Java exception

[Oomeh Khima]

hello
I have a quiz system that has a server, a player and a friend client. I have tried running them( by directly running their main classes) it works fine.

However when i load a player and a friend client from an interface by clicking on their respective jButton, the respective interfaces are loaded but i get the following error:

Exception in thread "Thread-3" java.lang.NullPointerException
at playerQuiz.src.QuizClient.run(QuizClient.java:318)
at java.lang.Thread.run(Thread.java:595)

But as i have already mentioned the program works perfectly when not loaded from a button.

The code at line 318:

if ((serverInput = in.readLine()) == null)   

The complete class QuizClient :

  
package playerQuiz.src;
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.net.*;
import java.io.*;
import java.util.*;
 
 
public class QuizClient extends JFrame implements ActionListener, Runnable {
 
 
    String moneySum[] = {"5", "10", "20", "30", "40", "45", "50", "55",
                         "60", "70", "75", "80",
                         "85", "90", "95", "100 "};
 
  
    static PrintStream out;         // for sending to server.
    static BufferedReader in;       // for getting data from server.
    // socket for connection with the server:
    static Socket socket = null;
 
 
    // label to show status messages:
    JLabel lblStatus = new JLabel("Connected! Waiting for a friend...", JLabel.CENTER);
    // label to show questions:
    JLabel lblQuestion = new JLabel(" ", JLabel.CENTER);
    // label to just to show "wheels" label:
    JLabel lblWheels = new JLabel("  Wheels");
    // label to show how many friend wheels are left:
    JLabel lblFriend = new JLabel("  Friend:   ?");
    // label to show how many 50:50 wheels are left:
    JLabel lbl5050 = new JLabel("  50:50 :   ?");
    // label to show how many wheels of the public are left:
    JLabel lblThePublic = new JLabel("  Public:   ?");
    // answer boxes for possible answers:
    AnswerBox cbAnswers[] = new AnswerBox[4];
    // buttons:
    JButton btnFinal, btnFriend, btn5050, btnThePublic, btnRetire;
    // money boxes:
    PointBox cbMoney[] = new PointBox[15];
    // response chart for the public:
    Chart publicChart = new Chart();
 
 
    int selectedAnswer = -1; // last clicked answer.
 
    int moneyIndex = 0;    // money index, updated by server.
 
    // will be true when friend will also be connected:
    public boolean ALL_CONNECTED = false;
 
    // thread which cares about receiving data from server:
    Thread thServer = null;
 
 
    /**
    * Default Constructor.
    */
    public QuizClient()
    {
        super ("The Revision Quiz");
 
        // look & feel setup:
        try {
            UIManager.setLookAndFeel(
                UIManager.getSystemLookAndFeelClassName() );
        } catch (Exception e) {
            System.err.println("Couldn't use the system "
                             + "look and feel: " + e);
        }
 
        // update look & feel for those components created in
        //     declaration (if required):
        lblStatus.updateUI();
        lblQuestion.updateUI();
        lblWheels.updateUI();
        lblFriend.updateUI();
        lbl5050.updateUI();
        lblThePublic.updateUI();
 
        // processing window events:
        WindowListener L= new WindowAdapter () {
            public void windowClosing(WindowEvent e) {
                closeApplication();
            }
        };
        addWindowListener(L);
 
        // prepare the layout:
        JPanel  pMain = new JPanel ();
        pMain.setLayout(new BorderLayout());
 
 
        JPanel pUpper = new JPanel();           // for labels.
        pUpper.setLayout(new GridLayout(2,1));
 
        JPanel  pCenter = new JPanel ();        // for answers.
        pCenter.setLayout(new BorderLayout());
 
        JPanel pAnswers = new JPanel();           // for answers & the public chart.
        pAnswers.setLayout(new GridLayout(2,2, 20,20));
 
        // for buttons:
        JPanel pButtons = new JPanel(new FlowLayout(FlowLayout.CENTER,10,10));
 
        JPanel pLeft = new JPanel(new GridLayout(15,1, 10,10));  // for wheels counters.
        JPanel pRight = new JPanel(new GridLayout(15,1, 1,1));   // for money boxes.
 
        pUpper.add(lblStatus);
        pUpper.add(lblQuestion);
 
        pLeft.add(lblWheels);
        pLeft.add(lblFriend);
        pLeft.add(lbl5050);
        pLeft.add(lblThePublic);
 
        // make & add check boxes for answers:
        cbAnswers[0] = new AnswerBox ("A:",pAnswers);   // first possible answer.
        pAnswers.add(cbAnswers[0]);
        cbAnswers[0].addActionListener(this);
 
        cbAnswers[1] = new AnswerBox ("B:",pAnswers);   // second possible answer.
        pAnswers.add(cbAnswers[1]);
        cbAnswers[1].addActionListener(this);
 
        cbAnswers[2] = new AnswerBox ("C:",pAnswers);   // third possible answer.
        pAnswers.add(cbAnswers[2]);
        cbAnswers[2].addActionListener(this);
 
        cbAnswers[3] = new AnswerBox ("D:",pAnswers);   // fourth possible answer.
        pAnswers.add(cbAnswers[3]);
        cbAnswers[3].addActionListener(this);
 
 
        // make & add buttons:
        btnFinal = new JButton("Final");        // for confirmation of answer.
        pButtons.add(btnFinal);
        btnFinal.addActionListener(this);
 
        btnFriend = new JButton("Friend");      // for friend request.
        pButtons.add(btnFriend);
        btnFriend.addActionListener(this);
 
        btn5050 = new JButton("50:50");         // for 50:50 request.
        pButtons.add(btn5050);
        btn5050.addActionListener(this);
 
        btnThePublic = new JButton("The Public");   // for request of the public.
        pButtons.add(btnThePublic);
        btnThePublic.addActionListener(this);
 
        btnRetire = new JButton("Retire");      // for request to retire.
        pButtons.add(btnRetire);
        btnRetire.addActionListener(this);
 
 
        enableButtons(false);     // disable all buttons.
 
        // make money boxes:
        for (int i=14; i>=0; i--) {
            cbMoney[i] = new PointBox (moneySum[i+1], pRight);
            pRight.add(cbMoney[i]);
        }
        cbMoney[4].setStopStation();    // "1,000"
        cbMoney[9].setStopStation();    // "32,000"
 
        pCenter.add (pAnswers, BorderLayout.SOUTH);
 
        // add response chart for the public:
        publicChart.setVisible(false);  // shown only when required.
        pCenter.add (publicChart, BorderLayout.CENTER);
 
        // Add sub layouts to main layout:
        pMain.add(pUpper, BorderLayout.NORTH);
        pMain.add(pButtons, BorderLayout.SOUTH);
        pMain.add(pCenter, BorderLayout.CENTER);
        pMain.add(pLeft, BorderLayout.WEST);
        pMain.add(pRight, BorderLayout.EAST);
 
        // -------- set colors and fonts:
        pButtons.setBackground(Color.lightGray );
        pAnswers.setBackground(Color.lightGray );
        pCenter.setBackground(Color.lightGray );
        pUpper.setBackground(Color.lightGray );
        pRight.setBackground(Color.lightGray) ;
        pLeft.setBackground(Color.lightGray);
        lblStatus.setForeground(Color.yellow);
        lblQuestion.setForeground(Color.black);
        lblWheels.setForeground(Color.green);
        lblFriend.setForeground(Color.white);
        lbl5050.setForeground(Color.white);
        lblThePublic.setForeground(Color.white);
        Font f1 = new Font("Arial", Font.BOLD, 12);
        Font f2 = new Font("Arial", Font.BOLD, 20);
        lblStatus.setFont(f1);
        lblQuestion.setFont(f2);
        lblWheels.setFont(f1);
        lblFriend.setFont(f1);
        lbl5050.setFont(f1);
        lblThePublic.setFont(f1);
 
        // show the window:
        setContentPane (pMain);
        setSize(590,450);
        setVisible(true);
 
 
        // start the thread that cares about receiving data from server:
        thServer = new Thread (this);
        thServer.start();
 
    }
 
 
 
    /**
     * Main function, from where QuizClient starts.
     */
    public static void main(String args[]) {
 
     String serverAddress = "172.22.45.120";
     int serverPort = 4450;
     try {
socket = new Socket(serverAddress, serverPort);
} catch (UnknownHostException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
 
        System.out.println(socket);
 
        // open in and out streams for talking with the server:
        try {
            in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            out = new PrintStream(socket.getOutputStream());
        }
        catch (IOException e) {
            System.err.println("Open streams -> " + e);
            System.exit(1);
        }
 
 
        // print the address of player - for verification:
        InetAddress localHost = null;
        try {
            localHost = InetAddress.getLocalHost();
        } catch (UnknownHostException e) {
            System.out.println("Unknown host - probably localhost with no IP!");
            // no exit, since can work on "localhost" without internet.
        }
        System.out.println("Player's local address: " + localHost);
 
        // create QuizClient and show its window:
        new QuizClient();
    }
 
 
 
  
    private void pause(int time) {
        try {
            Thread.sleep(time);
        }
        catch (InterruptedException e) {
            System.err.println(e.getMessage());
        }
    }
 
 
 
    /**
    * The run method of that is used by a thread.
    */
    public void run() {
 
        Thread thisThread = Thread.currentThread();
 
        // thread which cares about receiving data from server:
        while (thServer == thisThread) {
            String serverInput;
            try {
            if ((serverInput = in.readLine()) != null)
                gotMessageFromServer(serverInput);  // trigger "event".
            }
            catch (IOException ioe) {
              JOptionPane.showMessageDialog(this,
                   ioe + "\nNot regular game termination!",
                   "Cannot read from server!",
                   JOptionPane.ERROR_MESSAGE);
              closeApplication(); // force to close.
            }
        }
 
    } // end of run().
 
 
 
    public void actionPerformed(ActionEvent e){
      Object src=e.getSource();
 
      if (src == btnFinal) {            // confirmation of answer.
          if (selectedAnswer != -1) {
            enableButtons(false);     // disable all buttons.
            lblStatus.setText("Waiting for server's response...");
            sendToServer("FINAL:" + selectedAnswer);
          }
          else {
             JOptionPane.showMessageDialog(this,
                   "Please select an answer!",
                   "Not selected!", JOptionPane.ERROR_MESSAGE);
          }
      }
      else if  (src == btnFriend) {     // friend request.
          enableButtons(false);     // disable all buttons.
          lblStatus.setText("Waiting for friend's response...");
          sendToServer("FRIEND");
      }
      else if (src == btn5050) {        // 50:50 request.
          enableButtons(false);     // disable all buttons.
          lblStatus.setText("Waiting for server's response...");
          sendToServer("50:50");
      }
      else if (src == btnThePublic) {   // request of the public.
          enableButtons(false);     // disable all buttons.
          publicChart.setChart(0, 0, 0, 0); // reset the chart from any data.
          publicChart.setVisible(true);     // show the chart.
          lblStatus.setText("Waiting for server's response...");
          sendToServer("THE_PUBLIC");
      }
      else if  (src == btnRetire) {     // request to retire.
          enableButtons(false);     // disable all buttons.
          lblStatus.setText("Waiting for server's response...");
          sendToServer("RETIRE");
      }
      else{
 
          // clicks on check-boxes:
          for (int i=0; i<4; i++)
            if (cbAnswers[i] == src) {  // click on one of the answer boxes.
                selectedAnswer = i+1; // +1 because first is 1.
 
                  // deselect any other selected check-boxes
                  //      except the last one:
                  for (int k=0; k<4; k++) {
                      if (k != i)       // not equal to selected.
                        cbAnswers[k].setSelected(false);
                      else
                        cbAnswers[k].setSelected(true);
                  }
 
                return;         // no need to continue.
            }
 
       }
 
    }  // end of actionPerformed().
 
 
 
    
    private void setMoney(String sm) {
        int k;
        try {
            k = Integer.parseInt(sm);
        }
        catch(NumberFormatException e) {
            System.err.println("setMoney()--> " + e);
            return;
        }
 
        // validation (should be between 0 and 15):
        if ((k > 15) || (k < 0)) {
            System.err.println("setMoney() out of bounds --> " + sm);
            return;
        }
 
        moneyIndex = k;  // store in global variable.
 
        updateEarnedMoney();
    }
 
 
    private void updateEarnedMoney() {
        // select earned stations:
        for (int i=0; i<moneyIndex; i++)
            cbMoney[i].setSelected(true);
 
        // clear all the others:
        for (int i=moneyIndex; i<15; i++)
            cbMoney[i].setSelected(false);
    }
 
    private void gotMessageFromServer(String msg) {
        System.out.println("SERVER: [" + msg + "]");
 
        if (msg.equals("ALL_CONNECTED")) {   // game started.
            ALL_CONNECTED = true;
            sendToServer("RESTART");        // reset everything.
            sendToServer("QUESTION");       // ask for first question.
        }
        else if (msg.startsWith("Q:")) {    // question.
            prepareForNextQuestion();
            lblQuestion.setText(extractData(msg));
        }
        else if (msg.startsWith("A1:")) {       // first possible answer.
            cbAnswers[0].setText(extractData(msg));
        }
        else if (msg.startsWith("A2:")) {       // second possible answer.
            cbAnswers[1].setText(extractData(msg));
        }
        else if (msg.startsWith("A3:")) {       // third possible answer.
            cbAnswers[2].setText(extractData(msg));
        }
        else if (msg.startsWith("A4:")) {       // fourth possible answer.
            cbAnswers[3].setText(extractData(msg));
            // got last possible answer, so enable buttons:
            enableButtons(true);
            lblStatus.setText(" ");          // clear the status.
        }
        else if (msg.startsWith("MONEY:")) {    // earned money index.
            setMoney(extractData(msg));
        }
        else if (msg.startsWith("FRIEND:")) {  // how many friend tips left.
            lblFriend.setText ("  Friend: " + extractData(msg));
        }
        else if (msg.startsWith("50_50:")) {  // how many 50:50 tips left.
            lbl5050.setText ("  50:50 : " + extractData(msg));
        }
        else if (msg.startsWith("THE_PUBLIC:")) {  // how many tips of the public left.
            lblThePublic.setText ("  Public: " + extractData(msg));
        }
        else if (msg.equals("NOFRIEND!")) {     // error - no more friend tips!
               lblStatus.setText(" ");          // clear the status.
               JOptionPane.showMessageDialog(this,
                     "Server says you have no more friend tips, sorry.",
                   "No more friend tips", JOptionPane.ERROR_MESSAGE);
               enableButtons(true);
        }
        else if (msg.equals("NO50:50!")) {      // error - no more 50:50 tips!
               lblStatus.setText(" ");          // clear the status.
               JOptionPane.showMessageDialog(this,
                     "Server says you have no more 50:50 tips, sorry.",
                   "No more 50:50 tips", JOptionPane.ERROR_MESSAGE);
               enableButtons(true);
        }
        else if (msg.equals("50:50TWICE!")) {   // error - 50:50 done twice already!
               lblStatus.setText(" ");          // clear the status.
               JOptionPane.showMessageDialog(this,
                 "Server says you have done 50:50 twice already for this question!",
                 "Cannot do 50:50 three times for one question", JOptionPane.ERROR_MESSAGE);
               enableButtons(true);
        }
        else if (msg.equals("NO_PUBLIC!")) {    // error - no more tips of the public!
               lblStatus.setText(" ");          // clear the status.
               JOptionPane.showMessageDialog(this,
                     "Server says you have no more tips of the public, sorry.",
                   "No more tips of the public!", JOptionPane.ERROR_MESSAGE);
               enableButtons(true);
               publicChart.setVisible(false);   // hide the chart (it's empty).
        }
        else if (msg.startsWith("GREEN:")) {   // set right answer be green.
            try { // generally, server sends the right data, check anyway
                int k = Integer.parseInt(extractData(msg));
                cbAnswers[k-1].setCorrect();
            } catch (Exception e) {
                System.out.println("GREEN: -> " + e);
            }
        }
        else if (msg.equals("RIGHT!")) {      // last question was answered correctly!
             lblStatus.setText(" ");          // clear the status.
             JOptionPane.showMessageDialog(this,
                   "This is the right answer!",
                   "Congratulations!", JOptionPane.INFORMATION_MESSAGE);
             lblStatus.setText("Waiting for server's response...");
             sendToServer("QUESTION");       // ask for next question.
        }
        else if (msg.equals("MILLION!")) {    // correct, and you won a million!
             lblStatus.setText(" ");          // clear the status.
             JOptionPane.showMessageDialog(this,
                   "You passed with Distinction!",
                   "Congratulations! - Game over.", JOptionPane.INFORMATION_MESSAGE);
             askToRestart();
        }
        else if (msg.startsWith("WRONG!:")) { // last question was answered WRONG!
             lblStatus.setText(" ");          // clear the status.
             JOptionPane.showMessageDialog(this,
                            "Your answer is wrong!\n Correct answer is " +
                                indexToLetter( extractData(msg) ) +
                            "\nYour current marks is \t" + moneySum[moneyIndex],
                   "Game over!", JOptionPane.ERROR_MESSAGE);
             askToRestart();
        }
        else if (msg.startsWith("RETIRED:")) { // retire request confirmed.
             lblStatus.setText(" ");           // clear the status.
             JOptionPane.showMessageDialog(this,
                   "Your current marks is" + moneySum[moneyIndex] + ",\n" +
                   "and the right answer for the last question is " +
                            indexToLetter ( extractData(msg) ),
                   "Retired!", JOptionPane.INFORMATION_MESSAGE);
             askToRestart();
        }
        else if (msg.startsWith("FRIEND_ANSWER:")) {    // getting friend answer.
            lblStatus.setText("Your friend thinks answer is: " +
                                indexToLetter( extractData(msg) ) );
            // got answer from friend, so enable the buttons:
            enableButtons(true);
        }
        else if (msg.equals("FRIEND_DONTKNOW")) {       // friend doesn't know!
            lblStatus.setText("Your friend doesn't know the answer...");
            // friend says he doesn't know, so enable the buttons:
            enableButtons(true);
        }
        else if (msg.equals("FRIEND_TIMEUP")) {         // friend's time is up!
            lblStatus.setText("Your friend failed to give answer in 30 seconds...");
            // time is up, so enable the buttons:
            enableButtons(true);
        }
        else if (msg.startsWith("TIMER:")) {            // timer state for friend.
            lblStatus.setText("Your friend has: " + extractData(msg) + " seconds.");
        }
        else if (msg.startsWith("Message from Server:")) {
            lblStatus.setText(msg);
        }
        else if (msg.equals("FRIEND_DISCONECTED")) {    // error - friend disconected.
            // though the game can be continued without the friend,
            // we decided to end the game to avoid processing the state
            // when friend is required and it's not there....
            lblStatus.setText(" ");          // clear the status.
            JOptionPane.showMessageDialog(this,
                    "Your Friend has disconnected!\n" +
                    "Server terminated the game.",
                    "Friend disconnected!",
                    JOptionPane.ERROR_MESSAGE);
            closeApplication();     // force to close.
        }
        else if (msg.equals("WATING_PREV_FRIEND")) {    // error - server not ready!
            lblStatus.setText(" ");          // clear the status.
            JOptionPane.showMessageDialog(this,
                    "Server still waits for a connection of Friend" +
                    " for previous game that has not connected yet!\n" +
                    "Try again later...",
                    "Server not ready!",
                    JOptionPane.ERROR_MESSAGE);
            closeApplication();     // force to close.
        }
        else if (msg.equals("TERMINATED")) {        // server terminates our game!
            lblStatus.setText(" ");          // clear the status.
            JOptionPane.showMessageDialog(this,
                    "Server terminated your game for no obvious reason!",
                    "Game terminated!",
                    JOptionPane.ERROR_MESSAGE);
            closeApplication();     // force to close.
        }
        else if (msg.startsWith("CANCEL1:")) {   // 50:50 first canceled answer
            try {
                String index = extractData(msg);
                cbAnswers[Integer.parseInt(index)-1].setText(" ");  // clear.
            } catch (Exception e) {
                System.err.println("CANCEL1: -> " + e);
            }
        }
        else if (msg.startsWith("CANCEL2:")) {   // 50:50 another canceled answer
            try {
                String index = extractData(msg);
                cbAnswers[Integer.parseInt(index)-1].setText(" ");  // clear.
            } catch (Exception e) {
                System.err.println("CANCEL2: -> " + e);
            }
            // clear the status & enable the buttons:
            lblStatus.setText(" ");
            enableButtons(true);
        }
        else if (msg.startsWith("BAR1:")) {  // help of the public (% for "A").
            publicChart.setChart(extractData(msg), 1);
        }
        else if (msg.startsWith("BAR2:")) {  // help of the public (% for "B").
            publicChart.setChart(extractData(msg), 2);
        }
        else if (msg.startsWith("BAR3:")) {  // help of the public (% for "C").
            publicChart.setChart(extractData(msg), 3);
        }
        else if (msg.startsWith("BAR4:")) {  // help of the public (% for "D").
            publicChart.setChart(extractData(msg), 4);
            // assumed that "BAR4:?" is received as the last bar, so
            //   clear the status & enable the buttons:
            lblStatus.setText(" ");
            enableButtons(true);
        }
    }
 
 
 
 
    public void sendToServer(String msg) {
        if (thServer == null)   // shuting down?
            return;
 
        
        out.println(msg);
 
        // Flush the stream and check its error state:
        if (out.checkError())
            System.err.println("Cannot send -> " + msg);
        else
            System.out.println("SEND: " + msg);
    }
 
 
 
    /**
    * Asks user if he/she want's to restart the game.
    * If the answer is "yes", starts new game.
    */
    private void askToRestart() {
            int result;
            result = JOptionPane.showConfirmDialog(this,
                       "Would you like to play again?",
                       "Play again?",
                       JOptionPane.YES_NO_OPTION,
                       JOptionPane.QUESTION_MESSAGE);
 
            if (result == 0) {  // yes, restart.
                lblStatus.setText("Waiting for server's response...");
                sendToServer("RESTART");        // reset everything.
                sendToServer("QUESTION");       // ask for a question.
            }
            else {              // no, close connection.
                closeApplication(); // close connection, and exit.
            }
    }
 
 
 
    /**
    * Closes this Game Client station.
    */
    private void closeApplication() {
        // inform the server:
        sendToServer("CLOSE");
 
        thServer = null;  // stop the thread.
 
        try {
              // close server streams:
              out.close();      // close stream that sends data to server.
              in.close();       // close stream that gets data from server.
              // close socket connected to server:
              if (socket != null)
                 socket.close();
        } catch (IOException e) {}
 
        // close everything:
        System.exit(0);
    }
 
 
 
    /**
    * Enables (or disables) all buttons.
    *
    * @param b when true enables, otherwise disables.
    */
    private void enableButtons (boolean b) {
        btnFinal.setEnabled(b);
        btnFriend.setEnabled(b);
        btn5050.setEnabled(b);
        btnThePublic.setEnabled(b);
        btnRetire.setEnabled(b);
    }
 
 
 
    /**
    * Prepares the GUI for the next question,
    * hides the chart, resets question field, and possible
    * answers.
    */
    private void prepareForNextQuestion() {
        publicChart.setVisible(false);
 
        lblQuestion.setText(" ");
 
        for (int i=0; i<cbAnswers.length; i++) {
            cbAnswers[i].setText("");
            cbAnswers[i].setSelected(false); // reset selected check box.
        }
 
        selectedAnswer = -1;
    }
 
 
 
 
    
    private String indexToLetter(String index) {
        String sANSWERS[] = {"\"A\"", "\"B\"", "\"C\"", "\"D\""};
        try {
            return sANSWERS[Integer.parseInt(index)-1];
        } catch (Exception e) {
            System.err.println("indexToLetter -> " + e);
            return "?";
        }
    }
 
 
 
    private static String extractData(String msg) {
            try {
                int i = msg.indexOf(':');   // get index of first ':'.
                return msg.substring(i+1);  // return second token.
            }
            catch(Exception e) {
               return "";               // in case of error return empty string.
            }
    }
 
}   // end of class.
 
  

[Ulf Dittmer]

Welcome to JavaRanch.

That's too much code to work through all of it, but keep in mind that the code in the main method is executed only if it is run from the command line. So if the "in" and "out" streams are not initialized anywhere else, they will be null if the code is not run from the command line.

[Oomeh Khima]

Thank you for your reply. Where should the "in" and "out" be initialzed and to what, so that they will be null if the code is not run from the command line but from GUI.

[Ulf Dittmer]

Either initialize everything directly in the constructor, or move it into an "init()" method that's called after the constructor is done.