need help with 2 bugs

[Randall Twede]

My Applet has 2 problems. Both are in the displayQuiz method.
1) This line doesnt seem to work(81).
answers = (String[])theAnswers.elementAt(i);
The array answers doesnt seem to get reassigned. Are Arrays immutable?
2) A NullPointerException is thrown from this line(104).
if(((cbg.getSelectedCheckbox()).getLabel()).equals(correctAnswer)) {
here is the code:

import java.awt.*; 
import java.awt.event.*; 
import java.applet.*;
import java.io.*;
import java.net.*;
import java.util.*; 
public class Quiz extends Applet implements ActionListener {
  private int total = 10000;
  private boolean correct;
  private String correctAnswer;
  private String label;
  private String[] answers = new String[4];
  private Vector theQuestions = new Vector();
  private Vector theAnswers = new Vector();
  public void init() {
    setLayout(new BorderLayout());
    setBackground(Color.black);
    setForeground(Color.white);
    Label choose = new Label("Choose difficulty level");
    add(choose, "North");
    Button button1 = new Button("Easy");
    Button button2 = new Button("Medium");
    Button button3 = new Button("Hard");
    button1.setForeground(Color.black);
    button2.setForeground(Color.black);
    button3.setForeground(Color.black);
    button1.addActionListener(this);
    button2.addActionListener(this);
    button3.addActionListener(this);
    Panel choices = new Panel();
    choices.add(button1);
    choices.add(button2);
    choices.add(button3);
    add(choices, "Center");
  }
  
  public void actionPerformed(ActionEvent e) {
    label = ((Button)e.getSource()).getLabel();
    if(label.equals("Go")) {
      displayAnswer();
    }
    else if(label.equals("Next")) {
      displayQuiz();
    }
    else if(label.equals("Quit")) {
      quit();
    }
    else {
      try {
        loadQuiz(label);
      }
      catch(IOException ex) {
        System.out.println("Cannot load question bank");
      }
    }
  }
  
  public void loadQuiz(String selection) throws IOException {
    URL url = new URL( getDocumentBase(), selection + ".txt");
    BufferedReader in = new BufferedReader(new InputStreamReader(url.openStream()));
    String question = null;
    while((question = in.readLine())!= null) {
      answers[0] = in.readLine();
      answers[1] = in.readLine();
      answers[2] = in.readLine();
      answers[3] = in.readLine();
      theQuestions.add(question);
      theAnswers.add(answers);
    }
    in.close();
    displayQuiz();
  }
  public void displayQuiz() {
    removeAll();
    Label instructions = new Label("Select only one answer.");
    int i = (int)(Math.random() * theQuestions.size());
    Label question = new Label((String)theQuestions.elementAt(i));
    answers = (String[])theAnswers.elementAt(i);
    int j = (int)(Math.random()*4);
    String temp = answers[j];
    answers[j] = answers[0];
    answers[0] = temp;
    correctAnswer = answers[j];
    Button submit = new Button("Go");
    submit.setForeground(Color.black);
    submit.addActionListener(this);
    Panel choices = new Panel();
    choices.setLayout(new GridLayout(4,1));
    CheckboxGroup cbg = new CheckboxGroup();
    for(j = 0;j < 4;j++) {
      choices.add(new Checkbox(answers[j], cbg, false));
    }
    add(instructions, "North");
    add(question, "West");
    add(choices, "Center");
    add(submit, "South");
    validate();
    theQuestions.removeElementAt(i);
    theAnswers.removeElementAt(i);
    while(true) {
      if(((cbg.getSelectedCheckbox()).getLabel()).equals(correctAnswer)) {
        correct = true;
      }
      else {
        correct = false;
      }
    }
  }
  
  public void displayAnswer() {
    removeAll();
    Label answer = new Label("The correct answer is: " + correctAnswer); 
    String bal = "You still have " + total + " Gold Pieces.";
    String result = "Incorrect";
    if(correct) {
      result = "Correct";
      total += 10000;
      bal = "You now have " + total + " Gold Pieces.";
    }
    Label balance = new Label(bal);
    Panel message = new Panel();
    message.setLayout(new GridLayout(2, 1));
    message.add(answer);
    message.add(balance);
    add(message, "Center");
    add(new Label(result), "North");
    Button next = new Button("Next");
    next.setForeground(Color.black);
    next.addActionListener(this);
    Button quit = new Button("Quit");
    quit.setForeground(Color.black);
    quit.addActionListener(this);
    Panel choices = new Panel();
    choices.add(next);
    choices.add(quit);
    add(choices, "South");
    validate();
  }
  public void quit() {
  }
}

---

</BLOCKQUOTE>

[This message has been edited by Randall Twede (edited February 07, 2001).]

