Exception in thread "Thread-0" .. Please help me!

[kateyoz]

Hello all. I'm new in Java and i have a problem like
" Exception in thread "Thead-0" java.lang.NullPointerException at clientThread.run<Server.java:123>

I dont know why .. i try ( users.isEmpty() || users == null ) but no.
Do you have any idea to solve this problem?

if( users.isEmpty()){
users.put(name , addr);

}

[aussiemcgr]

users.isEmpty() || users == null
Swap these.
users == null || users.isEmpty()
You want to evaluate null BEFORE calling isEmpty() on a potentially null variable.

[kateyoz]

oh thanks a lot!
But no..again exception

[Norm]

Post the full text of the message and the source line where it happened.

[kateyoz]

The code is :

import java.io.*;
import java.net.*;
import java.util.*;
 
/*
 * A chat server that delivers public and private messages.
 */
public class Server {
 
  // The server socket.
  private static ServerSocket serverSocket = null;
  // The client socket.
  private static Socket clientSocket = null;
  private int serverPort = 10000;
 
  // This chat server can accept up to maxClientsCount clients' connections.
  private static final int maxClientsCount = 10;
  private static final clientThread[] threads = new clientThread[maxClientsCount];
 
  private Hashtable< String, String > users;
 
  public static void main(String args[]) {
 
    // The default port number.
    int serverPort = 4242;
    /*
     * Open a server socket on the serverPort (default 4242). Note that we can
     * not choose a port less than 1023 if we are not privileged users (root).
     */
    try {
 
        // dimiourgia enos socket na akouei sto port
      serverSocket = new ServerSocket(serverPort);
 
      System.out.println("Server is now online");
      try {
          Thread.sleep(200);
       } catch(InterruptedException ex) {
          Thread.currentThread().interrupt();
       }
       System.out.println("Waiting for clients...");
    } catch (IOException e) {
      System.out.println(e);
    }
 
    /*
     * Create a client socket for each connection and pass it to a new client
     * thread.
     */
    while (true) {
      try {
        clientSocket = serverSocket.accept();
        // eisodos kai elegxos gia ton arithmo twn clients
        int i = 0;
        for (i = 0; i < maxClientsCount; i++) {
          if (threads[i] == null) {
            (threads[i] = new clientThread(clientSocket, threads)).start();
            break;
          }
        }
        //clients = 10
        if (i == maxClientsCount) {
          PrintStream os = new PrintStream(clientSocket.getOutputStream());
          os.println("Server too busy. Try later.");
          os.close();
          clientSocket.close();
        }
      } catch (IOException e) {
        System.out.println(e);
      }
    }
  }
}
/*
 * The chat client thread. This client thread opens the input and the output
 * streams for a particular client, ask the client's name, informs all the
 * clients connected to the server about the fact that a new client has joined
 * the chat room, and as long as it receive data, echos that data back to all
 * other clients. When a client leaves the chat room this thread informs also
 * all the clients about that and terminates.
 */
class clientThread extends Thread {
 
  private DataInputStream is = null;
  private PrintStream os = null;
  private Socket clientSocket = null;
  private final clientThread[] threads;
  private int maxClientsCount;
  private Hashtable<String,String> users;
 
  public clientThread(Socket clientSocket, clientThread[] threads) {
    this.clientSocket = clientSocket;
    this.threads = threads;
    maxClientsCount = threads.length;
  }
 
 
  public void run() {
    int maxClientsCount = this.maxClientsCount;
    clientThread[] threads = this.threads;
 
    try {
      /*
       * Create input and output streams for this client.
       */
      is = new DataInputStream(clientSocket.getInputStream());
      os = new PrintStream(clientSocket.getOutputStream());
      os.println("Enter your name.");
      String name = is.readLine().trim(); // onoma
      InetAddress address = InetAddress.getLocalHost(); //address
      String addr = address.toString();
      byte[] ip = address.getAddress(); // ip
 
      // periptwsi poy den uparxei users
      if( users== null || users.isEmpty()){
            users.put(name , addr);
 
      } 
 
 
      os.println("Hello " + name
          + " to our chat room.\nTo leave enter '/quit' in a new line");
      for (int i = 0; i < maxClientsCount; i++) {
        if (threads[i] != null && threads[i] != this) {
          threads[i].os.println("*** A new user " + name
              + " entered the chat room !!! ^_^ ***");
        }
      }
      while (true) {
        String line = is.readLine();
        if (line.startsWith("/quit")) {
          break;
        }
        for (int i = 0; i < maxClientsCount; i++) {
          if (threads[i] != null) {
            threads[i].os.println("(" + name + ")" + line);
          }
        }
      }
      for (int i = 0; i < maxClientsCount; i++) {
        if (threads[i] != null && threads[i] != this) {
          threads[i].os.println("*** The user " + name
              + " is leaving the chat room !!! ***");
        }
      }
      os.println("*** Bye " + name + " *** ^_^");
 
      /*
       * Clean up. Set the current thread variable to null so that a new client
       * could be accepted by the server.
       */
      for (int i = 0; i < maxClientsCount; i++) {
        if (threads[i] == this) {
          threads[i] = null;
        }
      }
 
      /*
       * Close the output stream, close the input stream, close the socket.
       */
      is.close();
      os.close();
      clientSocket.close();
    } catch (IOException e) {
    }
  }
 
}

and the message :

Exception in thread "Thead-0" java.lang.NullPointerException at clientThread.run<Server.java:123>

[Norm]

Exception in thread "Thead-0" java.lang.NullPointerException at clientThread.run<Server.java:123>

There is a variable with a null value on line 123. Look at line 123 in the your source and see what variable is null. Then backtrack in the code to see why that variable does not have a valid value.
If you can not tell which variable it is, add a println just before line 123 and print out the values of all the variables on that line.

Please edit your post and wrap your code with code tags:
[code=java]
YOUR CODE HERE
[/code]
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