Android的listview问题

package com.example.listview;

import android.app.Activity;
 import android.content.Context;
 import android.database.DataSetObserver;
 import android.os.Bundle;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.BaseAdapter;
 import android.widget.ListAdapter;
 import android.widget.ListView;
 import android.widget.TextView;

public class MainActivity extends Activity {
private Context mContext;
private TextView textview;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //1找到listview
    ListView listview=(ListView)findViewById(R.id.listview);
    //2创建一个adapter对象，以为下面已经申明了一个类
    mylistadpater mylistadpater1=new mylistadpater();
    //3将adapter设置个listview
    listview.setAdapter(mylistadpater1);

}

class mylistadpater extends BaseAdapter{//default base simple

@Override
public int getCount() {
    // TODO Auto-generated method stub
    //集合里面又多少条目,告诉listview要显示多少条目
    return 10;
}

//根据position获取listview上面的bean书局，该方法不影响数据的展示，可以先不实现
@Override
public Object getItem(int position) {
    // TODO Auto-generated method stub
    return null;
}

@Override
public long getItemId(int position) {
    // TODO Auto-generated method stub
    return 0;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    textview=(TextView)findViewById(R.id.TextView);
    textview.setText("position:"+position);
    return textview;

}

 }
 }

不报错，但是安装到虚拟机上面就立刻崩溃