DataOutputStream keeps giving a Null Pointer Exception

Question

So I have tried using my code with DataInputStream and BufferedReader to send a String from a Client to a Server and have looked up resources all over the Internet for a solution to this. I am using a method that reads in a String from Client and it ALWAYS shows a NullPointerException every time. Here is a copy of the code on the Server side:

package WOF;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class TheBoard {

    private int port;
    Socket client;

    public TheBoard(int port)
    {
        this.port = port;
    }
    public void start() throws IOException 
    {
            System.out.println("Now Determining if guess is correct");

            ServerSocket serverSocket = new ServerSocket(port);

            Socket client = serverSocket.accept();
            sendWelcomeMessage(client); 
    }
    private void sendWelcomeMessage(Socket client) throws IOException
    {
        DataOutputStream out = new DataOutputStream(client.getOutputStream());
        out.writeBytes("Hello, You are now Playing Wheel of Fortune, Command Line Style!!");
    }
    public void getGuess(Socket client) throws IOException
    {
        String userInput;
        DataInputStream getGuessFromClient = new DataInputStream(client.getInputStream()); //NPE Occurs Right on this Line.
        System.out.println(getGuessFromClient);
        userInput = getGuessFromClient.readUTF();
        System.out.println("Guess is: " + userInput);
    }   
    public static void main(String[] args) {

        int portNumber = 9990;
        try 
        {
            TheBoard socketServer = new TheBoard(portNumber);
            socketServer.start();
            socketServer.getGuess(socketServer.client); //This Throws an NPE
        } 
        catch (IOException e)
        {
            System.err.println("UH OH!! Can't Connect.." + e.getMessage());
        }
    }
}

I commented in the code where the stacktrace occurs and have done a lot of my own troubleshooting but can't seem to get past the line that creates getGuessFromClient. I'm sure this is a trivial solution but my research has come up empty handed. Does anyone have any suggestions?

By the way, here is the stacktrace:

Now Determining if guess is correct
Exception in thread "main" java.lang.NullPointerException
    at WOF.TheBoard.getGuess(TheBoard.java:40)
    at WOF.TheBoard.main(TheBoard.java:52)

you are using class variable `client` in your main where as in your `start` method `client` is not initialized rather you created new local variable `client` — Rustam, Sep 23 '15 at 06:14
possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — Andy Turner, Sep 23 '15 at 06:17

score 2 · Accepted Answer · answered Sep 23 '15 at 06:10

2

Instead of

Socket client = serverSocket.accept();

Use:

client = serverSocket.accept();

You are assigning the client to a local variable, rather than the member variable, so you are currently not assigning the member variable before you dereference it.

answered Sep 23 '15 at 06:10

Andy Turner

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score 1 · Answer 2 · answered Sep 23 '15 at 06:15

You have declared "client" variable twice :

Inside TheBoard class
Inside start() method.Which is nothing but a local variable.

But you should use same instance variable inside start() method. So instead of :

Socket client = serverSocket.accept();

write

client = serverSocket.accept();

inside start() method.

DataOutputStream keeps giving a Null Pointer Exception

2 Answers2