вероятно совсем не понимаю условие задачи :(
вывод
abkny
abc
package com.javarush.task.task36.task3605;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.TreeSet;
/*
Использование TreeSet
*/
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader reader = new BufferedReader(new FileReader("C:/ARC/file2.txt"));
String line;
TreeSet<Character> treeSet = new TreeSet<>();
while ((line = reader.readLine()) != null) {
String output = line.replaceAll("[^a-zA-Z]", "");
String str1 = new String (output.toLowerCase());
char[] array = str1.toCharArray();
for(char l : array) {
treeSet.add(l);}
}
while(treeSet.size() > 5) {
treeSet.remove(treeSet.last());
}
for (char c : treeSet) {
System.out.print(c);
}
}
}