Task: SQL-based authentication
C#: using System.Security.Cryptography;
using System.Text;

namespace rosettaMySQL
{
    class Hasher
    {
        private static string _BytesToHex(byte[] input)
        {
            var strBuilder = new StringBuilder();
            foreach (byte _byte in input)
            {
                strBuilder.Append(_byte.ToString("x2"));
            }
            return strBuilder.ToString();
        }

        public static string Hash(string salt, string input)
        {
            using (MD5 md5 = new MD5CryptoServiceProvider())
            {
                var bytes = Encoding.Default.GetBytes(salt + input);
                var data = md5.ComputeHash(bytes);
                return _BytesToHex(data);
            }
        }

        public static string GenSalt()
        {
            using (RandomNumberGenerator rng = new RNGCryptoServiceProvider())
            {
                var salt = new byte[16];
                rng.GetBytes(salt);
                return _BytesToHex(salt);
            }
        }
    }
}

Java: import java.io.UnsupportedEncodingException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.math.BigInteger;


class UserManager {
    private Connection dbConnection;

    public UserManager() {
    }

    private String md5(String aString) throws NoSuchAlgorithmException, UnsupportedEncodingException {
        MessageDigest md;
        String hex;
        StringBuffer hexString;
        byte[] bytesOfMessage;
        byte[] theDigest;

        hexString = new StringBuffer();
        bytesOfMessage = aString.getBytes("UTF-8");
        md = MessageDigest.getInstance("MD5");
        theDigest = md.digest(bytesOfMessage);

        for (int i = 0; i < theDigest.length; i++) {
            hex = Integer.toHexString(0xff & theDigest[i]);
            if (hex.length() == 1) {
                hexString.append('0');
            }
            hexString.append(hex);
        }

        return hexString.toString();
    }

    public void connectDB(String host, int port, String db, String user, String password)
      throws ClassNotFoundException, SQLException {

        Class.forName("com.mysql.jdbc.Driver");

        this.dbConnection =  DriverManager.getConnection("jdbc:mysql:
                                + host
                                + ":"
                                + port
                                + "/"
                                + db, user, password);
    }

    public boolean createUser(String user, String password) {
        SecureRandom random;
        String insert;
        String salt;

        random = new SecureRandom();
        salt =  new BigInteger(130, random).toString(16);

        insert = "INSERT INTO users "
            + "(username, pass_salt, pass_md5) "
            + "VALUES (?, ?, ?)";

        try (PreparedStatement pstmt = this.dbConnection.prepareStatement(insert)) {
            pstmt.setString(1, user);
            pstmt.setString(2, salt);
            pstmt.setString(3, this.md5(salt + password));
            pstmt.executeUpdate();

            return true;
        } catch(NoSuchAlgorithmException | SQLException | UnsupportedEncodingException ex) {
            return false;
        }
    }

    public boolean authenticateUser(String user, String password) {
        String pass_md5;
        String pass_salt;
        String select;
        ResultSet res;

        select = "SELECT pass_salt, pass_md5 FROM users WHERE username = ?";
        res = null;

        try(PreparedStatement pstmt = this.dbConnection.prepareStatement(select)) {
            pstmt.setString(1, user);
            res = pstmt.executeQuery();

            res.next(); 

            pass_salt = res.getString(1);
            pass_md5 = res.getString(2);

            if (pass_md5.equals(this.md5(pass_salt + password))) {
                return true;
            } else {
                return false;
            }

        } catch(NoSuchAlgorithmException | SQLException | UnsupportedEncodingException ex) {
            return false;
        } finally {
            try {
                if (res instanceof ResultSet && !res.isClosed()) {
                    res.close();
                }
            } catch(SQLException ex) {
            }
        }
    }

    public void closeConnection() {
        try {
            this.dbConnection.close();
        } catch(NullPointerException | SQLException ex) {
        }
    }

    public static void main(String[] args) {
        UserManager um;

        um = new UserManager();
        try {
            um.connectDB("localhost", 3306, "test", "root", "admin");

            if (um.createUser("johndoe", "test")) {
                System.out.println("User created");
            }

            if (um.authenticateUser("johndoe", "test")) {
                System.out.println("User authenticated");
            }
        } catch(ClassNotFoundException | SQLException ex) {
            ex.printStackTrace();
        } finally {
            um.closeConnection();
        }
    }
}