照文档上fragment事例出现java.lang.NullPointerException

照文档上fragment例子出现java.lang.NullPointerException
我按照android文档写了个例子，但是出现
java.lang.NullPointerException.
at com.example.testfragmentcom.fragmentb.UpdateTextView(fragmentb.java:35)
at com.example.testfragmentcom.MainActivity.onArticleSeleted(MainActivity.java:33)
at com.example.testfragmentcom.fragmenta$1.onClick(fragmenta.java:58)
有一个mainActivity,fragment a和fragment b.
1.mainActivity的代码：
public class MainActivity extends FragmentActivity implements fragmenta.OnHeadlineSelectedListener {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Fragment fa=(Fragment)fragmenta.newInstance();
FragmentTransaction ft=getSupportFragmentManager().beginTransaction();
ft.add(R.id.framelayouta, fa).commit();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public void onArticleSeleted(int position) {
// TODO Auto-generated method stub
fragmentb fb=(fragmentb)getSupportFragmentManager().findFragmentById(R.id.framelayoutb);
if(fb!=null){
if(position==0)
fb.UpdateTextView(0);
if(position==1)
fb.UpdateTextView(1);
}else{
fragmentb new_fb=fragmentb.newInstance();
}
}
}
2.fragment a的代码：
public class fragmenta extends Fragment {
private static final String TAG="fragment A";
OnHeadlineSelectedListener mCallback;
public interface OnHeadlineSelectedListener{
public void onArticleSeleted(int position);
}
public static fragmenta newInstance(){
fragmenta f=new fragmenta();
return f;
}
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try{
mCallback =(OnHeadlineSelectedListener)activity;
}catch(ClassCastException e){
throw new ClassCastException(activity.toString()+"must implement OnHeadlineSelectedListener");
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v= inflater.inflate(R.layout.frangmenta_layout, container,false);
Button bt1=(Button)(v.findViewById(R.id.button1));
Button bt2=(Button)(v.findViewById(R.id.button2));