Error:(72, 42) error: incompatible types: FragmentActivity cannot be converted to Fragment

问题

I am having so issues with my project, i am getting the error:

Error:(72, 42) error: incompatible types: FragmentActivity cannot be converted to Fragment.

Could anyone assist me on this issues?

This is one of my Fragments

public class contact_Fragment extends FragmentActivity {
  View rootview;
  @Nullable 
 public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle          savedInstanceState) {
  rootview = inflater.inflate(R.layout.contact_layout, container, false);
  return rootview;}


Button sendEmail;
EditText msg;

@Override
protected void onCreate( Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.contact_layout);



    sendEmail = (Button) findViewById(R.id.sendBtn);
    sendEmail.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            msg = (EditText) findViewById(R.id.msgTxt);
            String message = msg.getText().toString();
            sendEmail(message);
        }
    });
}
protected void sendEmail(String message) {
    String[] to=new String[]{"samsul1993@hotmail.co.uk"};
    String subject =("A message from your app!");
    Intent emailIntent = new Intent(Intent.ACTION_SEND);
    emailIntent.putExtra(Intent.EXTRA_EMAIL, to);
    emailIntent.putExtra(Intent.EXTRA_SUBJECT, subject);
    emailIntent.putExtra(Intent.EXTRA_TEXT, message);
    emailIntent.setType("message/rfc822");//rfc822 email protocol
    startActivity(Intent.createChooser(emailIntent,"Email"));


}}

This is my MainActivity

public class MainActivity extends ActionBarActivity
    implements NavigationDrawerFragment.NavigationDrawerCallbacks {

/**
 * Fragment managing the behaviors, interactions and presentation of the navigation drawer.
 */
private NavigationDrawerFragment mNavigationDrawerFragment;

/**
 * Used to store the last screen title. For use in {@link #restoreActionBar()}.
 */
private CharSequence mTitle;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mNavigationDrawerFragment = (NavigationDrawerFragment)
            getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
    mTitle = getTitle();

    // Set up the drawer.
    mNavigationDrawerFragment.setUp(
            R.id.navigation_drawer,
            (DrawerLayout) findViewById(R.id.drawer_layout));
}

@Override
public void onNavigationDrawerItemSelected(int position) {

    FragmentActivity objFragment = null;

    switch (position){
        case 0:
            objFragment = new home_Fragment();
            break;
        case 1:
            objFragment = new Reservation_Fragment();
            break;
        case 2:
            objFragment = new contact_Fragment();
            break;


    }

    // update the main content by replacing fragments
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.container, objFragment)
            .commit();
}

public void onSectionAttached(int number) {
    switch (number) {
        case 1:
            mTitle = getString(R.string.title_section1);
            break;
        case 2:
            mTitle = getString(R.string.title_section2);
            break;
        case 3:
            mTitle = getString(R.string.title_section3);
            break;
    }
}

public void restoreActionBar() {
    ActionBar actionBar = getSupportActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
    actionBar.setDisplayShowTitleEnabled(true);
    actionBar.setTitle(mTitle);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    if (!mNavigationDrawerFragment.isDrawerOpen()) {
        // Only show items in the action bar relevant to this screen
        // if the drawer is not showing. Otherwise, let the drawer
        // decide what to show in the action bar.
        getMenuInflater().inflate(R.menu.main, menu);
        restoreActionBar();
        return true;
    }
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        ((MainActivity) activity).onSectionAttached(
                getArguments().getInt(ARG_SECTION_NUMBER));
    }
}

}

the error appears here:

FragmentManager fragmentManager = getSupportFragmentManager();
 fragmentManager.beginTransaction()
      **.replace(R.id.container, objFragment)**
        .commit();

Lastly I am using the android.support.v4 imports.