Any Idea what is wrong here?

package pl.codegym.task.task18.task1804; import java.io.BufferedReader; import java.io.FileInputStream; import java.io.InputStreamReader; import java.util.HashMap; /* Najrzadziej występujące bajty */ public class Solution { public static void main(String[] args) throws Exception { FileInputStream fileToReadFrom = new FileInputStream(getFileName()); HashMap<Character, Integer> characterMap = new HashMap<>(); while (fileToReadFrom.available() > 0) { Character symbol = (char) fileToReadFrom.read(); if (characterMap.containsKey(symbol)){ int newValue = characterMap.get(symbol) + 1; characterMap.replace(symbol, characterMap.get(symbol), newValue); } else characterMap.put(symbol, 1); } characterMap.put((char) fileToReadFrom.read(), 1); fileToReadFrom.close(); int minimum = setMinimum(characterMap); displayLetters(characterMap, minimum); } public static String getFileName() throws Exception { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); return reader.readLine(); } public static int setMinimum(HashMap<Character, Integer> map) { int minimum = Integer.MAX_VALUE; for (Character symbol : map.keySet()) if (minimum > map.get(symbol)) minimum = map.get(symbol); return minimum; } public static void displayLetters(HashMap<Character, Integer> map, int min) { String toDisplay = ""; for (Character symbol : map.keySet()) if (min == map.get(symbol)) toDisplay = toDisplay + " " + symbol; System.out.println(toDisplay); } }