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PONullPointerException when using DAO class
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  1. PONullPointerException when using DAO class
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    copied!<p>I tried out DAO Pattern after reading <a href="http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html" rel="nofollow">http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html</a> but I get NullPointerException when I run the servlet</p> <p><strong>DAOFactory</strong></p> <pre><code>package dao; import java.sql.SQLException; public abstract class DAOFactory { // List of DAO types supported by the factory public static final int MYSQL = 1; public abstract UserDAO getUserDAO() throws SQLException; public static DAOFactory getDAOFactory(int whichFactory) { switch (whichFactory) { case MYSQL: return new MySQLDAOFactory(); default : return null; } } } </code></pre> <p><strong>MySQLDAOFactory</strong></p> <pre><code>package dao; import java.sql.*; public class MySQLDAOFactory extends DAOFactory { public MySQLDAOFactory() { } public static final String DRIVER="com.mysql.jdbc.Driver"; public static final String DBURL="jdbc:mysql://localhost:3306/musicompany"; public static Connection createConnection() { Connection conn=null; try{ Class.forName(DRIVER); conn = DriverManager.getConnection(DBURL,"root","toor"); }catch(Exception e){} return conn; } public UserDAO getUserDAO() throws SQLException{ return new MySQLUserDAO(); } } </code></pre> <p><strong>MySQLUserDAO</strong></p> <pre><code>package dao; import java.sql.*; import java.util.ArrayList; import model.User; public class MySQLUserDAO implements UserDAO { Connection conn=null; Statement s=null; public MySQLUserDAO() throws SQLException{ conn = MySQLDAOFactory.createConnection(); s = conn.createStatement(); } public int insertUser(String fname, String lname) { try{ //code to insertUser }catch(Exception e){} return key; } public ArrayList&lt;User&gt; selectUsers() { ResultSet rs=null; ArrayList customerList=null; try{ rs =s.executeQuery("select * from users"); customerList = new ArrayList&lt;User&gt;(); while(rs.next()) { customerList.add(new User(rs.getString("name"), rs.getString("pass"), rs.getInt("type"), rs.getString("email"))); } return customerList; }catch(Exception e){} return customerList; } } </code></pre> <p><strong>UserDAO</strong></p> <pre><code>package dao; import java.util.ArrayList; import model.User; public interface UserDAO { public int insertUser(String fname, String lname); public ArrayList&lt;User&gt; selectUsers(); } </code></pre> <p><strong>model.User.java</strong></p> <p>This class has the following fields and the corresponding accessor methods.</p> <pre><code>String name; String pass; short type; String email; </code></pre> <p><strong>My Servlet file</strong></p> <pre><code>public class AddRetrieve extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{ response.setContentType("text/html;charset=UTF-8"); PrintWriter out = response.getWriter(); try { // create the required DAO Factory DAOFactory factoryObj = DAOFactory.getDAOFactory(DAOFactory.MYSQL); // Create a DAO UserDAO custDAO = factoryObj.getUserDAO(); // print existing users ArrayList list = custDAO.selectUsers(); Iterator i = list.iterator(); while(i.hasNext()) { User o = (User)i.next(); out.println(o.getName()); } } catch(Exception e) { out.println(e); } finally { out.close(); } } </code></pre>
 

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