MySql数据库的Java连接问题

Question

When I am trying retrieve values into a JTable it says "cast connectdb to connection", on the line con=Connect.ConnectDB() . But I have declared my Connect class without any error, and are able to insert values from another form successfully. This is my code:

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
  try{
    con= Connect.ConnectDB();
    String sql="select * from pharmacy";
    pst = con.prepareStatement(sql);
    pst.execute();
    jTable1.setModel(DbUtils.resultSetToTableModel(rs));
    JOptionPane.showMessageDialog(this,"Succesfully stored","User",JOptionPane.INFORMATION_MESSAGE);

  } catch(SQLException ex){
    JOptionPane.showMessageDialog(this,ex);
  }
}  

This is my Connect class:

public class Connect {
  Connection con=null;

  public static Connection ConnectDB(){
    try{

      Class.forName("com.mysql.jdbc.Driver");
      Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/hms_db1","root","root");
      return con;

    }catch(ClassNotFoundException | SQLException e){
      JOptionPane.showMessageDialog(null, e);
      return null;
    }      
  }
}

Answer 1

jTable1.setModel(DbUtils.resultSetToTableModel(rs));

This is the line you're getting NullPointerException from.. because rs is null

change this line

pst.execute();

to

rs = pst.executeQuery();

You haven't assigned the result from the executeQuery() to the resultSet reference!! and since you're passing a null reference thats why you're getting a NPE. Hope this answers your question.

Answer 2

try pst.executeQuery() in place of pst.execute()

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    try {
        con = Connect.ConnectDB();
        String sql = "select * from pharmacy";
        pst = con.prepareStatement(sql);
        pst.executeQuery();
        jTable1.setModel(DbUtils.resultSetToTableModel(rs));
        JOptionPane.showMessageDialog(this, "
            Succesfully stored" , "User", JOptionPane.INFORMATION_MESSAGE);
    } catch (SQLException ex) {
        JOptionPane.showMessageDialog(this, ex);
    }
}