Task: SQL-based authentication
Go: package main

import (
    "bytes"
    "crypto/md5"
    "crypto/rand"
    "database/sql"
    "fmt"

    _ "github.com/go-sql-driver/mysql"
)

func connectDB() (*sql.DB, error) {
    return sql.Open("mysql", "rosetta:code@/rc")
}

func createUser(db *sql.DB, user, pwd string) error {
    salt := make([]byte, 16)
    rand.Reader.Read(salt)
    _, err := db.Exec(`insert into users (username, pass_salt, pass_md5)
        values (?, ?, ?)`, user, salt, saltHash(salt, pwd))
    if err != nil {
        return fmt.Errorf("User %s already exits", user)
    }
    return nil
}

func authenticateUser(db *sql.DB, user, pwd string) error {
    var salt, hash []byte
    row := db.QueryRow(`select pass_salt, pass_md5 from users
        where username=?`, user)
    if err := row.Scan(&salt, &hash); err != nil {
        return fmt.Errorf("User %s unknown", user)
    }
    if !bytes.Equal(saltHash(salt, pwd), hash) {
        return fmt.Errorf("User %s invalid password", user)
    }
    return nil
}

func saltHash(salt []byte, pwd string) []byte {
    h := md5.New()
    h.Write(salt)
    h.Write([]byte(pwd))
    return h.Sum(nil)
}

func main() {
    
    db, err := connectDB()
    defer db.Close()
    createUser(db, "sam", "123")
    err = authenticateUser(db, "sam", "123")
    if err == nil {
        fmt.Println("User sam authenticated")
    }

    
    fmt.Println()
    
    rows, _ := db.Query(`select username, pass_salt, pass_md5 from users`)
    var user string
    var salt, hash []byte
    for rows.Next() {
        rows.Scan(&user, &salt, &hash)
        fmt.Printf("%s %x %x\n", user, salt, hash)
    }
    
    err = createUser(db, "sam", "123")
    fmt.Println(err)
    
    err = authenticateUser(db, "pam", "123")
    fmt.Println(err)
    
    err = authenticateUser(db, "sam", "1234")
    fmt.Println(err)
    
    db.Exec(`truncate table users`)
}

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();
        }
    }
}