![When I click the drop button, it shows me this: Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at StudentInformation.RegistryDialog$ButtonListener.actionPerformed(RegistryDialog.java:184)] 1
以下是代码：
![![When I click the drop button, it shows me this: Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at StudentInformation.RegistryDialog$ButtonListener.actionPerformed(RegistryDialog.java:184)]](http://i.stack.imgur.com/oWp7P.png){kind=link}
import java.awt.*;
import javax.swing.*;
import java.util.*;
import java.awt.event.*;
import javax.swing.table.*;
import javax.swing.event.*;
public class RegistryDialog extends JDialog {
//model
static Student selected = null;
Vector addedSbj = new Vector();
Subject slctdsbj;
//view
static JTable tbrsbj;
JScrollPane sprsbj;
JLabel lbrid = new JLabel("id here");
JLabel lbrnme = new JLabel("name here");
JLabel lbrcrs = new JLabel("Course");
JLabel lbryr = new JLabel("Year");
SpinnerModel mod = new SpinnerNumberModel(1,1,4,1);
JSpinner srryr = new JSpinner(mod);
String[] crs = {"BS-CS","BS-IT","BS-IS"};
JComboBox cbrcrs = new JComboBox(crs);
JButton btrok = new JButton("Save");
JButton btrcan = new JButton("Close");
JButton btradd = new JButton("Add Subject");
JButton btrdrp = new JButton("Drop Subject");
RegistryDialog(JFrame parent, Student selectedStud) {
super(parent, true);
btrdrp.setEnabled(true);
selected = selectedStud;
setSbjTable();
setTitle("Student Registration Information");
Container cont = getContentPane();
EasierGridLayout layout = new EasierGridLayout();
cont.setLayout(layout);
cont.add(lbrid); cont.add(lbrnme);
cont.add(btrok); cont.add(btrcan);
cont.add(lbryr); cont.add(srryr);
cont.add(lbrcrs); cont.add(cbrcrs);
cont.add(sprsbj); cont.add(btradd);
cont.add(btrdrp);
layout.setConstraints(lbrid,1,1,2,1);
layout.setConstraints(lbrnme,2,1,2,1);
layout.setConstraints(lbrcrs,3,1,1,1);
layout.setConstraints(lbryr,4,1,1,1);
layout.setConstraints(sprsbj,5,1,3,1);
layout.defineOwnConstraints(cbrcrs,3,2,1,1, 0,0, 0,17, 0,0,0,0);
layout.defineOwnConstraints(srryr,4,2,1,1, 0,0, 0,17, 0,0,0,0);
layout.defineOwnConstraints(btradd,4,2,2,1, 0,0, 0,13, 0,0,0,0);
layout.defineOwnConstraints(btrdrp,6,1,2,1, 0,0, 0,17, 0,0,0,0);
layout.defineOwnConstraints(btrok,6,3,1,1, 0,0, 0,13, 5,0,0,0);
layout.defineOwnConstraints(btrcan,7,3,1,1, 0,0, 0,13, 5,0,0,0);
fillFormWith(selected);
btrok.addActionListener(new ButtonListener());
btrcan.addActionListener(new ButtonListener());
btradd.addActionListener(new ButtonListener());
btrdrp.addActionListener(new ButtonListener());
tbrsbj.setModel(new SubjectModel());
ListSelectionModel tbModel = tbrsbj.getSelectionModel();
tbModel.addListSelectionListener(new SubjTableListener());
Toolkit thKit = getToolkit();
Dimension wndSze = thKit.getScreenSize();
pack();
int wd = getWidth();
int ht = getHeight();
int x = (int)((wndSze.getWidth()/2)-(wd/2));
int y = (int)((wndSze.getHeight()/2)-(ht/2));
setBounds(x,y,wd,ht);
//setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
//fill up for with necessary information.
private void fillFormWith(Student thisStudent){
Person temp = new Person(thisStudent.getFirstName(),
thisStudent.getLastName(),
thisStudent.getMiddleName());
lbrnme.setText("Full Name: "+temp);
lbrid.setText("ID Number: "+thisStudent.getIdNumber());
String stdCourse = thisStudent.getCourse();
int index = -1;
if(stdCourse.trim().equals("BS-CS"))
index = 0;
else if(stdCourse.trim().equals("BS-IT"))
index = 1;
else if(stdCourse.trim().equals("BS-IM"))
index = 2;
cbrcrs.setSelectedIndex(index);
srryr.setValue(new Integer(thisStudent.getYear()));
}
private String getSCode(String thisToString){
StringTokenizer str = new StringTokenizer(thisToString," -");
String thisCode = str.nextToken().trim();
return thisCode;
}
//setup table and table model
public void setSbjTable(){
tbrsbj = new JTable(new SubjectModel());
sprsbj = new JScrollPane(tbrsbj);
tbrsbj.setPreferredScrollableViewportSize(new Dimension(550,150));
tbrsbj.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
}
static class SubjectModel extends AbstractTableModel {
private String[] columnNames = {"SUBJECT","UNITS","MAJOR?"};
private Object[][] data = storeObject();
private Object[][] storeObject(){
Hashtable subj = selected.getSubjects();
Set sKeys = subj.keySet();
Vector keys = new Vector(sKeys);
Collections.sort(keys);
Object[][] temp = new Object[keys.size()][3];
int a=0;
for(Iterator i=keys.iterator();i.hasNext();a++){
String thisKey = ""+i.next();
Subject temp2 = (Subject)subj.get(thisKey);
temp[a][0] = ""+temp2;
temp[a][1] = ""+temp2.getUnits();
if(temp2.isMajor())
temp[a][2] = "YES";
else
temp[a][2] = "NO";
}
return temp;
}
public Object[][] getData(){
return data;
}
public int getColumnCount() {
return columnNames.length;
}
public int getRowCount() {
return data.length;
}
public String getColumnName(int col) {
return columnNames[col];
}
public Object getValueAt(int row, int col) {
return data[row][col];
}
public boolean isCellEditable(int row, int col) {
return false;
}
}
//controller
private class ButtonListener implements ActionListener {
public void actionPerformed(ActionEvent ev) {
if(ev.getSource()==btrok){
String crs = ""+cbrcrs.getSelectedItem();
int yr = Integer.parseInt(""+srryr.getValue());
selected.setCourse(crs);
selected.setYear(yr);
ViewElements.tb4grd.setModel(new StudRec.EnrModel());
ViewElements.tb3stud.setModel(new StudRec.EnrModel());
StudRec.writeFiles(); StudRec.readFiles();
dispose();
}
if(ev.getSource()==btrcan){
dispose();
}
if(ev.getSource()==btradd){
AddSubjectDialog asbj = new AddSubjectDialog(new JFrame(),selected);
asbj.show();
}
if(ev.getSource()==btrdrp){
selected.dropSubject(slctdsbj.getCode());
ViewElements.currentSem.enrollStudent(selected);
StudRec.writeFiles(); StudRec.readFiles();
tbrsbj.setModel(new SubjectModel());
System.out.println("subject dropped.");
}
}
}
private class SubjTableListener implements ListSelectionListener {
public void valueChanged(ListSelectionEvent e) {
if(e.getValueIsAdjusting()) return;
ListSelectionModel lsm = (ListSelectionModel)e.getSource();
if(lsm.isSelectionEmpty()) {
btrdrp.setEnabled(false);
}
else {
btrdrp.setEnabled(true);
int selectedRow = lsm.getMinSelectionIndex();
SubjectModel tempMod = new SubjectModel();
String subj = ""+tempMod.getValueAt(selectedRow,0);
String code = getSCode(subj);
Hashtable ht = ViewElements.currentSem.getSubjectsOpen();
slctdsbj = (Subject)ht.get(code);
}
}
}
private class MyWindowAdapter extends WindowAdapter {
public void windowClosing(WindowEvent we) {
dispose();
}
}
}
对于第184行，这里是代码： selected.dropSubject（slctdsbj.getCode（））;