我有一个我无法弄清楚的问题，我确信这很容易，但我的谷歌搜索和 Stackoverflow 搜索并没有让我到达那里。java文档也没有帮助。我发现了很多人们在 SQL 语句或代码中出现语法错误的问题。我认为我的在代码中。它是关于通过查询从数据库中获取一些信息，用户可以在其中填写自己的搜索信息（数字）。
这是代码：（我已经尝试了多种方法，所以我的代码可能不再是最好的了。另外我是java新手，所以请考虑到这一点。）
/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package dbConnection;
import data.Mail;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.logging.Level;
import java.util.logging.Logger;
import view.SelectionScreenCRA;
/**
*
* @author vm
*/
public class dbConnect {
public static Connection getConnection() {
String driver = "sun.jdbc.odbc.JdbcOdbcDriver";
String url = "jdbc:odbc:Driver={Microsoft Access Driver (*.mdb)};DBQ=C:\\Users\\Jeroen Veldhuijzen\\locaties.mdb;";
String username = "";
String password = "";
try {
Class.forName(driver);
return DriverManager.getConnection(url, "", "");
} catch (Exception ex) {
Logger.getLogger(dbConnect.class.getName()).log(Level.SEVERE, null, ex);
System.out.println("\nError: " + ex.toString() + ex.getMessage());
}
return null;
}
public static ArrayList<dbConnect> executeQuery(int cra) {
ArrayList<dbConnect> locatie = new ArrayList<dbConnect>();
Mail mail = new Mail();
try{
Connection conn = getConnection();
PreparedStatement pst = conn.prepareStatement(
"SELECT Locatiegegevens.[Locatiegegevens], "
+ "Locatiegegevens.[ISP - Straatnaam], "
+ "Locatiegegevens.[ISP - Huisnummer],"
+ " Locatiegegevens.[ISP - Postcode], "
+ "Locatiegegevens.[ISP - Plaatsnaam], "
+ "Locatiegegevens.[G4S - Transportcode] "
+ "FROM Locatiegegevens "
+ "WHERE Locatiegegevens.[Locatiegegevens] = '?';");
pst.setInt(1, cra);
pst.execute();
pst.close();
conn.close();
}catch (Exception e){
System.out.println("ERRROOOOR");
System.out.println(e.getMessage());
System.out.println(e.getClass());
Logger.getLogger(dbConnect.class.getName()).log(Level.SEVERE, null, e);
}
return locatie;
}
}
int cra 将在此处设置：
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
dbConnect dbc = new dbConnect();
dbConnect.executeQuery(Integer.parseInt(getCRA().getText()));
}
cra 将作为 String 检索，然后在传递给 executeQuery 命令时解析为 int。如果我发出 cra int，它会给我 int 9501，它会在文本框中填写。但是，如果我通过按下按钮 jButton1 来运行查询，则会出现 null 错误：
null
class java.lang.NullPointerException
aug 09, 2012 12:43:04 PM dbConnection.dbConnect executeQuery
SEVERE: null
java.lang.NullPointerException
at sun.jdbc.odbc.JdbcOdbcPreparedStatement.clearParameter(JdbcOdbcPreparedStatement.java:1022)
at sun.jdbc.odbc.JdbcOdbcPreparedStatement.setInt(JdbcOdbcPreparedStatement.java:526)
at dbConnection.dbConnect.executeQuery(dbConnect.java:62)
at view.SelectionScreenCRA.jButton1ActionPerformed(SelectionScreenCRA.java:502)
at view.SelectionScreenCRA.access$900(SelectionScreenCRA.java:29)
at view.SelectionScreenCRA$11.actionPerformed(SelectionScreenCRA.java:333)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6504)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6269)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4860)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4686)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2713)
at java.awt.Component.dispatchEvent(Component.java:4686)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
at java.awt.EventQueue.access$000(EventQueue.java:101)
at java.awt.EventQueue$3.run(EventQueue.java:666)
at java.awt.EventQueue$3.run(EventQueue.java:664)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:680)
at java.awt.EventQueue$4.run(EventQueue.java:678)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
有没有人可以指出我正确的方向？