面板不会显示在applet查看器中，但并没有错误

Question

我已经写了这个简单的JApplet（是的，我是个初学者）。它左边有4个按钮，中间有一个卡片布局。左边的每个按钮都应该在中间打开一个不同的面板。我的程序编译时没有任何错误（使用Drjava和Eclipse）。但显示的appletviewer完全是空白的。有人能帮我弄清楚我做错了什么吗？

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ApplicationCentre extends JApplet implements ActionListener {

JButton b1, b2, b3, b4, b5, b6;
JRadioButton rb1 ;
JLabel l1, l2, l3, l4, l5, l6, l7, l8, l9, l10, l11;
JTextField t1,t2,t3,t4,t5,t6;
JTextArea ta1, ta2;
JList j1;
JComboBox studentList;
JRadioButton accept[], reject[];
ButtonGroup radioGroup[];  
JPanel deck;
CardLayout cardManager = new CardLayout();
Student s[];
String uniList [] = {"Toronto", "York", "Western", "Brock", "Guelph", "Waterloo",       "McGill", "Concordia", "Laval", "Macmaster"};
public void init(){
s= new Student [100];
Container container = getContentPane();

deck = new JPanel();
cardManager = new CardLayout(); 
deck.setLayout( cardManager ); 
//add panels to deck
deck.add( inputPanel(), "input" );
deck.add( admitPanel(), "admit" );
deck.add( allPanel(), "displayAll" );
deck.add( onePanel(), "displayOne" );
//Left-side Buttons
JPanel P1 = new JPanel();
P1.setLayout (new GridLayout(4, 1));
b1 = new JButton("INPUT");
b1.addActionListener(this);
P1.add(b1);
b2 = new JButton("ADMIT");
b2.addActionListener(this);
P1.add(b2);
b3 = new JButton("DISPLAY ALL");
b3.addActionListener(this);
P1.add(b3);
b4 = new JButton("DISPLAY ONE");
b4.addActionListener(this);    
P1.add(b4);
container.add(P1, BorderLayout.WEST);
// add deck to container

container.add(deck, BorderLayout.CENTER); 

 }
// Input panel
public JPanel inputPanel (){
JPanel P2 = new JPanel();
P2.setLayout (new GridLayout(6,2));
l1 = new JLabel ("Name");    
P2.add(l1);    
t1 = new JTextField (15);
P2.add(t1);    
l2 = new JLabel ("Name of Program");
P2.add(l2);    
t2 = new JTextField ();
P2.add(t2);
l3 = new JLabel ("Average Mark");
P2.add(l3);    
t3 = new JTextField ();
P2.add(t3);
l4 = new JLabel ("Type of postgrad degree");
P2.add(l4);
t4 = new JTextField ();
P2.add(t4);    
l5 = new JLabel ("Universities (select 3)");
P2.add(l5);
j1 = new JList (uniList);
j1.setVisibleRowCount(3);
j1.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);
P2.add(j1);
b5 = new JButton ("Submit");
P2.add(b5);
l7 = new JLabel ();
P2.add(l7);
return P2;
}
// Admit panel
public JPanel admitPanel(){
JPanel P3 = new JPanel();
P3.setLayout (new BorderLayout());

studentList = new JComboBox(s);
studentList.setMaximumRowCount(8);

P3.add(studentList, BorderLayout.NORTH);

JPanel P3a = new JPanel();
P3a.setLayout (new GridLayout (3,3));
  for (int j=0; j<3; j++) {
    radioGroup[j] = new ButtonGroup();
    accept[j] = new JRadioButton("Accept", false);
    reject[j] = new JRadioButton("Reject", true);
    radioGroup[j].add(accept[j]);
    radioGroup[j].add(reject[j]);
  }
 l8 = new JLabel ();
 P3a.add(l8);
 P3a.add(accept[1]);
 P3a.add(reject[1]);
 l9 = new JLabel ();
 P3a.add(l9);     
 P3a.add(accept[2]);
 P3a.add(reject[2]);
 l10 = new JLabel ();
 P3a.add(l10);     
 P3a.add(accept[3]);
 P3a.add(reject[3]);
 P3.add(P3a, BorderLayout.CENTER);
 b6 = new JButton ("Submit");
 P3.add(b6, BorderLayout.SOUTH);
 return P3;
}
// DisplayAll panel
public JPanel allPanel(){
 JPanel P4 = new JPanel();
 ta1 = new JTextArea ();
 ta1.setFont(new Font("Serif", Font.ITALIC, 12));
 ta1.setEditable(false);
 P4.add(ta1);
 return P4;
}
//DisplayOne Panel
public JPanel onePanel(){
 JPanel P5 = new JPanel();
 P5.setLayout (new BorderLayout());
 JPanel P5a = new JPanel();
 l11 = new JLabel ("Enter Student's name");
 P5a.add(l11);
 t6 = new JTextField ();
 P5a.add(t6);
 P5.add(P5a, BorderLayout.NORTH);
 ta2 = new JTextArea();
 ta2.setFont(new Font("Arial", Font.PLAIN, 14));
 ta2.setEditable(false);
 P5.add(ta2, BorderLayout.CENTER);

