java.lang.NullPointerException when made a required change to the code (Java Netbeans IDE 7.4 & JDK 1.7)

My initial piece of code is the following.

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);
         
     }
 }

And the required java class for the ArrayList in the above code.

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;
    }
       
}

The above code executed correctly. It showed all data that resided in the "Marks" database table.

So, I copied and pasted this and the relevant class to create another JFrame and java class that functions the exact same way. Relevant changes were made to have distinct changes between the original and the copied JFrames and java classes.

In this new JFrame, I wanted to extract from the database, only the data that contained a certain Student ID.
So, I changed the select query to the following.

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

And when executing this new code, I received the following errors.

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)

Tried making changes to the lines of code, pinpointed by the error.
But had no luck in solving this.

Would really appreciate your assistance.

Because you created the SQL query out of a Java String, you need to use SQL syntax. SQL uses single-quotes to delimit a string, so you need to add those into your query manually.

This is the string that is needed:

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