Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

[Anders Kviback]

I can't figure out why I keep on getting the message Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException.

I have a server client program where the server sends out a mathnumber to the client. The number is shown up on a gui textfield. I put in my answer and hit the sendbutton.

Here is part of the so called MathClientHandler class.

package Lektion5;
/**
 * @author Anders Kvibäck
 * Kurs: JavaIII-DT066G SO11
 * Datum: 120607
 * Del: Lektion5, uppgift1 och tentauppgift
 * @file MathClientHandler.java
 */
import java.io.IOException;
import java.lang.ArithmeticException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.net.SocketException;


public class MathClientHandlerNy extends Thread{
	// Den socket kommunikation sker på
	private Socket socket;
	private int number1;
	private int number2;
	private String operand;
	private int kSvar;
	// Klientens adress
	private String address;
	private MatteTalNy talSvar;
	// Konstruktor
	public MathClientHandlerNy(Socket s)
	{
		socket = s;
		address = socket.getInetAddress().getHostAddress();
	}
	//Starta en tråd
	public void run()
	{
		// Deklarerar inström och utström
		ObjectInputStream in = null;
		ObjectOutputStream out = null;
		try
		{
			// Skapar in- och utströmmar för kommunikation
			out = new ObjectOutputStream(socket.getOutputStream());
			in = new ObjectInputStream(socket.getInputStream());
		}
		catch (IOException ioe)
		{
			System.out.println("MathClientHandlerNy:" + address + ": Kunde inte skapa strömmar");
			return; // Bör stänga strömmar och sockets först
		}

		// Vi har nu en uppkopplad förbindelse med klienten
		// Dags att ta emot talet som vi ska räkna ut

		while(true){
			try{
				// Servern skapar ett nytt mattetal med slumpade värden
				// och skickar objektet till klienten
				out.writeObject(new MatteTalNy());
				out.flush();
			}
			catch(IOException ioe){
				System.out.println("MathClientHandlerNy:" + address + ": Nedkopplad");
				return;
			}
			catch(ArithmeticException ae){
				System.out.println("Division med noll!\nVänta på nytt tal!");
			}

			//Servern tar emot mattetalet(som nu innehåller användarens svar) 
			try{
				talSvar = (MatteTalNy) in.readObject();
			}
			catch (SocketException e){
				System.out.println(address + ": Nedkopplad");
				
			}
			catch (ClassNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} 

			catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			try{
				String klientSvar = talSvar.getAnswer();
				kSvar = Integer.parseInt(klientSvar);
				number1 = talSvar.getNumber1();
				number2 = talSvar.getNumber2();
				operand = talSvar.getOperand(); 

				MatteTalNy tal2 = new MatteTalNy(number1,number2,operand);
				int serverCalc = tal2.calculate();
				if (serverCalc == kSvar){
					boolean answer = true;
					out.writeBoolean(answer);
					out.flush(); 
				}
				else {
					boolean answer = false;
					out.writeBoolean(answer);
					out.flush();
				}
			}
			catch (SocketException se){
				System.out.println(address + ": Nedkopplad");
			}
			catch(NullPointerException ne){
				System.out.println("MathClientHandlerNy: Klienten har inte angett något tal");
			}
			catch(NumberFormatException ne){
				System.out.println("Klienten har inte angett något tal");
			}
			catch(ArithmeticException ae){
				System.out.println("Division med noll!\nVänta på nytt tal!\n");
			}
			catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			// Sover en stund
			try
			{
			sleep(1000);
			}
			catch (InterruptedException ie)
			{
			break;				
			}

		}//end-while
	
		// Stänger strömmar och socket
		try
		{
			in.close();
			out.close();
			socket.close();
			System.out.println( address + "mathclienthandler: Nedkopplad");
		}
		catch(IOException ioe)
		{
			System.out.println(address + " Kunde inte stänga strömmar och/eller sockets");
		}
	}//end-run
}//end-class

And here is the Client class which has an inner class:

package Lektion5;

import java.net.*;

import java.util.Scanner;
import java.io.*;

public class MathClientNy {

	private String serverAddress;
	private int serverPort;
	public Thread aktivitet;
	private MatteTalNy talC;
	private MathClientGUINy gui;
	//private ObjectInputStream in;
	//private ObjectOutputStream out;
	private Socket s;
	// Deklarerar de komponenter som ska användas

