SQL Database in Java

package podaci;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.HashMap;

import javax.swing.JFrame;
import javax.swing.JOptionPane;


public abstract class Baze
{
    private static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    private static final String DB_URL = "jdbc:mysql://localhost:3306/skola";
    private static final String USER = "root";
    private static final String PASS = "12345";

    private static Connection conn = null;
    private static java.sql.Statement stmt = null;
    private static ResultSet rs = null;
    private static PreparedStatement preparedStatement = null;

    private static void close()
    {
        try
        {
            if(conn != null)
            {
                conn.close();
            }

            if(stmt != null)
            {
                conn.close();
            }

            if(rs != null)
            {
                rs.close();
            }

            if(preparedStatement != null)
            {
                preparedStatement.close();
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

    private static void start(boolean ind, String sql)
    {
        try
        {
            conn = DriverManager.getConnection(DB_URL, USER, PASS);
            stmt = conn.createStatement();
            if(ind == true)
            {
                rs = stmt.executeQuery(sql);
            }
            else
            {
                preparedStatement = conn.prepareStatement(sql);
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
            JOptionPane.showMessageDialog(new JFrame(), "Nemoguca konencija sa bazom!", "Greška", JOptionPane.ERROR_MESSAGE);
            System.exit(0);
        }
    }

    public static HashMap<Integer, Ucenik> iscitaj()
    {
        HashMap<Integer, Ucenik> mapa = new HashMap<Integer, Ucenik>();
        try
        {
            Class.forName(JDBC_DRIVER);

            String sql = "select ucenik.idUcenik, ucenik.ime, ucenik.prezime, ucenik.prosek, ucenik.datumRodjenja, ucenik.mestoRodjenja, ucenik.status from ucenik";
            start(true, sql);

            while(rs.next())
            {
                int id = rs.getInt("ucenik.idUcenik");
                String ime = rs.getString("ucenik.ime");
                String prezime = rs.getString("ucenik.prezime");
                double prosek = rs.getDouble("ucenik.prosek");
                String datumRodjenja = rs.getString("ucenik.datumRodjenja");
                String mestoRodjenja = rs.getString("ucenik.mestoRodjenja");
                int status = rs.getInt("ucenik.status");

                mapa.put(id, new Ucenik(ime, prezime, prosek, datumRodjenja, mestoRodjenja, status));
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            close();
        }

        return mapa;
    }

    public static void upisi(Ucenik u)
    {
        try
        {
            Class.forName(JDBC_DRIVER);

            String sql = "insert into ucenik(ucenik.ime, ucenik.prezime, ucenik.prosek, ucenik.datumRodjenja, ucenik.mestoRodjenja, ucenik.status) values(?, ?, ?, ?, ?, ?)";
            start(false, sql);

            preparedStatement.setString(1, u.getIme());
            preparedStatement.setString(2, u.getPrezime());
            preparedStatement.setDouble(3, u.getProsek());
            preparedStatement.setString(4, u.getDatum());
            preparedStatement.setString(5, u.getMesto());
            preparedStatement.setInt(6, u.getStatus());

            preparedStatement.execute();
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            close();
        }
    }

    public static void izmeni(HashMap.Entry<Integer, Ucenik> par)
    {
        try
        {
            Class.forName(JDBC_DRIVER);

                String sql = "update ucenik set ime = ?, prezime = ?, prosek = ?, datumRodjenja = ?, mestoRodjenja = ?, status = ? where idUcenik = ?";
                start(false, sql);

                preparedStatement.setString(1, par.getValue().getIme());
                preparedStatement.setString(2, par.getValue().getPrezime());
                preparedStatement.setDouble(3, par.getValue().getProsek());
                preparedStatement.setString(4, par.getValue().getDatum());
                preparedStatement.setString(5, par.getValue().getMesto());
                preparedStatement.setInt(6, par.getValue().getStatus());

                preparedStatement.setInt(7, par.getKey());
                preparedStatement.execute();
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            close();
        }
    }

    public static void obrisi(HashMap.Entry<Integer, Ucenik> par)
    {
        try
        {
            Class.forName(JDBC_DRIVER);

            String sql = "delete from ucenik where idUcenik = ?";
            start(false, sql);

            preparedStatement.setInt(1, par.getKey());
            preparedStatement.execute();
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            close();
        }
    }
}