下面代码显示空指针异常？请问哪里出错了？

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;

import android.widget.AdapterView;
import android.widget.Spinner;
import android.widget.TextView;

public class OnItemSelectedListener extends ActionBarActivity {
private Spinner city;
private TextView show;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_on_item_selected_listener);
this.city=(Spinner)super.findViewById(R.id.city);
this.show=(TextView)super.findViewById(R.id.show);
this.city.setOnItemSelectedListener(new OnItemSelectedListenerImpl());它说这里空指针异常
}
private class OnItemSelectedListenerImpl implements android.widget.AdapterView.OnItemSelectedListener
{

    @Override
    public void onItemSelected(AdapterView<?> parent, View view,
            int position, long id) {                                             
        // TODO Auto-generated method stub
           String value=parent.getItemAtPosition(position).toString();
           OnItemSelectedListener.this.show.setText("您喜欢的城市是："+value);
    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) {
        // TODO Auto-generated method stub

    }

}

R.layout.activity_on_item_selected_listener 存在不？
后面应该还有控件初始化的代码吧，如果不是上面的错误，可能是控件在 XML 中没有定义了。

代码不全，建议自己调试下，找到丢出异常的行，然后仔细检查这一行的每个变量，看看哪个是null