"Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException" error in socket programming in Java

Question

I'm trying to make a basic chat application with GUI, it works nearly perfect without GUI, but I want to build the application on GUI. What I want to do is that send a message that I wrote on JTextField when I pressed on the button. But the app throws this error whenever I try to click the button to send data to the server through the button.

gui.java

package program;

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.IOException;

public class gui
{
    Client client;
    server _server;
    JFrame pencere;
    JButton button;
    JTextArea area;
    JTextField type;
    public gui(){

        pencere = new JFrame("oxChat");
        pencere.setSize(640,480);
        pencere.setLayout(null);
        button = new JButton("gönder");
        button.addActionListener( new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                client.sendData("Hello");
            }
        });
        area = new JTextArea();
        type = new JTextField();
        pencere.add(type);
        pencere.add(area);
        pencere.add(button);
        area.setBounds(0,0,640,350);
        type.setBounds(0,370,640,25);
        button.setBounds(640/2-80/2,400,80,30);
        pencere.setVisible(true);
        pencere.setResizable(false);
        pencere.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        _server = new server();
        _server.start(4000);
        Client client = new Client("127.0.0.1",4000);
    }

    public static void main(String[] args) throws IOException {
        gui app = new gui();
    }

}

server.java

package program;

// A Java program for a Server
import java.net.*;
import java.io.*;

public class server
{
    private Socket socket;
    private ServerSocket server;
    // constructor with port
    public void start(int port){
        try {
            server = new ServerSocket(port);
            while(true){
                socket = server.accept();
                new Thread (new ConnectionHandler(socket)).start();
            }
        }catch(IOException i){

        }
    }
}

class ConnectionHandler extends Thread{
    private Socket       socket = null;
    private DataInputStream in   = null;
    private DataOutputStream out     = null;
    public ConnectionHandler(Socket socket){
        this.socket=socket;
    }
    @Override
    public void run() {
        try
        {
            System.out.println("Waiting for a client ...");
            System.out.println("Client accepted");
            in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
            out = new DataOutputStream(socket.getOutputStream());

            String line = "";

            // reads message from client until "Over" is sent
            while (!line.equals("Over"))
            {
                try
                {
                    line = in.readUTF();
                    System.out.println(line);
                }
                catch(IOException i)
                {
                    System.out.println(i);
                }
            }
            System.out.println("Closing connection");

            // close connection
            socket.close();
            in.close();
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
    }
}

client.java

package program;

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

public class Client
{
    // initialize socket and input output streams
    private Socket socket        = null;
    private DataInputStream input = null;
    private DataOutputStream out     = null;

    // constructor to put ip address and port
    public Client(String address, int port)
    {
        // establish a connection
        try
        {
            socket = new Socket(address, port);
            System.out.println("Connected");

            // takes input from terminal
            input = new DataInputStream(socket.getInputStream());

            // sends output to the socket
            out = new DataOutputStream(socket.getOutputStream());
        }
        catch(UnknownHostException u)
        {
            System.out.println(u);
        }
        catch(IOException i)
        {
            System.out.println(i);
        }

    }

   void sendData(String data){
        try{
            out.writeUTF(data);
        }catch(IOException i)
        {
        }
   }
}

Full error code

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at program.gui$1.actionPerformed(gui.java:27) at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967) at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308) at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405) at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262) at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279) at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636) at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342) at java.desktop/java.awt.Component.processEvent(Component.java:6401) at java.desktop/java.awt.Container.processEvent(Container.java:2263) at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012) at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321) at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844) at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918) at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547) at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488) at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307) at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2762) at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844) at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772) at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721) at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715) at java.base/java.security.AccessController.doPrivileged(AccessController.java:391) at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85) at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95) at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745) at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743) at java.base/java.security.AccessController.doPrivileged(AccessController.java:391) at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85) at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742) at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203) at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124) at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113) at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109) at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

I've read it before but it does not have a solution to my problem. — ShooterLens Aim, Oct 29 '19 at 02:53

score 1 · Answer 1 · answered Oct 29 '19 at 02:43

1

This client

 Client client = new Client("127.0.0.1",4000);

is shadowing the field by the same name.

Change to

client = new Client("127.0.0.1",4000);

answered Oct 29 '19 at 02:43

Scary Wombat

44,617
6
35
64

I realized just now that codes I wrote below _server.start(4000); doesn't executing. – ShooterLens Aim Oct 29 '19 at 02:47
so `_server` itself is not a Thread and is blocking. But even if was the field `client` is never set – Scary Wombat Oct 29 '19 at 02:55
But it has "ConnectionHandler" Thread.İt starts off by Thread named "ConnectionHandler" when I create the server object. – ShooterLens Aim Oct 29 '19 at 02:59
Yes, you start a new Thread for `ConnectionHandler` but server loops `while(true)` and then blocks on `accept` – Scary Wombat Oct 29 '19 at 03:01
So what should I do to solve that problem, sorry for I am new to socket programming and Java. – ShooterLens Aim Oct 29 '19 at 03:03
Either make `_server` a Thread or only accept one connection and do not loop – Scary Wombat Oct 29 '19 at 03:28

"Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException" error in socket programming in Java

1 Answers1

Linked