[Randall Twede]

I changed the code making the String[] answers local in both loadQuiz and displayQuiz methods. But I get the same results. No matter which question is displayed the last set of answers is shown. I am lost.
I tried changing:
theQuestions.add(question); theAnswers.add(answers);
to
theQuestions.addElement(question); theAnswers.addElement(answers);
Same problem. Although add() is new to 1.2 and would have caused a problem with browsers.

import java.awt.*; 
import java.awt.event.*; 
import java.applet.*;
import java.io.*;
import java.net.*;
import java.util.*; 
public class Quiz extends Applet implements ActionListener {
  private int total = 10000;
  private boolean correct;
  private String correctAnswer;
  private String label;
  private Vector theQuestions = new Vector();
  private Vector theAnswers = new Vector();
  public void init() {
    setLayout(new BorderLayout());
    setBackground(Color.black);
    setForeground(Color.white);
    Label choose = new Label("Choose difficulty level");
    add(choose, "North");
    Button button1 = new Button("Easy");
    Button button2 = new Button("Medium");
    Button button3 = new Button("Hard");
    button1.setForeground(Color.black);
    button2.setForeground(Color.black);
    button3.setForeground(Color.black);
    button1.addActionListener(this);
    button2.addActionListener(this);
    button3.addActionListener(this);
    Panel choices = new Panel();
    choices.add(button1);
    choices.add(button2);
    choices.add(button3);
    add(choices, "Center");
  }
  
  public void actionPerformed(ActionEvent e) {
    label = ((Button)e.getSource()).getLabel();
    if(label.equals("Go")) {
      displayAnswer();
    }
    else if(label.equals("Next")) {
      displayQuiz();
    }
    else if(label.equals("Quit")) {
      quit();
    }
    else {
      try {
        loadQuiz(label);
      }
      catch(IOException ex) {
        System.out.println("Cannot load question bank");
      }
    }
  }
  
  public void loadQuiz(String selection) throws IOException {
    URL url = new URL( getDocumentBase(), selection + ".txt");
    BufferedReader in = new BufferedReader(new InputStreamReader(url.openStream()));
    String question = null;
    String[] answers = new String[4];
    while((question = in.readLine())!= null) {
      answers[0] = in.readLine();
      answers[1] = in.readLine();
      answers[2] = in.readLine();
      answers[3] = in.readLine();
      theQuestions.add(question);
      theAnswers.add(answers);
    }
    in.close();
    displayQuiz();
  }
  public void displayQuiz() {
    removeAll();
    Label instructions = new Label("Select only one answer.");
    int i = (int)(Math.random() * theQuestions.size());
    Label question = new Label((String)theQuestions.elementAt(i));
    String[] answers = new String[4];
    answers = (String[])theAnswers.elementAt(i);
    int j = (int)(Math.random()*4);
    String temp = answers[j];
    answers[j] = answers[0];
    answers[0] = temp;
    correctAnswer = answers[j];
    Button submit = new Button("Go");
    submit.setForeground(Color.black);
    submit.addActionListener(this);
    Panel choices = new Panel();
    choices.setLayout(new GridLayout(4,1));
    CheckboxGroup cbg = new CheckboxGroup();
    for(j = 0;j < 4;j++) {
      choices.add(new Checkbox(answers[j], cbg, false));
    }
    add(instructions, "North");
    add(question, "West");
    add(choices, "Center");
    add(submit, "South");
    validate();
    theQuestions.removeElementAt(i);
    theAnswers.removeElementAt(i);
    while(true) {
      if(((cbg.getSelectedCheckbox()).getLabel()).equals(correctAnswer)) {
        correct = true;
      }
      else {
        correct = false;
      }
    }
  }
  
  public void displayAnswer() {
    removeAll();
    Label answer = new Label("The correct answer is: " + correctAnswer); 
    String bal = "You still have " + total + " Gold Pieces.";
    String result = "Incorrect";
    if(correct) {
      result = "Correct";
      total += 10000;
      bal = "You now have " + total + " Gold Pieces.";
    }
    Label balance = new Label(bal);
    Panel message = new Panel();
    message.setLayout(new GridLayout(2, 1));
    message.add(answer);
    message.add(balance);
    add(message, "Center");
    add(new Label(result), "North");
    Button next = new Button("Next");
    next.setForeground(Color.black);
    next.addActionListener(this);
    Button quit = new Button("Quit");
    quit.setForeground(Color.black);
    quit.addActionListener(this);
    Panel choices = new Panel();
    choices.add(next);
    choices.add(quit);
    add(choices, "South");
    validate();
  }
  public void quit() {
  }
}

---

</BLOCKQUOTE>
[This message has been edited by Randall Twede (edited February 07, 2001).]

[Randall Twede]

OK, I found the problem with the array. The Vector hold a reference to the array not the array itself. I moved this line String[] answers = new String[4]; inside the while loop. I now get the correct set of answers.
I still get a NullPointerException though at this line:
if(((cbg.getSelectedCheckbox()).getLabel()).equals(correctAnswer)) {

[Randall Twede]

I tried changing this:
for(j = 0;j < 4;j++) {
choices.add(new Checkbox(answers[j], cbg, false));
}
to this:
Checkbox cb1 = new Checkbox(answers[0], cbg, false);
Checkbox cb2 = new Checkbox(answers[1], cbg, false);
Checkbox cb3 = new Checkbox(answers[2], cbg, false);
Checkbox cb4 = new Checkbox(answers[3], cbg, false);
choices.add(cb1);
choices.add(cb2);
choices.add(cb3);
choices.add(cb4);
but still a NullPointerException here:
if(((cbg.getSelectedCheckbox()).getLabel()).equals(correctAnswer)) {
even if i choose the right answer it says incorrect
the correct answer is "correctAnswer"
[This message has been edited by Randall Twede (edited February 07, 2001).]

[Randall Twede]

woohoo it works! If anyone is curious I had to put the offending line inside an ItemListener(instead of while loop)