如何在fragment中点击按钮切换到另一个fragment中

Viewpager 中有两个pages，在每一个 fragment中，我放入一个按钮到switching fragment中，但是如果我改变方向，switching不能运行。对于switching fragment，我使用的是 OnChangePageButtonClick 接口。为什么会出现这种情况呢？
ViewPager Activity:
public class ViewPagerMusic extends FragmentActivity implements OnChangePageButtonClick {
private ViewPager vp;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.view_pager_music);
ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(super.getSupportFragmentManager(), fragments);

vp = (ViewPager)findViewById(R.id.viewpager);
vp.setAdapter(viewPagerAdapter);
}
private class ViewPagerAdapter extends FragmentStatePagerAdapter {

public ViewPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
switch(position) {
case 0:
MainPage mainPage = new MainPage();
mainPage.setOnChengeButtonListener(ViewPagerMusic.this);
return mainPage;
case 1:
PlaylistPage playlistPage = new PlaylistPage();
playlistPage.setOnChengeButtonListener(ViewPagerMusic.this);
return playlistPage;
}
return null;
}
@Override
public int getCount() {
return 2;
}
}
@Override
public void selectPage(int page) {
vp.setCurrentItem(page);
}
}
Frgamnets:
public class MainPage extends Fragment {
public MainPage() {
setRetainInstance(true);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.music_main_page, container, false);
ImageButton btnPlaylist = (ImageButton)v.findViewById(R.id.btnGoPlaylist2);
btnPlaylist.setOnClickListener(onButtonListener);
return v;
}
private OnClickListener onButtonListener = new OnClickListener() {

@Override
public void onClick(View view) {
onChangePageButtonClick.selectPage(1);
}
};
public void setOnBackButtonListener(OnChangePageButtonClick onChangePageButtonClick) {
this.onChangePageButtonClick = onChangePageButtonClick;
}
private OnChangePageButtonClick onChangePageButtonClick;
}
Playlist fragment 与 MainPage fragment相似。