import java.io.*; import java.util.ArrayList; import java.util.Collections; import java.util.List; public class P7 { public static void main(String[] args) throws IOException { String filePath = "P7.in"; List lines = new ArrayList<>(); try (BufferedReader reader = new BufferedReader(new FileReader(filePath))) { String line; while ((line = reader.readLine()) != null) lines.add(line + '\n'); } Collections.sort(lines); //sort try (BufferedWriter writer = new BufferedWriter(new FileWriter(filePath))) { for (String line: lines) writer.write(line); } } }