Question

我已经写了代码，现在我得到了一个nullpointer异常，请你能帮助我，或者只是指出我正确的方向

我仍然是一个大型的java程序员，这是我的最后一个任务，因为tommorow

 import java.sql.*;
 import java.io.*;
 import java.util.*;

 public class QueryDB_N
 {
private Connection con = null;
String userID,firstName,lastName,name,symbol;

public QueryDB_N() throws Exception
{
    Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
    String url = "jdbc:odbc:StockTracker";
    Connection con = DriverManager.getConnection (url);
    Statement stmt = con.createStatement();
    PreparedStatement pStmt = null;

    BufferedReader dataIn = new BufferedReader(new InputStreamReader(System.in));
    System.out.print("Enter User ID: ");
    userID = dataIn.readLine();

    System.out.println("\nStock holdings for User: " + userID);
    System.out.println("Stock  - Description");
    System.out.println("------- ----------------------------");

    pStmt = con.prepareStatement("SELECT  FROM (Users INNER JOIN UserStocks ON Users.userID = UserStocks.userID) INNER JOIN Stocks ON UserStocks.symbol = Stocks.symbol");

        pStmt.setString(1, userID);
        pStmt.setString(2, symbol);

        System.out.println("\n" + userID + "\t" + firstName + "\t" + lastName + "\t");
        System.out.println("\t\t" + symbol + "\t" + name);

     System.out.println("\n\nReport done.");
}


public static void main(String[] args)throws Exception
{
    QueryDB_N q = new QueryDB_N();
}
}

我已经重新编写了代码，现在它将值返回为null而不是实际的符号和描述，我真的需要你的帮助，我已经尝试了所有我无法让它正常工作

/*
Chapter 11: the QueryDB_N Class
Programmer: Jacques Duvenage
Date: November, 2014
Program Name: QueryDB_N.java
 */

import java.sql.*;
import java.io.*;
import java.util.*;

public class QueryDB_N
{
private Connection con = null;
String userID,firstName,lastName,name,symbol;

public QueryDB_N() throws Exception
{
    Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
    String url = "jdbc:odbc:StockTracker";
    Connection con = DriverManager.getConnection (url);
    Statement stmt = con.createStatement();
    PreparedStatement pStmt = null;

    BufferedReader dataIn = new BufferedReader(new InputStreamReader(System.in));
    System.out.print("Enter User ID: ");
    userID = dataIn.readLine();

    System.out.println("\nStock holdings for User: " + userID);
    System.out.println("Stock  - Description");
    System.out.println("------- ----------------------------");

    pStmt = con.prepareStatement("SELECT * FROM (Users INNER JOIN UserStocks ON Users.? = UserStocks.?) INNER JOIN Stocks ON UserStocks.symbol = Stocks.symbol");

        pStmt.setString(1, userID);
        pStmt.setString(2, symbol);


        System.out.println(symbol + "\t" + name);

     System.out.println("\n\nReport done.");
}


public static void main(String[] args)throws Exception
{
    QueryDB_N q = new QueryDB_N();
}
}

Answer 1

这样只有你在同一个PS语句中运行2个查询。这是错误。

public static void getUserID(Connection con, PreparedStatement pStmt, String userID, String stockSymbol) throws Exception
{
    try
    {
        pStmt = con.prepareStatement("INSERT INTO UserStocks VALUES(?,?)");
        pStmt.setString(1, userID);
        pStmt.setString(2, stockSymbol);
        pStmt.executeQuery();
        pstmt.close();        
        pStmt = con.prepareStatement("INSERT INTO Stocks VALUES(?,?)");
        pStmt.setString(1, userID);
        pStmt.setString(2, stockSymbol);
        pStmt.executeQuery();
        pStmt.close();               
    }
    catch(Exception e)
    {
    }

使用准备好的语句进行动态查询

1 个答案: