我正在研究netbeans上的java applet，并正在创建一个在线咨询系统。实际上它是我的第一个java项目，所以我实际上是一个新手。
即时通讯使用最新的java版本（8SE）和netbeans。 使用拖放方法（时间限制：|）制作GUI的组件。
工作（应该如此）如下： - 3（J）列出： - 1.大学名单 2.可用课程列表 3. 这将根据所选大学和以前的名单中的课程进行
问题我在搜索有关在JList中添加元素的任何地方，都会产生错误 我尝试使用ListModel方法，但它显示
Error:**angularbracket**identifier**angularbracket** expected
其次我也尝试过Vector方法，但它似乎也不起作用。
这是CODE
import javax.swing.*;
import java.util.Vector;
public class PrefApp extends javax.swing.JApplet {
Vector insert=new Vector();
public String selColl=null; //name of college selected from CollegeList
public String SecCour=null; //name of course selected from CourseList
public static String collegeName[];//holds the college name according to preference list made
public static String courseName[]; //holds the course name according to preference list made | works like Computer Science & Engineering~CUTCSE
public static int count=0; //global var; used by collCode[]&courseName[] to prioritise according to preference list
/**
* Initializes the applet PrefApp
*/
@Override
public void init() {
/* Set the Nimbus look and feel */
//<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
/* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
* For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
*/
try {
for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
javax.swing.UIManager.setLookAndFeel(info.getClassName());
break;
}
}
} catch (ClassNotFoundException ex) {
java.util.logging.Logger.getLogger(PrefApp.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
java.util.logging.Logger.getLogger(PrefApp.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
java.util.logging.Logger.getLogger(PrefApp.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(PrefApp.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}
//</editor-fold>
/* Create and display the applet */
try {
java.awt.EventQueue.invokeAndWait(new Runnable() {
public void run() {
initComponents();
}
});
} catch (Exception ex) {
ex.printStackTrace();
}
}
/**
* This method is called from within the init() method to initialize the
* form. WARNING: Do NOT modify this code. The content of this method is
* always regenerated by the Form Editor.
*/
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">
private void initComponents() {
PageLabel = new javax.swing.JLabel();
DescTextLabel = new javax.swing.JLabel();
jScrollPane1 = new javax.swing.JScrollPane();
CollegeList = new javax.swing.JList<>();
CollegeLabel = new javax.swing.JLabel();
CoursesLabel = new javax.swing.JLabel();
jScrollPane2 = new javax.swing.JScrollPane();
CoursesList = new javax.swing.JList<>();
AddButton = new javax.swing.JButton();
RemoveButton = new javax.swing.JButton();
UpButton = new javax.swing.JButton();
DownButton = new javax.swing.JButton();
NoteLabel = new javax.swing.JLabel();
jScrollPane3 = new javax.swing.JScrollPane();
PreferenceList = new javax.swing.JList<>(insert);
PrefListMade = new javax.swing.JLabel();
SubmitButton = new javax.swing.JButton();
PageLabel.setFont(new java.awt.Font("Malgun Gothic", 1, 36)); // NOI18N
PageLabel.setText("PREFERENCE LIST");
DescTextLabel.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
DescTextLabel.setText("List the College & Courses of your choice according to the admission priority.");
CollegeList.setModel(new javax.swing.AbstractListModel<String>() {
String[] strings = { "Amity School of Engineering & Technology, 580, Delhi", "Bhagwan Parshuram Institute of Technology, P.S.P. - 4,Sector - 17, Rohini,Delhi - 110085", "Bharti Vidyapeeth's College of Engineering, A - 4,Paschim Vihar, New Delhi - 110063", "Delhi Technical Campus, 28/1, Knowledge Park - III,Greater NOIDA, UP", "Guru Tegh Bahadur Institute of Technology,G-8 Area, Rajouri Garden, Opp. Swarg Ashram Mandir, Delhi - 110064", "HMR Institute of Technology & Management, Hameed Pur, Delhi - 110036", "JIMS Engineering Management Technical Campus,48/4, Knowledge Park - III, Greater NOIDA", "Maharaja