void updateText(int currentItem, PagerAdapter adapter) {
    final int itemCount = adapter != null ? adapter.getCount() : 0;
    mUpdatingText = true;

    CharSequence text = null;
    if (currentItem >= 1 && adapter != null) {
      text = adapter.getPageTitle(currentItem - 1);
    }
    mPrevText.setText(text);

    mCurrText.setText(adapter != null ? adapter.getPageTitle(currentItem) : null);

    text = null;
    if (currentItem + 1 < itemCount && adapter != null) {
      text = adapter.getPageTitle(currentItem + 1);
    }
    mNextText.setText(text);

    // Measure everything
    final int width = getWidth() - getPaddingLeft() - getPaddingRight();
    final int childHeight = getHeight() - getPaddingTop() - getPaddingBottom();
    final int childWidthSpec =
        MeasureSpec.makeMeasureSpec((int) (width * 0.8f), MeasureSpec.AT_MOST);
    final int childHeightSpec = MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY);
    mPrevText.measure(childWidthSpec, childHeightSpec);
    mCurrText.measure(childWidthSpec, childHeightSpec);
    mNextText.measure(childWidthSpec, childHeightSpec);

    mLastKnownCurrentPage = currentItem;

    if (!mUpdatingPositions) {
      updateTextPositions(currentItem, mLastKnownPositionOffset);
    }

    mUpdatingText = false;
  }
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main_activity);

    final ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    viewPager = (ViewPager) findViewById(R.id.pager);
    PagerAdapter adapter = new PagerAdapter(getSupportFragmentManager(), this);
    viewPager.setAdapter(adapter);
    viewPager.setOnPageChangeListener(
        new ViewPager.SimpleOnPageChangeListener() {
          @Override
          public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
          }
        });

    for (int i = 0; i < adapter.getCount(); i++) {
      actionBar.addTab(actionBar.newTab().setText(adapter.getPageTitle(i)).setTabListener(this));
    }
  }