
        for (int i = 0; i < input.length(); i++) {
            if(charInput[i]=='"')
            {
                if(counter%2==0)
                {
                    System.out.print("``");
                }
                else
                {
                    System.out.print("''");
                }
                counter++;
            }
            else
            {
                System.out.print(charInput[i]);
            } 
        }
        System.out.print("\n");
    }

The program supposed to stopped when it's already reached the EOF, but I don't know why, for some reasons it keeps running and result a runtime error. Please help. By the way I'm new here, sorry if my question is not really clear to be understood, Thank you before :)

Also, check [this](http://stackoverflow.com/questions/16104616/using-bufferedreader-to-read-text-file) post on how to read a file using `FileReader` + `BufferedReader` — TheLostMind, Apr 09 '16 at 11:21
I believe the scanner will just return ```null``` instead of an EOF character. That would explain the runtime exception. — Jorn Vernee, Apr 09 '16 at 11:45
@JornVernee No hasNextLine will check the underlying read for null and return false. His program works as expected on my machine. — totoro, Apr 09 '16 at 11:48
You are reading input from the console (stdin), maybe it behaves strange on your host. Try reading from a file you open. — totoro, Apr 09 '16 at 11:50
how do you run your program? are you actually piping in content from a file or are you just running it without piping in content (and just inputting values by hand into the console)? — Michael Ritter, Jan 24 '17 at 17:29

user207421 · Accepted Answer · 2020-04-14T07:37:08.377

20

It keeps running because it hasn't encountered EOF. At end of stream:

read() returns -1.
read(byte[]) returns -1.
read(byte[], int, int) returns -1.
readLine() returns null.
readXXX() for any other X throws EOFException.
Scanner.hasNextXXX() returns false for any X.
Scanner.nextXXX() throws NoSuchElementException for any X.

Unless you've encountered one of these, your program hasn't encountered end of stream. NB \u001a is a Ctrl/z. Not EOF. EOF is not a character value.

edited Apr 14 '20 at 07:37

answered Apr 09 '16 at 11:55

user207421

305,947
44
307
483

It might be worth pointing out that you need to use the corresponding `hasNext...` call, not just `hasNextLine` everywhere, since that will return true if you read the last token with `next` and there's an empty line at the end. – Bernhard Barker Dec 17 '17 at 16:17

score 3 · Answer 2 · answered Nov 05 '17 at 10:35

3

This is what I did

Scanner s = new Scanner(f); //f is the file object

while(s.hasNext())
{
String ss = s.nextLine();
System.out.println(ss);
}

Worked for me

answered Nov 05 '17 at 10:35

Srijan Goyal

41
7

You should use `hasNextLine()` in conjunction with `nextLine()`, and similarly for the other method pairs. – user207421 Apr 14 '20 at 07:38

score 0 · Answer 3 · answered Aug 26 '21 at 21:58

Specifically for System.in you should be able to check whether the backing FileDescriptor is still open via FileDescriptor.in.valid().

Simple demo:

$ cat FD.java
public class FD {
  public static void main(String[] args) throws Exception {
    System.out.println(java.io.FileDescriptor.in.valid());
    System.in.close();
    System.out.println(java.io.FileDescriptor.in.valid());
  }
}

$ javac FD.java
$ java -cp . FD
true
false

score -3 · Answer 4 · answered May 07 '19 at 00:21

You can use try and catch.

Scanner n=new Scanner(System.in);
String input;
int counter=0;
input=n.nextLine();
try{
    while(input!=null)
    { 
        char[] charInput=input.toCharArray();
        for (int i = 0; i < input.length(); i++) {
            if(charInput[i]=='"')
            {
                if(counter%2==0)
                {
                    System.out.print("``");
                }
                else
                {
                    System.out.print("''");
                }
                counter++;
            }
            else
            {
                System.out.print(charInput[i]);
            } 
        }
        System.out.print("\n");
        input=n.nextLine();
    }
}catch(Exception e){

}

Here, when you will give Ctrl+z, Scanner.nextLine() will give you NoSuchElementException. Use this exception as a condition of EOF. To handle this exception use try and catch.

Hacky - it ignores the hasNextLine method intended to tell you when input is exhausted. — , May 07 '19 at 01:25
Incorrect. `nextLine()` doesn't return null at EOS, it throws `NoSuchElementException`. — user207421, Nov 30 '19 at 22:18

How to detect EOF in Java?

4 Answers4

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