About java.util.Scanner

Question

I wrote some code for my assignment, 'heap sort'.

1. I received input file name.

   Scanner in = new Scanner(System.in);
   System.out.print("insert 'input file name' : ");
   String fileName = in.nextLine();
   in.close();
   

2. Then I read that file.

   in = new Scanner(new File(fileName));
   ArrayList<Integer> source = new ArrayList<Integer>();
   MyIntHeap heap = new MyIntHeap();
   for(int idx=0; in.hasNextInt(); ++idx){
       source.add(in.nextInt());
       heap.add(source.get(idx));
   }
   in.close();
   

3. Finally, I tried to receive output file name...

   in = new Scanner(System.in);
   System.out.print("insert 'output file name' : ");
   fileName = in.nextLine();
   in.close();
   

At this point, program threw an error to me.

insert 'input file name' : abc.txt
insert 'output file name' : Exception in thread "main" java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Unknown Source)
    at Solution.main(Solution.java:13)

For now I solve problem as

    Scanner in = new Scanner(System.in);
    System.out.print("insert 'input file name' : ");
    String inputFile = in.nextLine();
    System.out.print("insert 'output file name' : ");
    String outputFile = in.nextLine();
    in.close();

But I'd like to know why the problem happened.

Answer 1

Scanner in = new Scanner(System.in);
...
in.close();

The problem here is that you are closing a Scanner which is reading System.in. This also closes System.in.

The solution is that you should just not close the Scanner.

You should always close resources that you've created. System.in isn't a resource you've created, so you don't need to close it.