public class ContactFragment extends Fragment implements OnClickListener {MainActivity activity = (MainActivity) getActivity();View view = null;Button btn_inner_contact;Button btn_inner_friends;Button btn_inner_search;Button btn_inner_more;@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {view = inflater.inflate(R.layout.frag_content, container, false);TextView textview_hint = (TextView) view.findViewById(R.id.textview_hint);textview_hint.setText("Contact");initComponent();return view;}private void initComponent() {btn_inner_contact = (Button) view.findViewById(R.id.button_inner_contact);btn_inner_friends = (Button) view.findViewById(R.id.button_inner_friends);btn_inner_search = (Button) view.findViewById(R.id.button_inner_search);btn_inner_more = (Button) view.findViewById(R.id.button_inner_more);btn_inner_contact.setOnClickListener(this);btn_inner_friends.setOnClickListener(this);btn_inner_search.setOnClickListener(this);btn_inner_more.setOnClickListener(this);}@Overridepublic void onClick(View v) {FragmentManager fm = activity.getFragmentManager();FragmentTransaction ft = fm.beginTransaction();switch (v.getId()) {case R.id.button_inner_contact:Toast.makeText(getActivity(), "点击了内部contact按钮", Toast.LENGTH_SHORT).show();if (activity.contactFragment == null) {activity.contactFragment = new ContactFragment();}ft.replace(R.id.framelayout_content, activity.contactFragment);break;case R.id.button_inner_friends:Toast.makeText(getActivity(), "点击了内部friends按钮", Toast.LENGTH_SHORT)if (activity.friendsFragment == null) {activity.friendsFragment = new FriendsFragment();}ft.replace(R.id.framelayout_content, activity.friendsFragment);break;case R.id.button_inner_more:if (activity.moreFragment == null) {activity.moreFragment = new MoreFragment();}ft.replace(R.id.framelayout_content, activity.moreFragment);break;case R.id.button_inner_search:if (activity.searchFragment == null) {activity.searchFragment = new SearchFragment();}ft.replace(R.id.framelayout_content, activity.searchFragment);break;default:break;}ft.commit();}}

public class MainActivity extends Activity implements OnClickListener {public ContactFragment contactFragment;public FriendsFragment friendsFragment;public MoreFragment moreFragment;public SearchFragment searchFragment;private Button btn_contact;private Button btn_friends;private Button btn_search;private Button btn_more;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);initcomponet();initview();}private void initview() {contactFragment = new ContactFragment();FragmentManager fm = getFragmentManager();FragmentTransaction ft = fm.beginTransaction();ft.add(R.id.framelayout_content, contactFragment);ft.commit();}private void initcomponet() {btn_contact = (Button) findViewById(R.id.button_contact);btn_friends = (Button) findViewById(R.id.button_friends);btn_more = (Button) findViewById(R.id.button_more);btn_search = (Button) findViewById(R.id.button_search);btn_contact.setOnClickListener(this);btn_friends.setOnClickListener(this);btn_more.setOnClickListener(this);btn_search.setOnClickListener(this);}@Overridepublic void onClick(View v) {FragmentManager fm = getFragmentManager();FragmentTransaction ft = fm.beginTransaction();switch (v.getId()) {case R.id.button_contact:if (contactFragment == null) {contactFragment = new ContactFragment();}ft.replace(R.id.framelayout_content, contactFragment);break;case R.id.button_friends:if (friendsFragment == null) {friendsFragment = new FriendsFragment();}ft.replace(R.id.framelayout_content, friendsFragment);break;case R.id.button_more:if (moreFragment == null) {moreFragment = new MoreFragment();}ft.replace(R.id.framelayout_content, moreFragment);break;case R.id.button_search:if (searchFragment == null) {searchFragment = new SearchFragment();}ft.replace(R.id.framelayout_content, searchFragment);break;default:break;}ft.commit();}}

我试了一下，把MainActivity activity=（MainActivity）getActivity（）；这行代码放到onclick方法了，就可以了！