Agrasen Institute of Technology, Sector - 22,Rohini, Delhi - 110085", "Maharaja Surajmal Institute of Technology, C - 4, Janakpuri, New Delhi - 110058", "Mahavir Swami Institute of Technology, Sonepat,Haryana (Minority Institution)", "Northern India Engineering College, FC-26, Shastri Park, Delhi - 110053", "Ambedkar Institute of Advanced Communication Technologies & Research,Geeta Colony, Delhi – 110092", "Ch. Brahm Prakash Govt. Engineering College,Behind Rao Tula Ram Hospital, Jaffarpur, New Delhi - 110073", "Delhi Institute of Tool Engineering, Okhla Industrial Area, Phase - II, Delhi - 20", "G. B. Pant Govt. Engineering College, Okhla Industrial Estate, Phase - III, Okhla, New Delhi - 110020", "National Power Training Institute, Badarpur,New Delhi" };
public int getSize() { return strings.length; }
public String getElementAt(int i) { return strings[i]; }
});
CollegeList.setSelectionMode(javax.swing.ListSelectionModel.SINGLE_SELECTION);
jScrollPane1.setViewportView(CollegeList);
CollegeLabel.setFont(new java.awt.Font("Tahoma", 1, 18)); // NOI18N
CollegeLabel.setText("Colleges");
CoursesLabel.setFont(new java.awt.Font("Tahoma", 1, 18)); // NOI18N
CoursesLabel.setText("Courses Available");
CoursesList.setModel(new javax.swing.AbstractListModel<String>() {
String[] strings = { "Computer Science & Engineering", "Information Technology", "Electronics & Communication Engineering", "Mechanical & Automation Engineering" };
public int getSize() { return strings.length; }
public String getElementAt(int i) { return strings[i]; }
});
CoursesList.setSelectionMode(javax.swing.ListSelectionModel.SINGLE_SELECTION);
jScrollPane2.setViewportView(CoursesList);
AddButton.setText("Add");
AddButton.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
AddButtonActionPerformed(evt);
}
});
RemoveButton.setText("Remove");
UpButton.setText("Up");
DownButton.setText("Down");
NoteLabel.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
NoteLabel.setForeground(new java.awt.Color(255, 0, 0));
NoteLabel.setText("NOTE: Review your admission preferences carefully. Once submitted, they can't be altered!");
PreferenceList.setSelectionMode(javax.swing.ListSelectionModel.SINGLE_SELECTION);
PreferenceList.addContainerListener(new java.awt.event.ContainerAdapter() {
public void componentAdded(java.awt.event.ContainerEvent evt) {
PreferenceListComponentAdded(evt);
}
});
jScrollPane3.setViewportView(PreferenceList);
PrefListMade.setFont(new java.awt.Font("Tahoma", 1, 18)); // NOI18N
PrefListMade.setText("Preferences of Admission");
SubmitButton.setFont(new java.awt.Font("Tahoma", 1, 14)); // NOI18N
SubmitButton.setText("Submit");
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addContainerGap()
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(PageLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 450, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(DescTextLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 480, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGroup(layout.createSequentialGroup()
.addComponent(CollegeLabel)
.addGap(225, 225, 225)
.addComponent(CoursesLabel)
.addGap(142, 142, 142)
.addComponent(PrefListMade))
.addGroup(layout.createSequentialGroup()
.addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 280, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(20, 20, 20)
.addComponent(jScrollPane2, javax.swing.GroupLayout.PREFERRED_SIZE, 170, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(20, 20, 20)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(AddButton, javax.swing.GroupLayout.PREFERRED_SIZE, 90, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(RemoveButton, javax.swing.GroupLayout.PREFERRED_SIZE, 90, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(UpButton, javax.swing.GroupLayout.PREFERRED_SIZE, 90, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(DownButton, javax.swing.GroupLayout.PREFERRED_SIZE, 90, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(20, 20, 20)
