Question

我正在为我的第一个学校数据库项目工作。我以为自己很顺利，但遇到了一个空指针异常，我不知道如何在我所处的情况下修复它。分配是打开并读取一个访问数据库，然后拉来自它的信息，以找到最小余额的客户。

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();
    }
}
}

这些行特别是导致错误：

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

Answer 1

在这一行

System.out.println（＆＃34;余额最低的客户是＆＃34; + lowestBalCust）;

您正在尝试打印lowestBalCust属性，此属性以null开头：

String lowestBalCust = null;

如果此条件为false，您将获得空指针执行，因此，您必须在数据库中检查Balance和CustomerName的值是否存在

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

最后...... rs = null是...... null！所以将结果集置于变量

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);

访问数据库时出现空指针异常

1 个答案: