out.println上的空指针异常（“username ,,”+ username）;

Question

我的代码用于与java进行简单的聊天。我在第out.println("username,," + username);行发送用户名时遇到空指针异常，我不知道为什么

package guc.protocols.checkers;

import android.app.Activity;
import android.content.Intent;
//import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class LoginActivity extends Activity implements View.OnClickListener,
        Runnable {

    static Socket socket;
    static BufferedReader in;
    static PrintWriter out;
    Activity act = this;
    String inputMessage;

    EditText edit_username;
    Button btn_connect;
    String username;

    boolean exitFromLoop = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        edit_username = (EditText) findViewById(R.id.editText_username);
        btn_connect = (Button) findViewById(R.id.button_connect);

        btn_connect.setOnClickListener(this);
        try {
            Thread thread = new Thread(this);
            thread.start();

        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    @Override
    public void onClick(View v) {

        if (v == btn_connect) {
            username = edit_username.getText().toString();
            if (username.equals("")) {
                Toast.makeText(getApplicationContext(),
                        "You must enter your username", Toast.LENGTH_LONG)
                        .show();
                return;
            }

            out.println("username,," + username);

        }

    }

    @Override
    public void run() {
        try {
            socket = new Socket("10.0.2.2", 5454);
            out = new PrintWriter(socket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(
                    socket.getInputStream()));
            out.println("Hello");

            while (!exitFromLoop) {
                inputMessage = in.readLine();
                if (inputMessage != null) {
                    Log.i("input", inputMessage);
                    if (inputMessage.equals("This is the server")) {
                        // everything is ok - do nothing
                    } else if (inputMessage.startsWith("username,,")) {

                        String response = inputMessage.split(",,")[1];
                        Log.i("input", response);
                        if (response.equals("yes")) {
                            exitFromLoop = true;
                            runOnUiThread(new Runnable() {
                                @Override
                                public void run() {
                                    Toast.makeText(getApplicationContext(),
                                            "Username Available",
                                            Toast.LENGTH_LONG).show();
                                    Intent intent = new Intent(
                                            getApplicationContext(),
                                            ConnectedUsersActivity.class);
                                    startActivity(intent);
                                }
                            });

                        } else {
                            runOnUiThread(new Runnable() {
                                @Override
                                public void run() {
                                    Toast.makeText(getApplicationContext(),
                                            "Username Already Taken !!",
                                            Toast.LENGTH_LONG).show();
                                }
                            });
                        }
                    }
                }
            }

        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public static void sendToServer(String message) {
        out.println(message);
    }

    public static String receiveFromServer() {

        String received = "";
        try {
            received = in.readLine();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return received;
    }
}