导航回上一个选定的片段

Question

我有3个片段的mainActivity。在导航时，从其中一个片段到另一个活动，并按下应用程序上的后退图标将我带到mainActivity的第一个片段。我如何回到我导航的那个而不是第一个？这是MainActivity的代码。

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {


private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
int counter = 0;

private String[] tabs = { "Home", "Top Locations", "Events"};
@SuppressLint("NewApi")
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //For hiding the title and preventing lag
    setTheme(R.style.Theme_Example);
    setContentView(R.layout.activity_main);

    // Initialization
    viewPager = (ViewPager) findViewById(R.id.pager);
    actionBar = getActionBar();
    //To not display the title on actionbar. can be done by android:label="" as well
    getActionBar().setDisplayShowTitleEnabled(false);

    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

    viewPager.setAdapter(mAdapter);
    actionBar.setHomeButtonEnabled(false);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

    // Changing color of action bar
    // actionBar.setBackgroundDrawable(new ColorDrawable(Color.parseColor("#f0c914")));


    // Adding Tabs
    for (String tab_name : tabs) {
        actionBar.addTab(actionBar.newTab().setText(tab_name)
                .setTabListener(this));
    }

    // This code puts action bar and tabs in 2 lines
    Method setHasEmbeddedTabsMethod=null;
    try
    {
      setHasEmbeddedTabsMethod = actionBar.getClass().getDeclaredMethod("setHasEmbeddedTabs", boolean.class); 
    }
    catch(NoSuchMethodException e1)
    {
       e1.printStackTrace();
    }
     setHasEmbeddedTabsMethod.setAccessible(true);

     try
     {
        setHasEmbeddedTabsMethod.invoke(actionBar, false);
     } catch(IllegalArgumentException e) {
        e.printStackTrace();
     } catch (IllegalAccessException e) {

        e.printStackTrace();
    } catch (InvocationTargetException e) {

        e.printStackTrace();
    }


    //On Tab selection the current tab should be displayed.
     viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

         @Override
         public void onPageSelected(int position) {
             // on changing the page
             // make respected tab selected
             actionBar.setSelectedNavigationItem(position);
         }

         @Override
         public void onPageScrolled(int arg0, float arg1, int arg2) {
         }

         @Override
         public void onPageScrollStateChanged(int arg0) {
         }
     });   
}

以下是其中一个片段的代码

public class TopLocations extends Fragment implements OnClickListener {
ImageButton b1,b2,b3,b4,b5,b6,b7,b8;
public static int flag;
  @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.top_locations, container, false);
        //We need object of View in Fragments since findViewById doesn't work with Fragments
        b1 = (ImageButton)rootView.findViewById(R.id.btnn);
        b1.setOnClickListener(this);



    }
@Override
public void onClick(View arg0) {
    // TODO Auto-generated method stub
    //We need getActivity() method for intents within Fragments
    switch(arg0.getId())
    {
    case R.id.btnn:

        Intent a =  new Intent (getActivity(),mjclass.class);
        startActivity(a);
        break;
}
}
}

当我使用app back图标从mjclass返回时，它会将我带到MainActivity的主片段，而不是从片段开始的“Top Location”片段