	// Konstruktorn i vilken vi skapar komponenterna
	public MathClientNy(String address, int port, MathClientGUINy gui){
		serverAddress = address;
		serverPort = port;
		this.gui = gui;
		 
		ClientListener cl = new ClientListener();
		cl.start();
	}
	
	
	class ClientListener implements Runnable {
		private ObjectInputStream in;
		private ObjectOutputStream out;
		private Socket s;
	@SuppressWarnings("static-access")
	public void run() {
			// Skapar socket och strömmar
			try{
				s = new Socket(serverAddress, serverPort);
				//s.setSoTimeout(10000);

				// Viktigt att ObjectOutputStream skapas först eftersom
				// ObjectInputStream blockerar tills att motsvarande
				// ObjectOutputStream skickar en header
				out = new ObjectOutputStream(s.getOutputStream());
				in = new ObjectInputStream(s.getInputStream());
			}
			catch (IOException ioe)
			{
				show("MathClient-Fel vid upprättande av förbindelse med server:\n" + ioe);
				show("\nMathClient-Ingen kontakt");
				return;
			}
			
			//Ta emot tal från server och vänta på att användaren ska skriva in sitt svar
			while(!aktivitet.interrupted()){
				
				try{
					talC = (MatteTalNy)in.readObject();
					String tal = talC.toString();
					showNbr(tal + " = ");
				}
				catch (ClassNotFoundException cnfe)
				{
					show("Hittar inte klassen MatteTal:\n" + cnfe);
					show("Avbryter\n");
					return; // Bör stänga strömmar/sockets först
				}
				catch (IOException ioe)
				{
					// Mest troligt har servern stängts ner
					show("MathClientNy-Kunde inte ta emot tal:\n" + ioe);
					show("MathClient-Ingen kontakt\n");
					break;	
				}
				System.out.println("MathClientBefore _interrupt()");
				stop();
				System.out.println("MathClientAfter _interrupt()");
				try{
					boolean b = in.readBoolean();
					if(b ==true){
						gui.clear();
						show("Rätt svar\n");
					}
					else if(b==false){
						gui.clear();
						show("Fel svar\n");
					}
					else show("Du måste ange ett tal!\n");
				} 
				catch (EOFException eof){
					gui.clear();
					show("Du måste ange ett tal!\n");
				}
				catch (NullPointerException npe){
					show("Du måste ange ett tal!\n");
				}
				catch(NumberFormatException ne){
					show("Du måste ange ett tal!\n");
				}
				catch (IOException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
				try{
					aktivitet.sleep(1000);
				}
				catch(InterruptedException ie){
					aktivitet = null;
					break;
				}
			}//end-while
			
			try// Stänger strömmar och socket
			{
				in.close();
				out.close();
				s.close();
			}
			catch (IOException iofel)
			{}
			
			}//end-run	
	
		public void sendAnswer(String tal){
			show("MathClientNy_Före _aktivitet.start()");
			//start();
			show("MathClientNy_Efter _aktivitet.start()");
			//Användaren skriver in svaret som lagras i objektet
			talC.setAnswer(tal);
			try{
			//Klienten skickar tillbaka objektet till servern
			out.writeObject(talC);
			out.flush();
			}
			catch (NullPointerException npe){
			show("Du måste ange ett tal!\n");
			}
			catch(NumberFormatException ne){
			show("Du måste ange ett tal!\n");
			}
			catch (IOException ioe)
			{
			// Mest troligt har servern stängts ner
			show("MathClient-Kunde inte ta emot tal:" + ioe + "\n");
			show("MathClient-Ingen kontakt\n");
			
			}
		}//sendAnswer
	
		public void start(){
			show("MathClient_start()");
			if(aktivitet == null){
				aktivitet = new Thread(this);
				aktivitet.start();
			}
		}
		public void stop(){
			show("MathClientFöreaktivitet _stop()\n");
			if(aktivitet != null){
				show("MathClient Efter_stop()\n");
				aktivitet.interrupt();
				aktivitet = null;
				show("MathClient aktivitet=null i_stop()\n");
			}
		}
	}//end-inner class
		public  void showNbr( String nbr){
			gui.show_Number(nbr);
		}
		@SuppressWarnings("static-access")
		public void show(String s){
			gui.showMessage(s);
		}
			
