FragmentPagerAdapter getItem() is returning more than it should

Question

I have a MainActivity that controls four fragments, each of which is a tab. When my main activity starts, I have a line being printed to the log to show me what fragment is being instantiated. Here is my FragmentPagerAdapter:

public class TabsPagerAdapter extends FragmentPagerAdapter {

public TabsPagerAdapter(FragmentManager fm) {
    super(fm);
}

@Override
public Fragment getItem(int index) {

    switch (index) {
        case 0:
            System.out.println("Returning new PrearrivalPlan()");
            return new PrearrivalPlan();
        case 1:
            System.out.println("Returning new PrimarySurvey()");
            return new PrimarySurvey();
        case 2:
            System.out.println("Returning new SecondarySurvey()");
            return new SecondarySurvey();
        case 3:
            System.out.println("Returning new PrepareForTravel()");
            return new PrepareForTravel();
    }

    return null;
}

@Override
public int getCount() {
    // get item count - equal to number of tabs
    return 4;
}

}

The tab bar has the following options in order:

Prearrival Plan | Primary Survey | Secondary Survey | Prepare for Travel

When my main activity starts, the following is printed to the screen:

Returning new PrearrivalPlan() Returning new PrimarySurvey()

What it seems to be doing is loading one tab ahead of the one I have selected. Since PrearrivalPlan is the first tab, I would think it should just return a new PrearrivalPlan() except it returns both. Another example, when I click on Primary Survey tab (second tab), the following is printed to the screen:

Returning new SecondarySurvey() // <- This is the third tab!?

Because PrimarySurvey was already instantiated when the activity first started (see output above), it jumped ahead just like it did before and loaded the third tab even though I only clicked on the second.

Here is my MainActivity:

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {

private CustomViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;

private String[] tabTitles = {"Pre-arrival Plan", "Primary Survey", "Secondary Survey", "Prepare for Travel"};

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

    viewPager = (CustomViewPager) findViewById(R.id.pager);
    actionBar = getActionBar();
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

    viewPager.setAdapter(mAdapter);
    viewPager.setPagingEnabled(false);

    actionBar.setHomeButtonEnabled(true);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);


    viewPager.setOnPageChangeListener(new CustomViewPager.OnPageChangeListener() {


        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }

    });

    // Remove Android icon from Action Bar
    getActionBar().setIcon(new ColorDrawable(getResources().getColor(android.R.color.transparent)));
    getActionBar().setDisplayShowHomeEnabled(false);

    // Add tabs to Action Bar
    for (String tab_name : tabTitles) {
        actionBar.addTab(actionBar.newTab().setText(tab_name).setTabListener(this));
    }

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

public void updateTabTitles(int tabNumber, int checkBoxesRemaining) {
    String text = tabTitles[tabNumber] + " \n             (" + checkBoxesRemaining + ")";
    actionBar.getTabAt(tabNumber).setText(text);
}

@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
    viewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {

}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {

}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            Intent intent = new Intent(this, MainMenu.class);
            startActivity(intent);
            return true;
        case R.id.complete:
            DialogInterface.OnClickListener dialogClickListener = new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    switch (which){
                        case DialogInterface.BUTTON_POSITIVE:
                            goToReport();
                            break;
                        case DialogInterface.BUTTON_NEGATIVE:
                            //No button clicked
                            break;
                    }
                }
            };
            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setMessage("Are you sure you want to complete the checklist?").setPositiveButton("Yes", dialogClickListener).setNegativeButton("No", dialogClickListener).show();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

public void goToReport() {
    Intent intent = new Intent(this, Report.class);
    startActivity(intent);
}

Answer 1

It is the expected behaviour of ViewPager. ViewPager always keeps one tab ahead and behind in memory in order to show slider animation. If you don't you have the next or previous tab (or fragment) in memory, trying to initiate the fragment during the slider transition will cause a performance lag or at the worse you will have the sliding fragment to be empty and get loaded later.

ViewPager also allows you to set page offset limit through setOffscreenPageLimit() which will control amount fragment to keep them in memory. By default, this is 1 which means one fragment ahead and behind will always be there in memory