Java Data Display from database?

Question

package grid;

import java.awt.*;
import java.sql.*;
import java.util.*;
import javax.swing.*;
import java.lang.String;

public class ViewData extends JFrame{
   JTable table;
   String rowData[][]=new String[100][100];
    String columnNames[]=new String[3] ;
     static ViewData r;
    public  ViewData() throws ClassNotFoundException, SQLException{
        setLayout(new FlowLayout());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);    

        columnNames[0] ="Name";
        columnNames[1] ="Regno";
        columnNames[2] ="Email";

        r.load();
        table=new JTable(rowData, columnNames);
        add(new JScrollPane(table));
        pack();
    }
   public static void main(String[] args) throws ClassNotFoundException, SQLException {
        r=new ViewData();
    }

     public void load() throws ClassNotFoundException, SQLException{
            Class.forName("org.apache.derby.jdbc.ClientDriver");
            Connection con=DriverManager.getConnection("jdbc:derby://localhost:1527/swap_db","swap","swap");
            String q="SELECT * FROM STUDENT";
            Statement stm=con.createStatement();
            ResultSet rs=stm.executeQuery(q);
            int i=0;
            while(rs.next()){
                String name=rs.getString(1);
                String regno=rs.getString(2);
                String email=rs.getString(3);

                rowData[i][0]=name;
                rowData[i][1]=regno;
                rowData[i][2]=email;
                i++;

            }        
        }
}

I am trying to display data in database in JTable but it showing following error.

Exception in thread "main" java.lang.NullPointerException
at grid.ViewData.(ViewData.java:23)
at grid.ViewData.main(ViewData.java:29)
BUILD STOPPED (total time: 5 seconds)

What is error in this program?

Something is null. Debug your code.

Manu
– Manu

2015-10-27 14:17:23 +00:00
Commented Oct 27, 2015 at 14:17 — Manu
– Manu, Commented Oct 27, 2015 at 14:17

Bon · Accepted Answer · 2015-10-27 14:18:04Z

3

The following statement invokes the constructor of ViewData;

r=new ViewData();

But inside the constructor, you called:

r.load();

r is null before the constructor returns, that's why it throws a NullPointerException.

So what can do is move r.load() from the constructor to the main method after the instance is created.

public static void main(String[] args) throws ClassNotFoundException, SQLException {
    r=new ViewData();
    r.load();
}

answered Oct 27, 2015 at 14:18

Bon

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