}//end-MathClientNy

And here is the gui class:

package Lektion5;
/**
 * @author Anders Kvibäck

 * Kurs: JavaIII-DT066G SO11
 * Datum: 120607
 * Del: Lektion5, uppgift1 och tentauppgift
 * @file MathClientGUI.java
 */
import java.awt.BorderLayout;


import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;


@SuppressWarnings("serial")
public class MathClientGUINy extends JFrame implements ActionListener
{
	private JButton sendButton;
	private JButton startButton;
	private JTextField addressFieldM;
	private JTextField portFieldM;
	private JTextField portFieldF;
	private JTextField mathNrField;
	private JTextField answerField;
	private static JTextArea messageArea;
	private Container cont;
	private String tal;
	private JScrollPane sp;
	private MathClientNy.ClientListener mcn;
	
	public MathClientGUINy() {

		initiateInstanceVariables();
		buildFrame();
		makeNorthPanel();
		makeCenterPanel();
		makeSouthPanel();
		makeMenu();

	}
	private void buildFrame(){
		//Sätter titel på fönstret
		setTitle("Klient");
		//Layouten blir en gridlayout med 4 rader och 1 kolumn
		cont.setLayout(new GridLayout(4,1));

		//Sätter storlek och placering av fönstret
		setSize(500, 500);
		setLocation(600, 100);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setVisible(true);
	}
	private void initiateInstanceVariables(){
		addressFieldM = new JTextField(10);
		addressFieldM.setBorder(BorderFactory.createTitledBorder(" Mottagaradress  "));
		portFieldM = new JTextField(5);
		portFieldM.setBorder(BorderFactory.createTitledBorder(" Port "));
		portFieldF = new JTextField(10);
		portFieldF.setBorder(BorderFactory.createTitledBorder(" Port filhantering "));

		mathNrField = new JTextField(10);
		mathNrField.setBorder(BorderFactory.createTitledBorder("Mattetal: "));

		answerField = new JTextField(12);
		answerField.setBorder(BorderFactory.createTitledBorder("Skriv in ditt svar här: "));

		messageArea = new JTextArea();
		
		messageArea.setBorder(BorderFactory.createTitledBorder(" Meddelande "));
		sp = new JScrollPane(messageArea);
		
		// Skapar JButton för uppdatering
		sendButton = new JButton("Skicka svar");
		sendButton.addActionListener(this);
		startButton = new JButton("Starta klient");
		startButton.addActionListener(this);
		cont = getContentPane();
	}
	private void makeNorthPanel()
	{
		// Placerar ut komponenten i fönstret
		JPanel northPanel = new JPanel(new FlowLayout());
		northPanel.add(addressFieldM );
		northPanel.add(portFieldM);
		northPanel.add(portFieldF);
		northPanel.add(startButton);
		cont.add(northPanel, BorderLayout.NORTH);
	}

	private void makeCenterPanel(){
		JPanel backPanel = new JPanel(new FlowLayout());
		backPanel.add(mathNrField);
		backPanel.add(answerField);
		backPanel.add(sendButton);
		cont.add(backPanel, BorderLayout.CENTER);
	}
	private void makeSouthPanel(){
		JPanel southPanel = new JPanel(new FlowLayout());
		southPanel.add(messageArea);
		messageArea.setPreferredSize(new Dimension(400,200));
		
		cont.add(southPanel, BorderLayout.SOUTH);
		cont.add(sp);
	}


	private void makeMenu()
	{
		JMenuBar menuBar = new JMenuBar();
		JMenu menu = new JMenu("Arkiv");

		JMenuItem menuItem = new JMenuItem("Skicka fil");
		menuItem.addActionListener(this);
		menu.add(menuItem);
		menuBar.add(menu);

		menuItem = new JMenuItem("Avsluta");
		menuItem.addActionListener(this);
		menu.add(menuItem);
		menuBar.add(menu);

		setJMenuBar(menuBar);
	}

	public void actionPerformed(ActionEvent ae){
		String text = ae.getActionCommand();
		if(text.equals("Skicka svar")){
			//mcn.start();
			tal = answerField.getText();
			System.out.println("Tal: " + tal);
			mcn.sendAnswer(tal);
			
