While loop won't exit Java

Question

I'm entering a huge number and adding each line of numbers to the string "hold". Once there's nothing left to read it should exit the while loop and turn the string to a BigInteger.

It will run through the loop till there is nothing left then nothing happens. It doesn't continue to BigInteger or go into the loop again. I've looked everywhere and can't figure out why it's stuck.

System.out.print("Enter the number you want looked at: ");
String hold = "";
String line = null;
BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
while ((line = read.readLine()) != null){
    hold = hold.concat(line);
}
BigInteger giantNumber = new BigInteger(hold);

System.out.print(giantNumber);

I want it to exit the loop without the user doing anything more then entering the number at the beginning.

It seems to get stuck because System.in is looking for more input and isn't receiving anything to compare in the while loop. I'm not sure what to do from there.

The number I'm entering is, if that helps:

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

have you debugged that piece of code. It's possible that while loop doesn't exit and continues forever — lightbringer, May 26 '15 at 03:19
@JigarJoshi the number is spread across several lines. 123 new line 456 etc. Does that make sense? — BarbarianBunny, May 26 '15 at 03:22
this code will read each line and wait for user to enter new line, if you want to terminate have a terminator code (exit, quit, stop..) or something to stop taking inputs and add that condition in while block — jmj, May 26 '15 at 03:23
@CommuSoft I'm using Intellij in the debug menu if that's what you mean. — BarbarianBunny, May 26 '15 at 03:24
@BarbarianBunny: no, how are you providing input? Using the terminal or with I/O redirection. — Willem Van Onsem, May 26 '15 at 03:24
If you want to quite by entering ENTER then probably you need to check for empty tring, not null — Tony Vu, May 26 '15 at 03:25
@JigarJoshi the whole number with the line breaks is entered at once so the user shouldn't also have to enter a terminator — BarbarianBunny, May 26 '15 at 03:27
what is the difference between single line input and the way you are describing — jmj, May 26 '15 at 03:28
@CommuSoft oh sorry I'm new to programming so I'm not used to the terms. I'm just entering it where it says Console in Intellij. — BarbarianBunny, May 26 '15 at 03:30
@TonyVu I tried using while ((line = read.readLine()) != "") but that doesn't work. How would you suggest I check for an empty string? — BarbarianBunny, May 26 '15 at 03:38
@TonyVu I tried using while ((line = read.readLine()).length() != 0) which runs through all the numbers but also stops working. I think it's cause System.in is looking for input and not returning anything to judge. — BarbarianBunny, May 26 '15 at 03:49

score 6 · Answer 1 · answered May 26 '15 at 03:26

System.in reads until the stream reaches its end. If you use the Java program in interactive mode, this means the string normally doesn't end, unless the user presses Ctrl+D or a key combination that is recognized by the terminal operator that will close the input stream.

You can do a few things:

use Ctrl+D to end the terminal input
Use a file as input feed (java -jar program.jar < inputfile). In that case the stream will terminate after the end of the file

implement a stop code:

while ((line = read.readLine()) != null){
    if(line.equals("stop")) {
        break;
    }
    hold = hold.concat(line);
}

Here the java program will end when you enter stop for a certain line.

Is there a way to have it stop if there's nothing left being input. like ((line = ...) != "") or something? — BarbarianBunny, May 26 '15 at 03:35

Nitin Dandriyal · Accepted Answer · 2015-05-26T04:09:36.210

1

If you want to implement break for an empty line, get the line trim and check if empty.

Your way:

    while ((line = read.readLine()) != null){
        if(line.trim().equals("")){
            break;
        }
        hold = hold.concat(line);
    }

Using Scanner instead:

...
    Scanner in = new Scanner(System.in);
    while(in.hasNextLine()){
        String line = in.nextLine();
        if(line.trim().equals("")){
            break;
        }
        hold = hold.concat(line);
...

edited May 26 '15 at 04:09

answered May 26 '15 at 03:44

Nitin Dandriyal

1,559
11
20

It can't resolve in. is that supposed to be System.in? – BarbarianBunny May 26 '15 at 03:53
Both of those work perfectly. Do you suggest using Scanner or BufferedReader generally or does it just dependent on the situation? – BarbarianBunny May 26 '15 at 04:20
1

Using `Scanner` is better in this situation, `scanner` has some built in functionalities like `hasNextLine()` `nextInt` ... and so on. `BufferedReader` does have its own use cases and can be used alongwith Scanner as well http://stackoverflow.com/questions/2231369/scanner-vs-bufferedreader – Nitin Dandriyal May 26 '15 at 04:24

score 0 · Answer 3 · answered May 26 '15 at 06:01

Assignment inside a condition is ok in this case, as the assignment is surrounded by an extra pair of parentheses – the comparison is obviously != null, there is no chance that we wanted to type line == reader.readLine().

However, a for loop might actually be more elegant here:

for (String line = reader.readLine(); line != null; line = reader.readLine()) {
    doSomething(line);
}

Alternatively, we could do this which also restricts the scope of line as with the for-loop, and additionally eliminates unnecessary repetition: Note: This method doesn't mutate any variables

while (true) {
    final String line = reader.readLine();
    if (line == null) break;

    doSomething(line);
}

score 0 · Answer 4 · answered May 26 '15 at 14:31

0

You can use isEmpty() function of string class to check if string is empty.

while ((line = read.readLine()) != null&&!line.isEmpty())
{
     hold = hold.concat(line);
}

System.out.print(hold);

answered May 26 '15 at 14:31

aakansha

695
5
18

While loop won't exit Java

4 Answers4