Why is nextLine() returning an empty string?

Question

This is probably one of the easiest things but I'm not seeing what I'm doing wrong.

My input consist of one first line with a number (the number of lines to read), a bunch of lines with data and a final line only with \n. I should process this input and after the last line, do some work.

I have this input:

5
test1
test2
test3
test4
test5
      /*this is a \n*/

And for reading the input I have this code.

int numberRegisters;
String line;

Scanner readInput = new Scanner(System.in);

numberRegisters = readInput.nextInt();

while (!(line = readInput.nextLine()).isEmpty()) {
    System.out.println(line + "<");
}

My question is why I'm not printing anything? Program reads the first line and then does nothing.

btw, isn't first number is number of tests? – RiaD Apr 16 '13 at 15:12 — RiaD, Apr 16 '13 at 15:12
Try to replace !line.isEmpty() with line!=null ? – user2147970 Apr 16 '13 at 15:12 — user2147970, Apr 16 '13 at 15:12
Yes it is the number of tests – Favolas Apr 16 '13 at 15:12 — Favolas, Apr 16 '13 at 15:12
so, you may read this int x and then read lines x times – RiaD Apr 16 '13 at 15:13 — RiaD, Apr 16 '13 at 15:13
@RiaD Yes. You´re correct Updated the question – Favolas Apr 16 '13 at 15:15 — Favolas, Apr 16 '13 at 15:15
@user2147970 Now it prints but does not stop the while loop – Favolas Apr 16 '13 at 15:16 — Favolas, Apr 16 '13 at 15:16

Bernhard Barker · Accepted Answer · 2015-04-08T11:37:13.793

nextInt doesn't read the following new-line character, so the first nextLine (which returns the rest of the current line) will always return an empty string.

This should work:

numberRegisters = readInput.nextInt();
readInput.nextLine();
while (!(line = readInput.nextLine()).isEmpty()) {
    System.out.println(line + "<");
}

But my advice is not to mix nextLine with nextInt / nextDouble / next / etc. because anyone trying to maintain the code (yourself included) may not be aware of, or have forgotten, the above, so may be somewhat confused by the above code.

So I suggest:

numberRegisters = Integer.parseInt(readInput.nextLine());

while (!(line = readInput.nextLine()).isEmpty()) {
    System.out.println(line + "<");
}

Damn! . Any explanation on why this behaviour? – Manish Ranjan Sep 30 '16 at 11:37 — Manish Ranjan, Sep 30 '16 at 11:37

score 1 · Answer 2 · answered Apr 16 '13 at 15:13

I think I've see this issue before. I think you need to add another readInput.nextLine() or else you're just reading between the end of the 5, and the \n after that

int numberRegisters;
String line;

Scanner readInput = new Scanner(System.in);

numberRegisters = readInput.nextInt();
readInput.nextLine();

while (!(line = readInput.nextLine()).isEmpty()) {
    System.out.println(line + "<");
}

score 0 · Answer 3 · answered Apr 16 '13 at 15:21

Actually it doesn't answer the question completely(why your code isn't working) but you may use following code.

int n = Integer.parseInt(readInput.readLine());
for(int i = 0; i < n; ++i) {
    String line = readInput().readLine();
    // use line here
}

As for me it's more readable and even may save your time in such rare cases when testcases are incorrect(with extra info at the end of file)

BTW, seems you take part in some programming competition. Make note, that Scanner may be quite slow to input a lot of data. you may consider using BufferedReader with possible StringTokenizer (not needed in this task)

Why is nextLine() returning an empty string?

3 Answers3

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