android fragment 跳到另一个fragment

一共有4个fragment，分别是contact（联系人），friends（朋友），search（查找），more（更多）。使用的都是同一个布局，每个fragment中都有四个内部按钮，可以切换到其他的3个fragment中。

现在只考虑在contact中，实现点4个（其实有效的是3个），切换到其他3个fragment中。

现在的情况是，点击contactfragment中的按钮，toast会响应，但是fragment不会跳转。这是怎么回事，下面是两个类的代码。

这是contactfragment。

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;

    @Override
    public 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);
    }

    @Override
    public 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();
    }

}

这是mainactivity类，mainactivity中的那四个按钮及其响应时间是fragment外部的四个按钮，与内部的inner按钮没关系

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;

    @Override
    protected 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);
    }

    @Override
    public 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！