Cfrgtkky

The following code is created by the Create New Project wizard of Android Studio 3.2.1 , it's a Tabbed Activity.

I'm so strange why Android Studio 3.2.1 want to add the code setSupportActionBar(toolbar), and why the App works well and can display 3 tabs after I delete setSupportActionBar(toolbar).

Could you tell me?

Code

class MainActivity : AppCompatActivity() {



    private var mSectionsPagerAdapter: SectionsPagerAdapter? = null



    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)

        setContentView(R.layout.activity_main)



        setSupportActionBar(toolbar)  //I can delete it





        mSectionsPagerAdapter = SectionsPagerAdapter(supportFragmentManager)



        // Set up the ViewPager with the sections adapter.

        container.adapter = mSectionsPagerAdapter



        container.addOnPageChangeListener(TabLayout.TabLayoutOnPageChangeListener(tabs))

        tabs.addOnTabSelectedListener(TabLayout.ViewPagerOnTabSelectedListener(container))



    }







    inner class SectionsPagerAdapter(fm: FragmentManager) : FragmentPagerAdapter(fm) {



        override fun getItem(position: Int): Fragment {



        }



        override fun getCount(): Int {

            // Show 3 total pages.

            return 3

        }

    }





    class PlaceholderFragment : Fragment() {





    }

}

Layout

<?xml version="1.0" encoding="utf-8"?>

<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"

                                                 xmlns:tools="http://schemas.android.com/tools"

                                                 xmlns:app="http://schemas.android.com/apk/res-auto"

                                                 android:id="@+id/main_content"

                                                 android:layout_width="match_parent"

                                                 android:layout_height="match_parent"

                                                 android:fitsSystemWindows="true"

                                                 tools:context=".MainActivity">



    <android.support.design.widget.AppBarLayout

            android:id="@+id/appbar"

            android:layout_width="match_parent"

            android:layout_height="wrap_content"

            android:paddingTop="@dimen/appbar_padding_top"

            android:theme="@style/AppTheme.AppBarOverlay">



        <android.support.v7.widget.Toolbar

                android:id="@+id/toolbar"

                app:title="@string/app_name"

                android:layout_width="match_parent"

                android:layout_height="?attr/actionBarSize"

                android:layout_weight="1"

                android:background="?attr/colorPrimary"

                app:popupTheme="@style/AppTheme.PopupOverlay"

                app:layout_scrollFlags="scroll|enterAlways">



        </android.support.v7.widget.Toolbar>



        <android.support.design.widget.TabLayout

                android:id="@+id/tabs"

                android:layout_width="match_parent"

                android:layout_height="wrap_content">



            <android.support.design.widget.TabItem

                    android:id="@+id/tabItem"

                    android:layout_width="wrap_content"

                    android:layout_height="wrap_content"

                    android:text="@string/tab_text_1"/>



            <android.support.design.widget.TabItem

                    android:id="@+id/tabItem2"

                    android:layout_width="wrap_content"

                    android:layout_height="wrap_content"

                    android:text="@string/tab_text_2"/>



            <android.support.design.widget.TabItem

                    android:id="@+id/tabItem3"

                    android:layout_width="wrap_content"

                    android:layout_height="wrap_content"

                    android:text="@string/tab_text_3"/>



        </android.support.design.widget.TabLayout>

    </android.support.design.widget.AppBarLayout>



    <android.support.v4.view.ViewPager

            android:id="@+id/container"

            android:layout_width="match_parent"

            android:layout_height="match_parent"

            app:layout_behavior="@string/appbar_scrolling_view_behavior"/>



    <android.support.design.widget.FloatingActionButton

            android:id="@+id/fab"

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:layout_gravity="end|bottom"

            android:layout_margin="@dimen/fab_margin"

            app:srcCompat="@android:drawable/ic_dialog_email"/>



</android.support.design.widget.CoordinatorLayout>

setSupportActionBar() method is used to designate the Toolbar as the ActionBar.

If you want to add backButton or add menu items, etc. in the Toolbar it should be designated as ActionBar.

Here you have not done any above things. So you were able to safely remove the line of code.

For details about Toolbar refer: Toolbar | Android Developers