How '[ ]' this bracket prints in output?

Thanks for the reply! In this problem inputs are 1 2 3 4 5 and the output is somewhat #1 [] like this. so here my next question arise, what should i do for printing the result string without containing brackets i.e [ ] .

This is how ArrayLists are converted to String:

public static class ReaderThread extends Thread {
   // array list on this line
   private List<String> result = new ArrayList<>();

   /* Code to solve the task */

   @Override
   public String toString() {
      // converted to String on this line
      return result.toString();
   }
}