unfortunately has stopped，求解决，会的大神提点下，感激不尽

这是log文件
03-16 12:47:02.671: E/AndroidRuntime(1146): at dalvik.system.NativeStart.main(Native Method)
03-16 12:47:02.671: E/AndroidRuntime(1146): Caused by: java.lang.NullPointerException
03-16 12:47:02.671: E/AndroidRuntime(1146): at com.yongxin.adstudyview316.MainActivity.onCreate(MainActivity.java:22)
03-16 12:47:02.671: E/AndroidRuntime(1146): at android.app.Activity.performCreate(Activity.java:5231)
03-16 12:47:02.671: E/AndroidRuntime(1146): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
03-16 12:47:02.671: E/AndroidRuntime(1146): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)

代码：
public class MainActivity extends ActionBarActivity {
private TextView textview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textview=(TextView)findViewById(R.id.textview);
textview.setText("wohaoyun");

}



    布局文件：
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.yongxin.adstudyview316.MainActivity$PlaceholderFragment" >

<TextView
    android:id="@+id/textview"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="#FF0000"
    android:text="@string/hello_world" />

看出来的大神说一下吧，弄了半天了，一点办法都没~~~~~谢谢

检查下控件id是不是没找到。

Caused by: java.lang.NullPointerException，空指针异常，看你是否findViewById啦