对代码进行必要的更改时出现 java.lang.NullPointerException(Java Netbeans IDE 7.4 和 JDK 1.7)

Question

我最初的代码如下。

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);

     }
 }

以及上面代码中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“Marks”数据库表中的所有数据。

因此，我复制并粘贴了该类和相关类来创建另一个功能完全相同的 JFrame 和 java 类。进行了相关更改，以便在原始 JFrame 和 Java 类与复制的 JFrame 和 java 类之间产生明显的变化。

在这个新的 JFrame 中，我想从数据库中仅提取包含特定学生 ID 的数据。因此，我将选择查询更改为以下内容。

String selectquery = "SELECT * FROM Marks WHERE StudentID=S001";

在执行这个新代码时，我收到以下错误。

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)

尝试更改错误所指出的代码行。但没能解决这个问题。

非常感谢您的帮助。

Answer 1

因为您是从 Java 字符串创建 SQL 查询，所以您需要使用 SQL 语法。 SQL 使用单引号来分隔字符串，因此您需要手动将它们添加到查询中。

这是所需的字符串:

String selectquery = "SELECT * FROM Marks WHERE StudentID='S001'";