Null Pointer Exception when accessing database

Question

I'm working on my first database project for school. I thought I was going along nicely, but ran into a null pointer exception and I'm not sure how to fix it in the situation I'm in. The assignment is to open and read an access database, and pull information from it to find the customer with the smallest balance.

package dbproject;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MyFirstDB {


public static void main(String[] args)
{
    ResultSet resultSet = null;
    double totalPref = 0.0;
    double lowestBalPref = 1000000;
    String lowestBalCust = null; 
    double highestBalPref = 0.0;
    String highestBalCust = null;
    double totalBalance = 0.0;
    ResultSet rs = null;
    String convertTo = "";
    double result = 0.0;
    long lngCN = 0;
    String query = null;

    String url = "jdbc:ucanaccess://c:/cps/ms121.accdb";

    try{
        Class.forName("net.ucanaccess.jdbc.UcanaccessDriver");

        Connection con = DriverManager.getConnection(url);

        if(con!=null){
            System.out.println("Connecton Successful!");

        }
        else{
            System.out.println("Connection Unsuccesful!");
            System.exit(0);
        }



        Statement statement = con.createStatement();
        if(statement != null)
            System.out.println("Statement is not null");

        resultSet = statement.executeQuery("SELECT * FROM CUSTOMER where preferred = 'y'");

        while(resultSet.next()){
//                System.out.println(resultSet.getString("CustomerName"));
            totalPref = totalPref + 1;
            if(rs.getDouble("Balance") < lowestBalPref){
                lowestBalCust = rs.getString("CustomerName");
//                System.out.println(resultSet.getDouble("Balance"));
            }
        }
        System.out.println("Total Preferred Customers is " + totalPref);
        System.out.println("The Customer with the lowest balance is " + lowestBalCust);

        if (con != null)
            con.close();

    }catch (Exception e)
    {
        e.printStackTrace();
    }
}
}

These lines in particular seem to be causing the error:

                if(rs.getDouble("Balance") < lowestBalPref){
                lowestBalCust = rs.getString("CustomerName");

Get rid of the uninitialized `rs` variable, use `resultSet` instead . — Arnaud, May 02 '17 at 15:46
Other than the null rs object, avoid performing logic in your data access classes. Pull data from the DB, return it to your app, and perform logic there. — Michael Peacock, May 02 '17 at 16:26

developer_hatch · Answer 1 · 2017-05-02T15:50:03.067

in this line

System.out.println("The Customer with the lowest balance is " + lowestBalCust);

you are trying to print lowestBalCust propertie, and this property starts in null:

String lowestBalCust = null;

If this condition is false, you will get a null pointer exeption, so, you must check in your database that the value for Balance and CustomerName exist

            if(rs.getDouble("Balance") < lowestBalPref){
                lowestBalCust = rs.getString("CustomerName");
            }

finally... rs = null is... null! so asign the result set at the variable

resultSet = statement.executeQuery("SELECT * FROM CUSTOMER where preferred = 'y'");

        while(resultSet.next()){
//                System.out.println(resultSet.getString("CustomerName"));
            totalPref = totalPref + 1;
            if(resultSet .getDouble("Balance") < lowestBalPref){
                lowestBalCust = resultSet .getString("CustomerName");
//                System.out.println(resultSet.getDouble("Balance"));
            }
        }
        System.out.println("Total Preferred Customers is " + totalPref);
        System.out.println("The Customer with the lowest balance is " + lowestBalCust);

check your result sets, (you have two by the way) – developer_hatch May 02 '17 at 15:51 — developer_hatch, May 02 '17 at 15:51

Null Pointer Exception when accessing database

1 Answers1