动态修改TextView 的问题

今天遇到一个问题，没办法动态修改TextView 的text 会有NullPointerException, 似乎只能在onCreate里面setConentView 之后才能修改，成员函数什么的都不行。最后无奈我只有把整个图都动态编写了。留着我以后慢慢研究

package com.example.contextawaredemo;


import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CompoundButton;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;
import android.widget.ToggleButton;


public class SwitchTV extends Activity{
    static TextView tv;
    static ToggleButton tb;
    LinearLayout linlayout;
    SimplePost post=new SimplePost();
    String success="0";
    
    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
//        setContentView(R.layout.switch_tv);
//        tv = (TextView) findViewById(R.id.TVswitch);
        tv=new TextView(this);
        tv.setText("the tv status");
        
        tb=new ToggleButton(this);
        tb.setVisibility(View.GONE);
        tb.setText("ON");
        linlayout=new LinearLayout(this);
        linlayout.setOrientation(LinearLayout.VERTICAL);
        LinearLayout.LayoutParams Params=new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,ViewGroup.LayoutParams.WRAP_CONTENT);
        linlayout.addView(tv,Params);
        linlayout.addView(tb,Params);
        setContentView(linlayout);
//        tb=(ToggleButton) findViewById(R.id.TBswitch);
        
        NetworkReceiver networkReceiver=new NetworkReceiver();
        System.out.println("new Network receiver created");
        tb.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener()

        {

            @Override
            public void onCheckedChanged(CompoundButton buttonView,
                    boolean isChecked) {
                if(post.sendMessage("powerOn").equals(success)){
                    Toast.makeText(getBaseContext(), "tv is on", Toast.LENGTH_LONG).show();
                }
                else{
                    Toast.makeText(getBaseContext(), "try again", Toast.LENGTH_LONG).show();
                }
                
                
            }});
        
    }
    @Override
    public void onDestroy() {
        super.onDestroy();
        
    }
    
    public void statusOn(){
        //tv = new TextView(this);
        
        //tv = (TextView) findViewById(R.id.TVswitch);
        tv.setText("the tv is in status on");
        
    }
    
    public void statusOff(){
        
        
        tv.setText("the tv is in status off, click the toggle to turn on it");
        tb.setVisibility(View.VISIBLE);
    }

}