[Android]尝试从服务器读取数据时出现NullPointerException

Question

我一直收到这个错误：

04-05 11:38:39.168  14995-14995/com.example.orestis.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.orestis.myapplication, PID: 14995
    java.lang.NullPointerException
            at com.example.orestis.myapplication.signup$1.onClick(signup.java:50)
            at android.view.View.performClick(View.java:4633)
            at android.view.View$PerformClick.run(View.java:19330)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5506)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at dalvik.system.NativeStart.main(Native Method) 

每次InputStreamReader inputStreamReader = new InputStreamReader(client.getInputStream());我对这些东西都是新手，所以我真的不知道发生了什么，会非常感谢你的帮助：）

 package com.example.orestis.myapplication;

    import android.os.AsyncTask;
    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.ServerSocket;
    import java.net.Socket;
    import java.net.UnknownHostException;


    public class signup extends ActionBarActivity {
        private EditText username;
        private EditText password;
        private Button button;
        private String messsage;
        private Socket client;
        private PrintWriter printwriter;
        private BufferedReader bufferedReader;

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

            username = (EditText) findViewById(R.id.editText1); // reference to the text field
            password = (EditText) findViewById(R.id.editText);
            button = (Button) findViewById(R.id.button1); // reference to the send button
            button.setOnClickListener(new View.OnClickListener() {

                public void onClick(View v) {
                    messsage = "SIGNUP-"+username.getText().toString() + "-" + password.getText().toString(); // get the text message on the text field
                    username.setText(""); // Reset the text field to blank
                    password.setText("");
                    SendMessage sendMessageTask = new SendMessage();
                    sendMessageTask.execute();
            resp:   while(true){
                        try {
                            InputStreamReader inputStreamReader = new InputStreamReader(client.getInputStream());
                            bufferedReader = new BufferedReader(inputStreamReader);
                            if (bufferedReader.ready()) {
                                String serverReply = bufferedReader.readLine();
                                if(serverReply.equals("SUCCESS")){
                                    //get into game
                                    break resp;
                                }else if(serverReply.equals("toast:Username exists")){
                                    Toast toast = Toast.makeText(getApplicationContext(),"Username exists!", Toast.LENGTH_SHORT);
                                    toast.show();
                                    break resp;
                                }
                            }
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
            }
                }
            });
        }

        private class SendMessage extends AsyncTask<Void, Void, Void> {

            @Override
            protected Void doInBackground(Void... params) {
                try {

                    client = new Socket("IPGOESHERE", 4444); // connect to the server
                    printwriter = new PrintWriter(client.getOutputStream(), true);
                    printwriter.write(messsage); // write the message to output stream

                    printwriter.flush();
                    //printwriter.close();

                } catch (UnknownHostException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                return null;
            }

        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_signup, menu);
            return true;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();

            //noinspection SimplifiableIfStatement
            if (id == R.id.action_settings) {
                return true;
            }

            return super.onOptionsItemSelected(item);
        }
    }