public void mOnClick(View v){
   FragmentManager fm = getFragmentManager();
   Fragment fragment = fm.findFragmentById(R.id.frame);
   FragmentTransaction transaction = fm.beginTransaction();
   
   Page1Activity pg1 = new Page1Activity();
   Page2Activity pg2 = new Page2Activity();
   Page3Activity pg3 = new Page3Activity();
   
   switch(v.getId()){
   case R.id.btn1:
    transaction.replace(R.id.frame, pg1).commit();
    break;
   case R.id.btn2:
    transaction.replace(R.id.frame, pg2).commit();
    break;
   case R.id.btn3:
    transaction.replace(R.id.frame, pg3).commit();
    break;
   }
  }
이런식으로 짯는데요
replace 쪽에 빨간줄 뜨면서 pg1,2,3 타입을 Fragment로 바꾸라네요
Page1Activity에 extent Fragment도 해줬거든요? 왜이럴까요...
 
public class Page1Activity extends Fragment {
 
 public View onCreateView(LayoutInflater inflater, ViewGroup container,
  Bundle saveedInstanceState){
  View root = inflater.inflate(R.layout.activity_page1, container, false);
  
  return root;
    }
}