		}
		if(text.equals("Skicka fil")){

			laddaFrånFil();
		}
		if(text.equals("Starta klient")){

			startUpClient();
		}
		else if (text.equals("Avsluta"))
		{
			System.exit(0);
		}
	}

	public void startUpClient(){
		// Skapar komponenten
		
		String address = addressFieldM.getText();
		String portM = portFieldM.getText();
		if( address.equals("")|| portM.equals("")){
			messageArea.setText("Du måste ange vilken port och mottagare som ska användas!\n");
		}
		else{
			int port = Integer.parseInt(portM);
			new MathClientNy(address, port, this);

		}
	}
	public void laddaFrånFil(){
		// Skapar komponenten
		String address = addressFieldM.getText();
		String portF = portFieldF.getText();
		if( address.equals("")|| portF.equals("")){
			messageArea.setText("Du måste ange vilken port och mottagare som ska användas!\n");
		}
		else{
			int port = Integer.parseInt(portF);
			new FileClientNy(address, port, this);
		}
	}

	//Metod för att kunna ta emot meddelanden på MathClientGUINy.
	public void show_Number(String tal){

		mathNrField.setText(tal);
		answerField.setText("");
	}

	 
	/*public void send_Answer(String tal){
		tal = answerField.getText();
	}*/
	
	
	//Metod för att visa meddelanden på textArea
	public static void showMessage(String message){
		messageArea.append(message);
	}

	/*public String sendFromClient(){
		tal = answerField.getText();
		//send_Answer(tal);	
		return tal;
	}*/
	public void clear(){
		messageArea.setText("");
	}

	public String toString(){
		return tal;
	}
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		new MathClientGUINy();
		messageArea.append("Använd port 10001 och 10011!\n");
	}
}
			

So, when I execute this I get the error message:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Lektion5.MathClientGUINy.actionPerformed(MathClientGUINy.java:147) <--- So, this is the problem
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)


Why do I get a NPE here?
There is something wrong with the line

mcn.sendAnswer(tal);

Why doesn't the code understand it, or mcn?

Could anyone please help me here?


Anders

[Henry Wong]

Anders Kviback wrote:I can't figure out why I keep on getting the message Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException.

I have a server client program where the server sends out a mathnumber to the client. The number is shown up on a gui textfield. I put in my answer and hit the sendbutton.

Here is part of the so called MathClientHandler class.

So, when I execute this I get the error message:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Lektion5.MathClientGUINy.actionPerformed(MathClientGUINy.java:147) <--- So, this is the problem
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)


Why do I get a NPE here?
There is something wrong with the line

mcn.sendAnswer(tal);

Why doesn't the code understand it, or mcn?

Could anyone please help me here?


Anders

That is what debugging is for -- the JVM is complaining that it can't dereference something on that line because something is null. But what is null??? Put a couple of print outs at the line -- print out the value of mcn. print out the value of tal. are any of them set to null?

And also, don't assume the line number is exact -- it may be off a bit, so put some debugging lines to make sure you are where you think it is.

Henry

[Chris Hurst]

mcn is null , where do you set it to not null ??

Just skimming the code but it just looks wrong .

... mcn is declared (null), you call mcn.sendAnswer(tal); at the offending line which is a null pointer exception.

[Anders Kviback]

Thank you for answering, but why is mcn declared null?
Could you please explain that?

Anders

[Anders Kviback]

Yes, you are right. Forget my previous message.

Thank you-I have to look into the code again.

Thanks, Anders

[Ulf Dittmer]

That's a question for you. It gets declared, but nothing is assigned to it, so it stays null. But you should know what should be assigned to it, because you put the declaration there, right?

I notice that in line 26 of MathClientNy you are instantiating an object that -at a quick glance- might fit there; is that the one that you want to use for "mcn"?

[Anders Kviback]

So, I've been working on this now. I believe that I managed to solve that problem.
I changed the way to solve the issue but now I've got stuck on another thing.

I have a server and a MathClientHandler class that is supposed to send out mathematical
numbers, for example 2/4.
On the other side I have a client MathClient that is supposed to put an answer to
these numbers.

