Query fromMysql with Java---java.lang.NullPointerException

Question

Database is named 'test'��in it, one of the table is named 'item'.It displays the following error:java.lang.NullPointerException when I do query with following code.
This java is used to connect mysql:
package Be;
import java.sql.*;
import java.util.*;
public class Dataconnection {
     public Connection conn;
     String sql;
public Connection makeC() throws SQLException,ClassNotFoundException,InstantiationException,IllegalAccessException
try
String dbDriver = "com.mysql.jdbc.Driver";
Class.forName(dbDriver).newInstance();
Connection conn= java.sql.DriverManager.getConnection("jdbc:mysql://localhost/test","root","");
     catch (ClassNotFoundException cnfe)
//System.err.println(cnfe.getMessage());
System.out.println(cnfe.getMessage());
     return conn;
this java is used to do query from table 'item'
ackage Be;
import java.util.*;
import java.sql.*;
import java.lang.*;
public class testForMysql{
public void testMysql() throws SQLException,ClassNotFoundException,InstantiationException,IllegalAccessException
DB = new Dataconnection();     
     Statement stmt = null;
     ResultSet rs = null;
try{  //try1
stmt = conn.createStatement();
rs = stmt.executeQuery("SELECT * FROM item");
System.out.println("SELECT * FROM item");
               while (rs.next()) //while1
          int invoiceno = rs.getInt("invoiceno");
     System.out.println(invoiceno);
     }//while1 over
     rs.close();
     conn.close();
          }//try1 over
     catch (SQLException e) {System.out.print(e);}
     catch (Exception e){System.out.print(e);}
private String sql;
public Dataconnection DB;
public Connection conn;
public static void main(String[] args)throws Exception
testForMysql sta =new testForMysql();
sta.testMysql();
Any idea or hint?
thanks a lot!

Answer 1

At what point did you get the exception? You say you got a NullPointerException, but you give no indication as to where it happened. That would give you a good clue.
Start following the Sun Java coding conventions:
http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html
When you post code, put it inside [ code][ code] tags (see the special tokens link up above).
Make that connection in your Dataconnection class private. Why public?
If I was writing this class, here's how I'd do it:
package be;
import java.sql.*;
import java.util.*;
public class DataConnection
    public static final String DEFAULT_DRIVER   = "sun.jdbc.odbc.JdbcOdbcDriver";
    public static final String DEFAULT_URL      = "jdbc:odbc:DRIVER={Microsoft Access Driver (*.mdb)};DBQ=c:\\Software\\Java\\Forum\\DataConnection.mdb";
    public static final String DEFAULT_USERNAME = "admin";
    public static final String DEFAULT_PASSWORD = "";
    /** Database connection */
    private Connection connection;
     * Driver for the DataConnection
     * @param command line arguments
     * <ol start='0'>
     * <li>SQL query string</li>
     * <li>JDBC driver class</li>
     * <li>database URL</li>
     * <li>username</li>
     * <li>password</li>
     * </ol>
    public static void main(String [] args)
        DataConnection db = null;
        try
            if (args.length > 0)
                String sql      = args[0];
                String driver   = ((args.length > 1) ? args[1] : DEFAULT_DRIVER);
                String url      = ((args.length > 2) ? args[2] : DEFAULT_URL);
                String username = ((args.length > 3) ? args[3] : DEFAULT_USERNAME);
                String password = ((args.length > 4) ? args[4] : DEFAULT_PASSWORD);
                db = new DataConnection(driver, url, username, password);
                List result = db.query(sql);
                System.out.println(result);
            else
                System.out.println("Usage: db.DataConnection <sql> <driver> <url> <username> <password>");
        catch (SQLException e)
            System.err.println("SQL error: " + e.getErrorCode());
            System.err.println("SQL state: " + e.getSQLState());
            e.printStackTrace(System.err);
        catch (Exception e)
            e.printStackTrace(System.err);
        finally
            if (db != null)
                db.close();
            db = null;
     * Create a DataConnection
     * @throws SQLException if the database connection fails
     * @throws ClassNotFoundException if the driver class can't be loaded
    public DataConnection() throws SQLException,ClassNotFoundException
        this(DEFAULT_DRIVER, DEFAULT_URL, DEFAULT_USERNAME, DEFAULT_PASSWORD);
     * Create a DataConnection
     * @throws SQLException if the database connection fails
     * @throws ClassNotFoundException if the driver class can't be loaded
    public DataConnection(final String driver,
                          final String url,
                          final String username,
                          final String password)
        throws SQLException,ClassNotFoundException
        Class.forName(driver);
        this.connection = DriverManager.getConnection(url, username, password);
     * Clean up the connection
    public void close()
        try
            this.connection.close();
        catch (Exception e)
            ; // do nothing; you've done your best
     * Execute an SQL query
     * @param SQL query to execute
     * @returns list of row values
     * @throws SQLException if the query fails
    public List query(final String sql) throws SQLException
        Statement statement     = this.connection.createStatement();
        ResultSet rs            = statement.executeQuery(sql);
        ResultSetMetaData meta  = rs.getMetaData();
        int numColumns          = meta.getColumnCount();
        List rows               = new ArrayList();
        while (rs.next())
            Map thisRow = new LinkedHashMap();
            for (int i = 1; i <= numColumns; ++i)
                String columnName   = meta.getColumnName(i);
                Object value        = rs.getObject(columnName);
                thisRow.put(columnName, value);
            rows.add(thisRow);
        rs.close();
        statement.close();
        return rows;
}This code compiled and ran on my Windows machine. I tested it against an Access database, and it worked fine. You'll have to change those static variables to point to your stuff if you try it. - MOD