javalab/P7.java

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<String> 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);
        }
    }
}