 return P5;
}



public void actionPerformed(ActionEvent e) {
 if ( e.getSource() == b1 )    
     cardManager.show( deck,"input" );
 else if ( e.getSource() == b2 )    
     cardManager.show( deck,"admit" );
 else if ( e.getSource() == b3 )    
     cardManager.show( deck,"displayAll" ); 
 else if ( e.getSource() == b4 )    
     cardManager.show( deck,"displayOne" ); 
}
}
// student class and extensions
abstract class Student {
  private String name, programName, uni1, uni2, uni3;
  public boolean u1a=false;
  public boolean u2a=false;
  public boolean u3a=false;
  public Student (String n, String p, String u1, String u2, String u3){
    setName(n);
    setProgramName(p);
    setUni1(u1);
    setUni2(u2);
    setUni3(u3);
  }
public String getProgramName() {
    return programName;
}
public void setProgramName(String programName) {
    this.programName = programName;
}
public String getUni3() {
    return uni3;
}
public void setUni3(String uni3) {
    this.uni3 = uni3;
}
public String getUni2() {
    return uni2;
}
public void setUni2(String uni2) {
    this.uni2 = uni2;
}
public String getUni1() {
    return uni1;
}
public void setUni1(String uni1) {
    this.uni1 = uni1;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
}

class Postgrad extends Student{
  private double avgMark;
  private String degree;
  public Postgrad (String n, String p, String u1, String u2, String u3, double avg,    String d){
    super(n, p, u1, u2, u3);
    setAvgMark(avg);
    setDegree(d);
  }
public double getAvgMark() {
    return avgMark;
}
public void setAvgMark(double avgMark) {
    this.avgMark = avgMark;
}
public String getDegree() {
    return degree;
}
public void setDegree(String degree) {
    this.degree = degree;
}
}

class Undergrad extends Student {
  private double avgMark;
  public Undergrad (String n, String p, String u1, String u2, String u3, double   avg){
    super( n, p, u1, u2, u3);
    setAvgMark(avg);
  }
public double getAvgMark() {
    return avgMark;
}
public void setAvgMark(double avgMark) {
    this.avgMark = avgMark;
}
}

Answer 1

您的代码中有几个问题，可能还有几个问题，但这里（目前）有一个"可运行"的版本：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ApplicationCentre extends JApplet implements ActionListener {

JButton b1, b2, b3, b4, b5, b6;
JRadioButton rb1 ;
JLabel l1, l2, l3, l4, l5, l6, l7, l8, l9, l10, l11;
JTextField t1,t2,t3,t4,t5,t6;
JTextArea ta1, ta2;
JList j1;
JComboBox studentList;
JRadioButton accept[], reject[];
ButtonGroup radioGroup[];  
JPanel deck;
CardLayout cardManager = new CardLayout();
Student s[];
String uniList [] = {"Toronto", "York", "Western", "Brock", "Guelph", "Waterloo",       "McGill", "Concordia", "Laval", "Macmaster"};
public void init(){
s= new Student [100];
Container container = getContentPane();

deck = new JPanel();
cardManager = new CardLayout(); 
deck.setLayout( cardManager ); 
//add panels to deck
deck.add( inputPanel(), "input" );
deck.add( admitPanel(), "admit" );
deck.add( allPanel(), "displayAll" );
deck.add( onePanel(), "displayOne" );
//Left-side Buttons
JPanel P1 = new JPanel();
P1.setLayout (new GridLayout(4, 1));
b1 = new JButton("INPUT");
b1.addActionListener(this);
P1.add(b1);
b2 = new JButton("ADMIT");
b2.addActionListener(this);
P1.add(b2);
b3 = new JButton("DISPLAY ALL");
b3.addActionListener(this);
P1.add(b3);
b4 = new JButton("DISPLAY ONE");
b4.addActionListener(this);    
P1.add(b4);
container.add(P1, BorderLayout.WEST);
// add deck to container

container.add(deck, BorderLayout.CENTER); 

 }
// Input panel
public JPanel inputPanel (){
JPanel P2 = new JPanel();
P2.setLayout (new GridLayout(6,2));
l1 = new JLabel ("Name");    
P2.add(l1);    
t1 = new JTextField (15);
P2.add(t1);    
l2 = new JLabel ("Name of Program");
P2.add(l2);    
t2 = new JTextField ();
P2.add(t2);
l3 = new JLabel ("Average Mark");
P2.add(l3);    
t3 = new JTextField ();
P2.add(t3);
l4 = new JLabel ("Type of postgrad degree");
P2.add(l4);
t4 = new JTextField ();
P2.add(t4);    
l5 = new JLabel ("Universities (select 3)");
P2.add(l5);
j1 = new JList (uniList);
j1.setVisibleRowCount(3);
j1.setSelectionMode(ListSelectionModel.MULTIPLE_INTERVAL_SELECTION);
P2.add(j1);
b5 = new JButton ("Submit");
P2.add(b5);
l7 = new JLabel ();
P2.add(l7);
return P2;
}
// Admit panel
public JPanel admitPanel(){
JPanel P3 = new JPanel();
P3.setLayout (new BorderLayout());

studentList = new JComboBox(s);
studentList.setMaximumRowCount(8);

