Java - Error de conexión

import java.sql.*;

import java.util.logging.Level;

import java.util.logging.Logger;

import javax.swing.JOptionPane;

import javax.swing.table.DefaultTableModel;

public class frmDepartamento extends javax.swing.JFrame {

Conectar cc;

Connection cn;

int id;

    public frmDepartamento() {

        initComponents();

        this.setTitle("DEPARTAMENTOS");

        this.setLocationRelativeTo(null);

        lblContraseña.setVisible(false);

    }

private void btnAltaActionPerformed(java.awt.event.ActionEvent evt) {

        try {

        // TODO add your handling code here:

        Statement st=cn.createStatement();

        st.executeUpdate("INSERT INTO departamentos (Nombre_dpto) VALUES('"+txtDpto.getText()+"')");

        JOptionPane.showMessageDialog(this, "Departamento dado de alta", "Departamentos", JOptionPane.INFORMATION_MESSAGE);

        txtDpto.setText("");

        llenarTabla();

        st.close();

    } catch (SQLException ex) {

        JOptionPane.showMessageDialog(this, "El departamento no se dió de alta", "Departamentos", JOptionPane.ERROR_MESSAGE);

        Logger.getLogger(frmDepartamento.class.getName()).log(Level.SEVERE, null, ex);

    }

    }

import java.sql.*;

public class Conexion {

   private static Connection cnx = null;

   public static Connection obtener() throws SQLException, ClassNotFoundException {

      if (cnx == null) {

         try {

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

            cnx = DriverManager.getConnection("jdbc:mysql://localhost/java_mysql", "root", "");

         } catch (SQLException ex) {

            throw new SQLException(ex);

         } catch (ClassNotFoundException ex) {

            throw new ClassCastException(ex.getMessage());

         }

      }

      return cnx;

   }

   public static void cerrar() throws SQLException {

      if (cnx != null) {

         cnx.close();

      }

   }

}

import java.sql.*;

import com.mysql.jdbc.Connection;

import java.util.logging.Level;

import java.util.logging.Logger;

import javax.swing.JOptionPane;

public class Conectar {

Connection conectar=null;

public String usuario;

public String contraseña;

public Conectar(String user, String contraseña){

    this.usuario=user;

    this.contraseña=contraseña;

}

public Connection conexion(){

    try {

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

        conectar=(Connection) DriverManager.getConnection("jdbc:mysql://localhost/archivo_busq",this.usuario,this.contraseña);

    } catch (ClassNotFoundException ex) {

        Logger.getLogger(Conectar.class.getName()).log(Level.SEVERE, null, ex);

    } catch (SQLException ex) {

//      Logger.getLogger(Conectar.class.getName()).log(Level.SEVERE, null, ex);

        JOptionPane.showMessageDialog(null, ex,"No se pudo conectar", JOptionPane.ERROR_MESSAGE);

    }

    return conectar;

}

}