ResultSet.next() throws NullPointerException

Question

Hello,
When running the following code:
import java.sql.*;
class DatabaseDriver {
     private int                    databaseType;
     private String               error;
     private Connection     con;
     private Statement          statement;
     private ResultSet          rs;
     static final int          MYSQL_DB_TYPE = 1;
     static final int          PGSQL_DB_TYPE = 2;
     static final int          MSSQL_DB_TYPE = 3;
     static final int          ORACLE_DB_TYPE = 4;
     public DatabaseDriver (int dbType) {
          databaseType = dbType;
          try {
               switch (dbType) {
                    case MYSQL_DB_TYPE:
                         Class.forName("com.mysql.jdbc.Driver").newInstance();
                         break;
                    case PGSQL_DB_TYPE:
                         Class.forName("org.postgresql.Driver").newInstance();
                         break;
                    case MSSQL_DB_TYPE:
                         Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver").newInstance();
                         break;
                    case ORACLE_DB_TYPE:
                         Class.forName("oracle.jdbc.driver.OracleDriver").newInstance();
                         break;
          } catch (Exception e) {
               error = "could not load driver";
     public boolean connect (String server, String user, String password) {
          try {
               con = DriverManager.getConnection("jdbc:mysql://" + server + "?user=" + user + "&password=" + password);
          } catch (Exception e) {
               error = "could not connect to the database";
               return false;
          return true;
     public boolean execQuery (String query) {
          try {
               statement = con.createStatement();
               if (statement.execute(query)) {
                    rs = statement.getResultSet();
               while (rs.next()) {
                    System.out.println(rs.getString("1") + "\t" + rs.getString("2"));
               rs.close();
               rs = null;
               statement.close();
               statement = null;
          } catch (Exception e) {
               error = e.toString();
               e.printStackTrace();
               return false;
          return true;
     public String getError () {
          return error;
     public boolean disconnect () {
          return true;
class Test {
     public static void main (String[] args) {
          DatabaseDriver db = new DatabaseDriver(1);
          db.connect(args[0],args[1],args[2]);
          db.execQuery("USE " + args[3] + ";");
          db.execQuery("SELECT * FROM te;");
          System.out.println(db.getError());
}I receive this in the printout:
java.lang.NullPointerException
     at DatabaseDriver.execQuery(DatabaseDriver.java:61)
     at Test.main(Test.java:7)
bla1     bla2
bla6     bla7
java.lang.NullPointerException
Line 61
while (rs.next()) {
I have read a lot of other posts that say that the connection object is null or the recordset is empty, but it seems that neither of those are the case considering I am able to printout the two rows properly.
Can someone please explain to me what I am doing wrong and how to fix it?
Thanks

Answer 1

You can pepper your code with System.out.println() statements to determine what value goes into the execute() statement before it runs.
In the following, I think the first statement will probably return false. What is this sql trying to accomplish that you think it sould return true? Even if it returned true, what value do you expect in the resultSet?
db.execQuery("USE " + args[3] + ";");
db.execQuery("SELECT * FROM te;");
As a side note, I strongly suggest you use connection pooling and not driverManager. here is an example of getting and returning a connection properly (all within a function)
public String[] myFunction(){
Connection conn=null;
PreparedStatement pstmt1= null;
ResultSet resultSet=null;
ArrayList list1; //array that can grow in size
try{
list1=new ArrayList();
conn= dataSource.getConnection();
pstmt1= conn.prepareStatement();
resultSet= pstmt1.executeUpdate();
while(resultSet.next()){
arrayList.add(resultSet.getString("lastName");
return ( String[] ) arrayList.toArray( new String[arrayList.size()] );
} catch (SqlException e){
e.printStackTrace();
} finally {
if(rsultSet!=null)
resultSet.close();
if(pstmt1!=null)
pstmt1.close();
if(conn!=null)
conn.close();
}