P3.add(studentList, BorderLayout.NORTH);

JPanel P3a = new JPanel();
P3a.setLayout (new GridLayout (3,3));
radioGroup = new ButtonGroup[3];
accept = new JRadioButton[3];
reject = new JRadioButton[3];
  for (int j=0; j<3; j++) {
    radioGroup[j] = new ButtonGroup();
    accept[j] = new JRadioButton("Accept", false);
    reject[j] = new JRadioButton("Reject", true);
    radioGroup[j].add(accept[j]);
    radioGroup[j].add(reject[j]);
  }
 l8 = new JLabel ();
 P3a.add(l8);
 P3a.add(accept[0]);
 P3a.add(reject[0]);
 l9 = new JLabel ();
 P3a.add(l9);     
 P3a.add(accept[1]);
 P3a.add(reject[1]);
 l10 = new JLabel ();
 P3a.add(l10);     
 P3a.add(accept[2]);
 P3a.add(reject[2]);
 P3.add(P3a, BorderLayout.CENTER);
 b6 = new JButton ("Submit");
 P3.add(b6, BorderLayout.SOUTH);
 return P3;
}
// DisplayAll panel
public JPanel allPanel(){
 JPanel P4 = new JPanel();
 ta1 = new JTextArea ();
 ta1.setFont(new Font("Serif", Font.ITALIC, 12));
 ta1.setEditable(false);
 P4.add(ta1);
 return P4;
}
//DisplayOne Panel
public JPanel onePanel(){
 JPanel P5 = new JPanel();
 P5.setLayout (new BorderLayout());
 JPanel P5a = new JPanel();
 l11 = new JLabel ("Enter Student's name");
 P5a.add(l11);
 t6 = new JTextField ();
 P5a.add(t6);
 P5.add(P5a, BorderLayout.NORTH);
 ta2 = new JTextArea();
 ta2.setFont(new Font("Arial", Font.PLAIN, 14));
 ta2.setEditable(false);
 P5.add(ta2, BorderLayout.CENTER);

 return P5;
}



public void actionPerformed(ActionEvent e) {
 if ( e.getSource() == b1 )    
     cardManager.show( deck,"input" );
 else if ( e.getSource() == b2 )    
     cardManager.show( deck,"admit" );
 else if ( e.getSource() == b3 )    
     cardManager.show( deck,"displayAll" ); 
 else if ( e.getSource() == b4 )    
     cardManager.show( deck,"displayOne" ); 
}
}
// student class and extensions
abstract class Student {
  private String name, programName, uni1, uni2, uni3;
  public boolean u1a=false;
  public boolean u2a=false;
  public boolean u3a=false;
  public Student (String n, String p, String u1, String u2, String u3){
    setName(n);
    setProgramName(p);
    setUni1(u1);
    setUni2(u2);
    setUni3(u3);
  }
public String getProgramName() {
    return programName;
}
public void setProgramName(String programName) {
    this.programName = programName;
}
public String getUni3() {
    return uni3;
}
public void setUni3(String uni3) {
    this.uni3 = uni3;
}
public String getUni2() {
    return uni2;
}
public void setUni2(String uni2) {
    this.uni2 = uni2;
}
public String getUni1() {
    return uni1;
}
public void setUni1(String uni1) {
    this.uni1 = uni1;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
}

class Postgrad extends Student{
  private double avgMark;
  private String degree;
  public Postgrad (String n, String p, String u1, String u2, String u3, double avg,    String d){
    super(n, p, u1, u2, u3);
    setAvgMark(avg);
    setDegree(d);
  }
public double getAvgMark() {
    return avgMark;
}
public void setAvgMark(double avgMark) {
    this.avgMark = avgMark;
}
public String getDegree() {
    return degree;
}
public void setDegree(String degree) {
    this.degree = degree;
}
}

class Undergrad extends Student {
  private double avgMark;
  public Undergrad (String n, String p, String u1, String u2, String u3, double   avg){
    super( n, p, u1, u2, u3);
    setAvgMark(avg);
  }
public double getAvgMark() {
    return avgMark;
}
public void setAvgMark(double avgMark) {
    this.avgMark = avgMark;
}
}

最重要的问题：您忘记初始化数组radioGroup、accept和reject，并试图访问索引1、2、3上的前三个元素，但Java（与大多数语言一样）开始以0计数，因此您必须通过索引0、1、2访问才能获得前三个元件。