swing java。lang.NullPointerException(Java Netbeans IDE 7.4和JDK 1.7)
我的第一段代码如下
public class JFrameAllMarks extends javax.swing.JFrame {
public JFrameAllMarks() {
initComponents();
setDefaultCloseOperation(JFrameAllMarks.DISPOSE_ON_CLOSE);
showMark();
}
public ArrayList<Marks> markList1()
{
ArrayList<Marks> marksList = new ArrayList<>();
try {
DBConnection obj = new DBConnection();
String selectquery = "SELECT * FROM Marks";
ResultSet rs = obj.getdata(selectquery);
Marks objmk;
while(rs.next())
{
objmk = new Marks(rs.getString("StudentID"), rs.getString("StudentName"), rs.getString("SubjectID"), rs.getString("SubjectName"), rs.getString("BatchID"), rs.getInt("Marks"));
marksList.add(objmk);
}
}
catch (SQLException ex) {
Logger.getLogger(JFrameAllMarks.class.getName()).log(Level.SEVERE, null, ex);
}
return marksList;
}
public void showMark()
{
ArrayList<Marks> list = markList1();
DefaultTableModel dtm = (DefaultTableModel)jTableAllMarks.getModel();
Object[] row = new Object[6];
for(int i=0; i<list.size();i++)
{
row[0] = list.get(i).getStudentID();
row[1] = list.get(i).getStudentName();
row[2] = list.get(i).getSubjectID();
row[3] = list.get(i).getSubjectName();
row[4] = list.get(i).getBatchID();
row[5] = list.get(i).getMarks();
dtm.addRow(row);
}
}
以及上述代码中ArrayList所需的java类
package AllMarksList;
/**
*
* @author Admin
*/
class Marks {
private String StudentID, StudentName, SubjectID, SubjectName, BatchID;
private int Marks;
public Marks (String StudentID, String StudentName, String SubjectID, String SubjectName, String BatchID, int Marks)
{
this.StudentID=StudentID;
this.StudentName=StudentName;
this.SubjectID=SubjectID;
this.SubjectName=SubjectName;
this.BatchID=BatchID;
this.Marks=Marks;
}
public String getStudentID()
{
return StudentID;
}
public String getStudentName()
{
return StudentName;
}
public String getSubjectID()
{
return SubjectID;
}
public String getSubjectName()
{
return SubjectName;
}
public String getBatchID()
{
return BatchID;
}
public int getMarks()
{
return Marks;
}
}
以上代码执行正确。它显示了“标记”数据库表中的所有数据
所以,我复制并粘贴了这个和相关的类,以创建另一个JFrame和java类,它们的功能完全相同。进行了相关更改,以便在原始和复制的JFrames和java类之间进行明显的更改
在这个新的JFrame中,我想从数据库中提取包含特定学生ID的数据。 因此,我将select查询更改为以下内容
String selectquery = "SELECT * FROM Marks WHERE StudentID=S001";
在执行这段新代码时,我收到了以下错误
SEVERE: null
java.lang.NullPointerException
at StudentMarks.JFrameStudentAllMarks.markList1(JFrameStudentAllMarks.java:47)
at StudentMarks.JFrameStudentAllMarks.showMark1(JFrameStudentAllMarks.java:63)
at StudentMarks.JFrameStudentAllMarks.<init>(JFrameStudentAllMarks.java:30)
at StudentMarks.JFrameStudentAllMarks$3.run(JFrameStudentAllMarks.java:224)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:312)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:745)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:715)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
尝试对错误所指出的代码行进行更改。 但没能解决这个问题
非常感谢你的帮助
# 1 楼答案
因为您是用Java字符串创建SQL查询的,所以需要使用SQL语法。SQL使用单引号来分隔字符串,因此需要手动将它们添加到查询中
这是需要的字符串: