Question

My application throws NullPointerException. I created connection between MySQL database and my second class.

I can’t call DefaultTableModel by method in my second class.
How can I solve this problem?

public class MySQL extends javax.swing.JFrame {
    private DefaultTableModel modelTabeli;

    public MySQL(){
        initComponents();
        BazaDana bd = new BazaDana();

        try{
            modelTabeli = bd.map();
            jTable1.setModel(modelTabeli);
        }
        catch(Exception e){
            System.out.println(e.toString());
        }
    }

    public static void main(String args[]) throws Exception{
        BazaDana bd = new BazaDana();
        bd.readDataBase();
    }
}

Second class

import java.sql.*;
import javax.swing.table.DefaultTableModel;

public class BazaDana{    
    public DefaultTableModel map() throws SQLException
    {
        defaultTableModel = new DefaultTableModel();
        int numberOfColumns = resultSetMetaData.getColumnCount();
        while (resultSet.next())
        {
            Object [] rowData = new Object[numberOfColumns];
            for (int i = 0; i < rowData.length; ++i)
            {
                rowData[i] = resultSet.getObject(i+1);
            }
            defaultTableModel.addRow(rowData);
        }
        return defaultTableModel;
    }
}

Answer 1

Ok, may I give you an alternative solution if your goal is to separate GUI class from class which queries database. Don’t return DefaultTableModel, return just values from ResultSet through some collection, like this:

Create MySql GUI class:

import java.awt.EventQueue;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.DefaultTableModel;


public class MySql extends JFrame{
    JTable table = new JTable();
    DefaultTableModel model = new DefaultTableModel(new Object[][]{},new String[]{"First column","Second column"});

    public MySql(){
        table.setModel(model);
        add(new JScrollPane(table));

        //Populate table
        BazaDana bd = new BazaDana();
        List<Value> values = bd.selectAll();
        for(Value v : values){
            model.addRow(new Object[]{v.getFirstValue(),v.getSecondValue()});
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable(){
            public void run() {
                MySql ms = new MySql();
                ms.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                ms.pack();
                ms.setVisible(true);
            }});
    }
}

Then create Java bean class:

public class Value {
    private int id;
    private String firstValue;
    private String secondValue;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getFirstValue() {
        return firstValue;
    }
    public void setFirstValue(String firstValue) {
        this.firstValue = firstValue;
    }
    public String getSecondValue() {
        return secondValue;
    }
    public void setSecondValue(String secondValue) {
        this.secondValue = secondValue;
    }   
}

And finally your BazaDana class which queries database:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;

public class BazaDana {

    public List<Value> selectAll(){
        Connection conn = null;
        Statement st = null;
        ResultSet rs = null;
        //Create list of values
        List<Value> values = new ArrayList<Value>();

        try{
            Class.forName("com.mysql.Driver");
            conn =     DriverManager.getConnection("jdbc:mysql://localhost:3306/nameofdb","user","pass");
            st = conn.createStatement();
            rs = st.executeQuery("SELECT * FROM mytable");

            while(rs.next()){
                Value v = new Value();
                v.setFirstValue(rs.getString("first_column"));
                v.setSecondValue(rs.getString("second_column"));
                values.add(v);
            }
        }
        catch(Exception e){
            e.printStackTrace();
        }
        finally{
            try {
                rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
            try {
                st.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }

            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }

        }
        return values;
    }
}