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

Answer 1

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.

Answer 2

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);
...