I was given some sample code by my professor to make a rudimentary chat program and one of the requirements was to allow the user to connect and disconnect by pressing buttons. I created the buttons and added action listeners for both but when I click the "connect" button, I get this:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Client.actionPerformed(Client.java:64)
at java.awt.Button.processActionEvent(Unknown Source)
at java.awt.Button.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
I traced the error back to this block:
try{
myOutputStream.reset(); //this is line 64 in the full Java file
myOutputStream.writeObject(myObject);
}catch(IOException ioe){
System.out.println(ioe.getMessage());
}
I will say that I did make changes to the sample code, so that part above was working before I made some edits, which was moving a block a code that starts the connection into an if statement that checks to see if the button was clicked. Before it used to be in the "Client" constructor (what my prof. made for the program) and would instantly run and connect upon starting the Java program.
The full Client class is here:
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;
//THINGS TO DO:
//1: Add a means to add usernames
//2: Add something that adds a list of users
//3: Button to disconnect/connect
//4: Panel that allows you to draw things
public class Client extends Thread implements ActionListener{
ChatMessage myObject;
boolean sendingdone = false, receivingdone = false;
Scanner scan;
Socket socketToServer;
ObjectOutputStream myOutputStream;
ObjectInputStream myInputStream;
Button connect,disconnect;
Frame f;
TextField tf;
TextArea ta;
public Client(){
f = new Frame();
f.setSize(300,400);
f.setTitle("Chat Client");
f.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent we){
System.exit(0);
}
});
tf = new TextField();
tf.addActionListener(this);
f.add(tf, BorderLayout.NORTH);
ta = new TextArea();
f.add(ta, BorderLayout.CENTER);
/////////////////////////
/*
try{
socketToServer = new Socket("127.0.0.1", 4000);
myOutputStream = new ObjectOutputStream(socketToServer.getOutputStream());
myInputStream = new ObjectInputStream(socketToServer.getInputStream());
start();
}
catch(Exception e){
System.out.println(e.getMessage());
}
*/this is the original spot
Panel b = new Panel();
connect = new Button("Connect");
disconnect = new Button("Disconnect");
connect.addActionListener(this);
disconnect.addActionListener(this);
b.add(connect);
b.add(disconnect);
f.add(b, BorderLayout.SOUTH);
f.setVisible(true);
}
public void actionPerformed(ActionEvent ae){
myObject = new ChatMessage();
myObject.setMessage(tf.getText());
tf.setText("");
try{
myOutputStream.reset();
myOutputStream.writeObject(myObject);
}catch(IOException ioe){
System.out.println(ioe.getMessage());
}
if (ae.getSource() == connect){
try{
socketToServer = new Socket("127.0.0.1", 4000);
myOutputStream =
new ObjectOutputStream(socketToServer.getOutputStream());
myInputStream =
new ObjectInputStream(socketToServer.getInputStream());
start();
}
catch(Exception e){
System.out.println(e.getMessage());
}
}
}
public void run(){
System.out.println("Listening for messages from server . . . ");
try{
while(!receivingdone){
myObject = (ChatMessage)myInputStream.readObject();
ta.append(myObject.getMessage() + "\n");
}
}catch(IOException ioe){
System.out.println("IOE: " + ioe.getMessage());
}catch(ClassNotFoundException cnf){
System.out.println(cnf.getMessage());
}
}
public static void main(String[] arg){
Client c = new Client();
}
}
I think you'll also need these two Java classes as well, ChatServer, which has to be running and the Client must connect to, and ChatMessage, what the Client uses to create messages:
import java.io.*;
import java.net.*;
import java.util.*;
public class ChatServer{
public static void main(String[] args )
{
ArrayList<ChatHandler> AllHandlers = new ArrayList<ChatHandler>();
try
{ ServerSocket s = new ServerSocket(4000);
for (;;)
{ Socket incoming = s.accept( );
new ChatHandler(incoming, AllHandlers).start();
}
}
catch (Exception e)
{ System.out.println(e);
}
}
}
class ChatHandler extends Thread
{ public ChatHandler(Socket i, ArrayList<ChatHandler> h)
{
incoming = i;
handlers = h;
handlers.add(this);
try{
in = new ObjectInputStream(incoming.getInputStream());
out = new ObjectOutputStream(incoming.getOutputStream());
}catch(IOException ioe){
System.out.println("Could not create streams.");
}
}
public synchronized void broadcast(){
ChatHandler left = null;
for(ChatHandler handler : handlers){
ChatMessage cm = new ChatMessage();
cm.setMessage(myObject.getMessage());
try{
handler.out.writeObject(cm);
System.out.println("Writing to handler outputstream: " + cm.getMessage());
}catch(IOException ioe){
//one of the other handlers hung up
left = handler; // remove that handler from the arraylist
}
}
handlers.remove(left);
if(myObject.getMessage().equals("bye")){ // my client wants to leave
done = true;
handlers.remove(this);
System.out.println("Removed handler. Number of handlers: " + handlers.size());
}
System.out.println("Number of handlers: " + handlers.size());
}
public void run()
{
try{
while(!done){
myObject = (ChatMessage)in.readObject();
System.out.println("Message read: " + myObject.getMessage());
broadcast();
}
} catch (IOException e){
if(e.getMessage().equals("Connection reset")){
System.out.println("A client terminated its connection.");
}else{
System.out.println("Problem receiving: " + e.getMessage());
}
}catch(ClassNotFoundException cnfe){
System.out.println(cnfe.getMessage());
}finally{
handlers.remove(this);
}
}
ChatMessage myObject = null;
private Socket incoming;
boolean done = false;
ArrayList<ChatHandler> handlers;
ObjectOutputStream out;
ObjectInputStream in;
}
(I have not made any changes to either of these)
import java.io.*;
public class ChatMessage implements Serializable{
public String name;
public String message;
public ChatMessage(){}
public ChatMessage(String name, String message){
setName(name);
setMessage(message);
}
public void setName(String name){
this.name = name;
}
public String getName(){
return name;
}
public void setMessage(String message){
this.message = message;
}
public String getMessage(){
return message;
}
}
