scanner.hasNext() results in an infinite loop

Question

I have a piece of code which should read input word by word and add the length of the word to an ArrayList recursively (for a school exercise).

I wrote this method:

ArrayList<Integer> wordLengths = new ArrayList<Integer>();
Scanner scanner = new Scanner(System.in);
// ...
private void readWords() {
    // read a word, if there are more words, read the next word
    this.wordLengths.add(this.scanner.next().length());

    if (this.scanner.hasNext()) {
        readWords();
    }
}

When I call this method, it keeps asking for new words. It won't stop the (recursive) loop. Why is this happening? this.scanner.hasNext() should only be true when there's a next word in the input so it should stop when there is no other word.

What *exactly* do you mean by "it keeps asking for new words"? I'd add that using recursion here is a very odd approach... you'd normally use a `while` loop instead. — Jon Skeet, Sep 30 '13 at 19:04
@JonSkeet I know, that's why I added (for a school exercise) What I mean is that the readWords() method never stops calling itself. Even when all the words are read. — Jochem Kuijpers, Sep 30 '13 at 19:05
"for a school exercise" doesn't automatically mean "we have to use recursion". It would have been clearer if you'd stated that. — Jon Skeet, Sep 30 '13 at 19:09
@JonSkeet I'm sorry if it was a bit confusing, that is what I meant though. — Jochem Kuijpers, Sep 30 '13 at 19:09

score 3 · Accepted Answer · answered Sep 30 '13 at 19:07

3

System.in is an input stream. It is never closed, so it will always return true for #hasNext()

answered Sep 30 '13 at 19:07

codethulhu

3,976
2
21
15

Thanks. I'll keep that in mind next time! I'll select your answer when I'm able to unless a better one shows up. – Jochem Kuijpers Sep 30 '13 at 19:07
2

Not quite true. `hasNext()` can return `false` if an end-of-file condition on the input is reached. I just tried it on a Windows system and I got the program to work (in a Command Prompt window) by typing control-Z and then . Redirecting standard input works too. This may be platform-dependent, and I don't know whether Eclipse's console input would be able to trigger an end-of-file condition. – ajb Sep 30 '13 at 19:13

score 0 · Answer 2 · answered Sep 30 '13 at 19:17

0

Use Ctrl+Z(+Enter) in Windows or Ctrl+D in Unix to send EOF and terminate input stream

answered Sep 30 '13 at 19:17

Mateusz Antkiewicz

88
1
6

Well, I haven't used Eclipse for years but I assume that you can just do it in console view (place your cursor in console view, press Ctrl+Z and then Enter) – Mateusz Antkiewicz Sep 30 '13 at 19:26

scanner.hasNext() results in an infinite loop

2 Answers2

Linked