Task: Append a record to the end of a text file
C#: using System;
using System.IO;

namespace AppendPwdRosetta
{
    class PasswordRecord
    {
        public string account, password, fullname, office, extension, homephone, email, directory, shell;
        public int UID, GID;
        public PasswordRecord(string account, string password, int UID, int GID, string fullname, string office, string extension, string homephone, 
            string email, string directory, string shell)
        {
            this.account = account; this.password = password; this.UID = UID; this.GID = GID; this.fullname = fullname; this.office = office;
            this.extension = extension; this.homephone = homephone; this.email = email; this.directory = directory; this.shell = shell;
        }
        public override string ToString()
        {
            var gecos = string.Join(",", new string[] { fullname, office, extension, homephone, email });
            return string.Join(":", new string[] { account, password, UID.ToString(), GID.ToString(), gecos, directory, shell });
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var jsmith = new PasswordRecord("jsmith", "x", 1001, 1000, "Joe Smith", "Room 1007", "(234)555-8917", "(234)555-0077", "jsmith@rosettacode.org", 
                "/home/jsmith", "/bin/bash");
            var jdoe = new PasswordRecord("jdoe", "x", 1002, 1000, "Jane Doe", "Room 1004", "(234)555-8914", "(234)555-0044", "jdoe@rosettacode.org", "/home/jdoe", 
                "/bin/bash");
            var xyz = new PasswordRecord("xyz", "x", 1003, 1000, "X Yz", "Room 1003", "(234)555-8913", "(234)555-0033", "xyz@rosettacode.org", "/home/xyz", "/bin/bash");

            
            File.WriteAllLines("passwd.txt", new string[] { jsmith.ToString(), jdoe.ToString() });

            
            File.AppendAllText("passwd.txt", xyz.ToString());

            
            string[] lines = File.ReadAllLines("passwd.txt");
            Console.WriteLine("Appended record: " + lines[2]);
        }
    }
}

Java: import static java.util.Objects.requireNonNull;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class RecordAppender {
    static class Record {
        private final String account;
        private final String password;
        private final int uid;
        private final int gid;
        private final List<String> gecos;
        private final String directory;
        private final String shell;

        public Record(String account, String password, int uid, int gid, List<String> gecos, String directory, String shell) {
            this.account = requireNonNull(account);
            this.password = requireNonNull(password);
            this.uid = uid;
            this.gid = gid;
            this.gecos = requireNonNull(gecos);
            this.directory = requireNonNull(directory);
            this.shell = requireNonNull(shell);
        }

        @Override
        public String toString() {
            return account + ':' + password + ':' + uid + ':' + gid + ':' + String.join(",", gecos) + ':' + directory + ':' + shell;
        }

        public static Record parse(String text) {
            String[] tokens = text.split(":");
            return new Record(
                    tokens[0],
                    tokens[1],
                    Integer.parseInt(tokens[2]),
                    Integer.parseInt(tokens[3]),
                    Arrays.asList(tokens[4].split(",")),
                    tokens[5],
                    tokens[6]);
        }
    }

    public static void main(String[] args) throws IOException {
        List<String> rawData = Arrays.asList(
                "jsmith:x:1001:1000:Joe Smith,Room 1007,(234)555-8917,(234)555-0077,[email protected]:/home/jsmith:/bin/bash",
                "jdoe:x:1002:1000:Jane Doe,Room 1004,(234)555-8914,(234)555-0044,[email protected]:/home/jdoe:/bin/bash",
                "xyz:x:1003:1000:X Yz,Room 1003,(234)555-8913,(234)555-0033,[email protected]:/home/xyz:/bin/bash"
        );

        List<Record> records = rawData.stream().map(Record::parse).collect(Collectors.toList());

        Path tmp = Paths.get("_rosetta", ".passwd");
        Files.createDirectories(tmp.getParent());
        Files.write(tmp, (Iterable<String>) records.stream().limit(2).map(Record::toString)::iterator);

        Files.write(tmp, Collections.singletonList(records.get(2).toString()), StandardOpenOption.APPEND);

        try (Stream<String> lines = Files.lines(tmp)) {
            lines.map(Record::parse).forEach(System.out::println);
        }
    }
}