The MathClientHandler can only send out numbers when the client has answered on them.
I e it has to wait until it is OK to send.
I have a class Numbergenerator that deliveres the numbers:

public class Numbergenerator extends Thread {
	
	private MathNumber nbr;

	
	public Numbergenerator(MathNumber nbr) {
		this.nbr = nbr;
	}

	public Numbergenerator(){
		
	}

	public void run() {
		
		while (!isInterrupted()) {
			
			try {
				this.sleep((int) (Math.random() * 1000));
			} catch (InterruptedException ie) {
				
				break;
			}

			nbr = new MathNumber();
			nbr.setNumber(op1, op, op2);
		}
	}
}  
 

The MathNumber class looks partly like this:

// Method in order to set the new number to calculate
		public synchronized void setNumber(int op1, String op, int op2) {
			
			while (operand != null) {
				try {
					
					wait();

				} catch (InterruptedException ie) {
				}
			}

			// It's Ok to set another number
			this.number1 = op1;
			this.operand = op;
			this.number2 = op2;

			// Waik up threads that are waiting to get the number
			notifyAll();

		}

		// Method that returns the actual mathnumber as a string
		public synchronized MathNumber getNumber() {
			
			while (operand == null) {
				try {
					System.out.print("Väntar på nytt tal");
					wait();
					System.out.println();
				} catch (InterruptedException ie) {
				}
			}
			
			 MathNumber nbr = new MathNumber(number1, number2, operand);
			 operand = null;
			// Waik up the threads that are waiting to set the new number
			notifyAll();

			// Return the  number
			return nbr;
		}

And then part of the server class:

public class MathClientHandler extends Thread{
	
	private Socket socket;
	private int number1;
	private int number2;
	private String operand;
	private int kSvar;
	
	private String address;
	private MatteTal talSvar;
	private MatteTal tal;
	private MatteTal nyttTal;
	// Deklarerar inström och utström
	private		ObjectInputStream in = null;
	private		ObjectOutputStream out = null;
	
	// Konstruktor
	public MathClientHandler(Socket s)
	{
		socket = s;
		address = socket.getInetAddress().getHostAddress();
	}

	public void run()
	{
		
		try
		{
			// Skapar in- och utströmmar för kommunikation
			out = new ObjectOutputStream(socket.getOutputStream());
			in = new ObjectInputStream(socket.getInputStream());
		}
		catch (IOException ioe)
		{
			System.out.println("MathClientHandler:" + address + ": Kunde inte skapa strömmar");
			return; // Bör stänga strömmar och sockets först
		}

		// Vi har nu en uppkopplad förbindelse med klienten
		// Dags att ta emot talet som vi ska räkna ut

		while(true){ 
			try{
				
				sendNumber();
			}
			
			catch(ArithmeticException ae){
				System.out.println("Division med noll!\nVänta på nytt tal!");
			} 
		.......more code

public synchronized void sendNumber(){

		operand = talSvar.getOperand();
		
		while( operand == null){
			try{
				System.out.println("Waiting for a new number to send");
				wait();
				System.out.println();
			}
			catch( InterruptedException eioe){}
		}
		notifyAll();
		
		try {
			out.writeObject(nbr.sendNumber());
			out.flush();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
					
			
	}

So, the problem now is that I get a NPE at line 55 in the MathClientHandler class
and I can't understand why.
And if I hide the statement, ie //operand = talSvar.getOperand();
I the get this result from the console:

Waiting for a new number to send

, i e operand = null, but how can it be null?

If I do a System.out.println("")
System.out.println("op1: " + nbr.getNumber1());
System.out.println("op2: " + nbr.getNumber2());
System.out.println("op: " + nbr.getOperand());

The result is for example
op1: 3
op2: 13
op: /

i e operand is not null.


What is wrong here?

As you can see I have three synchronized methods and two threads here. Is there something wrong with the synchronization?
Please, anyone?

[Ulf Dittmer]

Anders Kviback wrote:So, the problem now is that I get a NPE at line 55 in the MathClientHandler class
and I can't understand why.

Assuming that "Line 55" is exactly what is line 55 in what you posted -which I doubt because of your ".......more code " comment- then it would likely be the talSvar variable that contains null. The code you posted doesn't assign anything to it, but since that is not the entire code we can't really help with that.

And if I hide the statement, ie //operand = talSvar.getOperand();
...
, i e operand = null, but how can it be null?

If you comment out the statement that assigns something to operand, then it would be natural that operand is null, no?

[Anders Kviback]

Thank you for answering,

In MathClientHandler line 11 it should be

private MathNumber nbr;

(instead of tal, forget talSvar),
and then on line 55 it should be

operand = nbr.getOperand();

And as you say it is not assigned anything to it(nbr).

As you can see MathNumber is created in class Numbergenerator. So, then it must be possible in some way to get it to the method
sendNumber() in MathClientHandler class. How do I do that?

On my other question I just tried to show what happens when I hide the statement.

//Anders

[Ulf Dittmer]

That's hard to say, as I don't really understand how all these classes work together. There seems to be some thread synchonization, and also some socket communication. Are all classes running in the same JVM? Which class creates and/or calls which other class? Is it really necessary to create new MathNumber objects, or would calling its setNumber method with new parameters suffice?

[Anders Kviback]

Sorry, I forgot some peace of code that you need to see: Here is more of the MathNumber class:

public MathNumber(){
		this.number1 = (int)(Math.random()*19);
		this.number2 = (int)(Math.random()*19);

		int oand = (int)(Math.random()*5);

		switch(oand){

		case 0: this.operand = "+";break;
		case 1: this.operand = "-";break;
		case 2: this.operand = "*";break;
		case 3: this.operand = "/";break;
		default: this.operand = "+";break;
		}
	}

[Anders Kviback]

This is the way it works:
I start the Numbergenerator thread in MathServer class and the MathClientHandler thread. In Numbergenerator I then set the numbers to work with
through nbr.setNumber(nbr1, op, nbr1);
In sendNumber(), MathClientHandler I have to wait to send new numbers to the client before having answered the number quiz.
So, I have three synchronized methods(or, at least I believe they are synchronized). They are supposed to interact through the classes and threads
MathClientHandler(MCH) class and Numbergenerator class.

public synchronized void setNumber(int op1, String op, int op2) - MathNumber class

public synchronized MatteTal getNumber() - MathNumber class

public synchronized void sendNumber() - MCH


So;

sendNumber has to wait until it can send away its number. If I set the setNumber method, operand gets a value
and therefore the setNumber method waits. When I get the number from getNumber method the operand gets a null value.
In sendNumber I can't send the number away if operand is null(i e we don't have a number to send). So we wait in sendNumber.
The thing is that the number is set so operand shouldn't be null. But how do I write the code so that nbr gets a value
(because it has a value and I do not understand how to get it) and not null?


The server client communication works fine, it is all running in the same JVM. I have to have this constructor -
MathNumber() that creates the numbers.
I can send the whole code if you want to?

[Maxim Karvonen]

Hi, Anders. Your Numbergenerator does not make much sense. It creates a new MathNumber and sets it to a private variable. This new number cannot be accessed by other threads. Also you have too many synchronized methods for a simple "exchange" task. And these synchronized methods are in different objects. So, I can't say which of synchronized methods are related to a number-passing.

First thing to do is split data classes and logic classes. Let MathNumber be only a data container. Create another class to transfer data between objecst. You may use an already available BlockingQueue implementation with size 1 for that purpose. Or you may implement your own exchanger with two methods (put/get). This exchanger may be generic. This separation will help to see are transfer objects are properly shared between parts of program or not. Numbergenerator and consumer (mathClientHandler) should have reference to a same exchanger instance (probably passed as a constructor parameter to them).

[Anders Kviback]

Hello, again!

Thanks for answering. I have now been working on this issue and I solved it by doing

try{
					talC = (MatteTalNy)in.readObject();
					String tal = talC.toString();
					showNbr(tal + " = ");
					b = in.readBoolean();
				}

Hereby the code stops and waits at the last line.

So now that works and now everything works except one thing;

the system freezes. When starting up a client gui and after entering the port numbers a new thread starts. A new thread does also mean that a new gui starts. So,

most of the time the system can generate a math number or two maybe three but then it freezes. There are a lot of different threads here, gui, thread and then the main.
Do you have any idea were to start looking for the problem. Or do you need more info?