执行套接字编程时java.lang.nullPointerException ..如何使用相同的套接字对象

Question

Password.java

package com.example.vnc;

import java.io.IOException;
import java.io.PrintWriter;

import java.net.UnknownHostException;


import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class Password extends MainActivity{

 Button submit;
   EditText pass;
   String data;

  @Override
   public void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.sec);
submit=(Button) findViewById(R.id.submit);
submit.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View arg0) {
        // TODO Auto-generated method stub
        pass=(EditText)findViewById(R.id.editText1);
        data=pass.getText().toString();

            try{
                PrintWriter out=new PrintWriter(soc.getOutputStream(),true);
                out.write(data);
                out.close();
            }
            catch(UnknownHostException e)
            {
                e.printStackTrace();
            }
            catch (IOException e) {
                 e.printStackTrace();
                }



    Intent next2=new Intent(Password.this,Security.class);
        startActivity(next2);
    }
});

}

MainActivity.java

   package com.example.vnc;

   import java.io.IOException;
   import java.net.Socket;
   import java.net.UnknownHostException;

    import android.app.Activity;
    import android.content.Intent;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.Toast;

    public class MainActivity extends Activity
    {
   Button connect;
   EditText ipadd,portadd;
   Socket soc;
    String ip,port;
      int port1;
     public void onCreate(Bundle savedInstanceState){
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_main);
  connect=(Button) findViewById(R.id.button1);
  connect.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View arg0) {
        // TODO Auto-generated method stub
        ipadd=(EditText) findViewById(R.id.editText1);
        portadd=(EditText) findViewById(R.id.editText2);
        ip=ipadd.getText().toString();
        port=portadd.getText().toString();
        port1=Integer.parseInt(port);

    new Thread(new Runnable()
    {
        public void run()
        {
        try{
            soc=new Socket(ip,port1);
        }
        catch(UnknownHostException e)
        {
            e.printStackTrace();
        }
        catch (IOException e) {
             e.printStackTrace();
            }
        }
    }).start();

        Toast.makeText(MainActivity.this,"Connecting to Server"+ip,Toast.LENGTH_LONG).show();

        Intent next1=new Intent(MainActivity.this,Password.class);

        startActivity(next1);

    }
  });
}

}

This is in android. the MainActivity class has Socket connection and I am extending the MainActivity class. I am using the same socket soc at printwriter and it gives me error null pointer exception at printwriter line.

Please tell me what to do? how can I use same socket over 2 class

Answer 1

The instance of Password is different from the MainActivity instance. Hence they do nog share state, and thus the soc instance var is empty. You might try to pass it along when you start the Password activity. You can do that using Intent.putExtra .