вроде должно пройти )
package com.javarush.task.task19.task1925;
import java.io.*;
import java.util.*;
/*
Длинные слова
*/
public class Solution {
public static void main(String[] args) throws IOException{
BufferedReader red = new BufferedReader(new FileReader(args[0]));
BufferedWriter writer = new BufferedWriter(new FileWriter(args[1]));
String s="";
while(red.ready()){
s+=red.readLine();}
red.close();
String[] line = s.split(" ");
ArrayList<String> list = new ArrayList<>();
for(int i = 0;i<line.length;i++){
String h = line[i];
if(h.length()>6)
list.add(h);
}
for (int i = 0;i<list.size();i++){
if (i != (list.size()))
writer.write(list.get(i)+",");
else
writer.write(list.get(i));}
writer.close();
}
}