.addComponent(jScrollPane3, javax.swing.GroupLayout.PREFERRED_SIZE, 240, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGroup(layout.createSequentialGroup()
.addComponent(NoteLabel)
.addGap(188, 188, 188)
.addComponent(SubmitButton, javax.swing.GroupLayout.PREFERRED_SIZE, 100, javax.swing.GroupLayout.PREFERRED_SIZE)))
.addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addComponent(PageLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 50, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(10, 10, 10)
.addComponent(DescTextLabel, javax.swing.GroupLayout.PREFERRED_SIZE, 20, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGap(30, 30, 30)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(CollegeLabel)
.addComponent(CoursesLabel)
.addComponent(PrefListMade))
.addGap(8, 8, 8)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 210, javax.swing.GroupLayout.PREFERRED_SIZE)
.addComponent(jScrollPane2, javax.swing.GroupLayout.PREFERRED_SIZE, 210, javax.swing.GroupLayout.PREFERRED_SIZE)
.addGroup(layout.createSequentialGroup()
.addComponent(AddButton)
.addGap(7, 7, 7)
.addComponent(RemoveButton)
.addGap(107, 107, 107)
.addComponent(UpButton)
.addGap(7, 7, 7)
.addComponent(DownButton))
.addComponent(jScrollPane3, javax.swing.GroupLayout.PREFERRED_SIZE, 210, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(17, 17, 17)
.addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(10, 10, 10)
.addComponent(NoteLabel))
.addComponent(SubmitButton, javax.swing.GroupLayout.PREFERRED_SIZE, 30, javax.swing.GroupLayout.PREFERRED_SIZE))
.addGap(0, 10, Short.MAX_VALUE))
);
}// </editor-fold>
private void AddButtonActionPerformed(java.awt.event.ActionEvent evt) {
// TODO add your handling code here:
String collSelected=CollegeList.getSelectedValue(); //reads value of selected college in CollegeList
String courSelected=CoursesList.getSelectedValue(); //reads value of selected course in CoursesList
/*UPDATION OF PREFERENCE LIST*/
collegeName[count]=collSelected;
if(null != courSelected)
switch (courSelected) {
case "Computer Science & Engineering":
courseName[count]="CUTCSE";
break;
case "Information Technology":
courseName[count]="CUTIT";
break;
case "Electronics & Communication Engineering":
courseName[count]="CUTECE";
break;
default:
courseName[count]="CUTMAE";
break;
}
count++; //priority variable
/*UPDATION OF PREFEENCE LIST*/
String choice=courSelected+"@"+collSelected;
insert.addElement(choice);
}
private void PreferenceListComponentAdded(java.awt.event.ContainerEvent evt) {
// TODO add your handling code here:
}
// Variables declaration - do not modify
private javax.swing.JButton AddButton;
private javax.swing.JLabel CollegeLabel;
private javax.swing.JList<String> CollegeList;
private javax.swing.JLabel CoursesLabel;
private javax.swing.JList<String> CoursesList;
private javax.swing.JLabel DescTextLabel;
private javax.swing.JButton DownButton;
private javax.swing.JLabel NoteLabel;
private javax.swing.JLabel PageLabel;
private javax.swing.JLabel PrefListMade;
public javax.swing.JList<String> PreferenceList;
private javax.swing.JButton RemoveButton;
private javax.swing.JButton SubmitButton;
private javax.swing.JButton UpButton;
private javax.swing.JScrollPane jScrollPane1;
private javax.swing.JScrollPane jScrollPane2;
private javax.swing.JScrollPane jScrollPane3;
// End of variables declaration
}
这是错误
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at PrefApp.AddButtonActionPerformed(PrefApp.java:222)
at PrefApp.access$100(PrefApp.java:12)
at PrefApp$4.actionPerformed(PrefApp.java:121)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6535)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6300)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4891)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
JList properties | "insert" refers to the Vector variable being used
