Fragment$InstantiationException on inflating androidx fragment from android.app.Activity

I'm getting the following exception:

android.app.Fragment$InstantiationException: Trying to instantiate a class com.example.android.btVspTest.StageCollectionFragment that is not a Fragment

The exception triggers within the following call to android.app.Activity.setContentView from onCreate() of my VspTestActivity class, which extends android.app.Activity:
setContentView(R.layout.activity_vsp_test);

My com.example.android.btVspTest.StageCollectionFragment extends androidx.fragment.app.Fragment. I am guessing the issue has to do with incompatibility beween android.app.Activity and androidx.fragment.app.Fragment. Is there a fix or work-around?

Application level build.gradle:

apply plugin: 'com.android.application'
android {
compileSdkVersion 29
defaultConfig {
minSdkVersion 26
targetSdkVersion 29
}
compileOptions {
sourceCompatibility JavaVersion.VERSION_1_7
targetCompatibility JavaVersion.VERSION_1_7
}
}
dependencies {
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
implementation 'com.google.android.material:material:1.1.0'
/*implementation 'com.google.android.material:material:1.1.0-alpha08'*/
implementation 'androidx.viewpager2:viewpager2:1.0.0'
}

activity_vsp_test.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:id="@+id/test_layout">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="@dimen/margin_huge"
android:orientation="horizontal">
<Button
android:id="@+id/test_connect_button"
android:layout_width="126dp"
android:layout_height="match_parent"
android:text="@string/connect" />
<Button
android:id="@+id/test_disconnect_button"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="1"
android:text="@string/disconnect" />
<Button
android:id="@+id/test_bluetooth_button"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:layout_weight="1"
android:onClick="startBluetoothActivity"
android:text="Bluetooth" />
</LinearLayout>
<TextView
android:id="@+id/vsp_test_connection_text"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="@string/disconnected" />
<TextView
android:id="@+id/test_result_text"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<!-- 2 irrelevant linear layouts with buttons in them removed -->
<fragment class="com.example.android.btVspTest.StageCollectionFragment"
android:id="@+id/test_stage_collection_fragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="1"/>
</LinearLayout>

StageCollectionFragment.java:

package com.example.android.btVspTest;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.viewpager2.widget.ViewPager2;
import com.example.android.bluetoothvsp.R;
import com.google.android.material.tabs.TabLayout;
import com.google.android.material.tabs.TabLayoutMediator;
import com.google.android.material.tabs.TabLayoutMediator.TabConfigurationStrategy;
public class StageCollectionFragment extends Fragment implements TabConfigurationStrategy {
// When requested, this adapter returns a StageFragment,
// representing an object in the collection.
StageCollectionAdapter demoCollectionAdapter;
ViewPager2 viewPager;
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
return inflater.inflate(R.layout.test_stage_layout, container, false);
}
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
demoCollectionAdapter = new StageCollectionAdapter(this);
viewPager = view.findViewById(R.id.test_stage_pager);
viewPager.setAdapter(demoCollectionAdapter);
TabLayout tabLayout = view.findViewById(R.id.test_stage_tab_layout);
new TabLayoutMediator(tabLayout, viewPager, this).attach();
}
@Override
public void onConfigureTab(@NonNull TabLayout.Tab tab, int position) {
tab.setText("OBJECT " + (position + 1));
}
}

VspTestActivity.java:

package com.example.android.btVspTest;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import com.example.android.bluetoothvsp.ActivityMessageDataKey;
import com.example.android.bluetoothvsp.BluetoothActivity;
import com.example.android.bluetoothvsp.DeviceListActivity;
import com.example.android.bluetoothvsp.R;
import com.example.android.bluetoothvsp.enums.ConnectionState;
import com.example.android.bluetoothvsp.enums.TestState;
import com.example.android.common.logger.Log;
import com.example.android.common.logger.LogWrapper;
import com.example.android.common.logger.MessageOnlyLogFilter;
public class VspTestActivity extends Activity
{
/**
* Debugging.
*/
private static final String TAG = "VspTestActivity";
/**
* UI elements.
*/
private TextView mConnectionTextView;
private TextView mTestResultTextView;
private Button mConnectButton;
private Button mDisconnectButton;
private Button mSynchronousTestButton;
private Button mHostSpamButton;
private Button mReaderSpamButton;
private Button mBothSpamButton;
/**
* Objects provided by Android to facilitate Bluetooth operations.
*/
private BluetoothAdapter mBluetoothAdapter;
private BluetoothDevice mDevice;
/**
* Manger used to start and handle the communication tests.
*/
private BluetoothVspTestManager mVspTestManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_vsp_test);
if (savedInstanceState != null) {
return;
}
// Irrelevant code removed
}
// Irrelevant code removed
@Override
protected  void onStart() {
super.onStart();
// Irrelevant code removed
}
// Irrelevant code removed
}

Some of the code is copy->paste->edit of example at https://developer.android.com/guide/navigation/navigation-swipe-view-2, with following mapping:

• My StageCollectionFragment is their CollectionDemoFragment
• My StageFragment is their DemoObjectFragment

That developer.android.com example unfortunately isn't complete as the layout resources are missing. I haven't been able to find a full example on the internet, of combining TabLayout & ViewPager2.

I'm trying to do something like the draft screen at <https://imgur.com/a/k8Tr52y?>. I would eventually have 4 to 5 tabs with actual meaningful tab titles, with the tabs being added dynamically, and specific fragments in each of the tabs.

Your activity must extend androidx.fragment.app.FragmentActivity.

From the documentation,

> Base class for activities that want to use the support-based
> Fragments.

Also from Fragment's documentation,

> - Your activity must extend FragmentActivity
>
>
>
> - You must call FragmentActivity.getSupportFragmentManager() to get the FragmentManager

Since part of your code is using AndroidX libraries, I'll assume you are okay with going all-in and use AndroidX exclusively. You should. Right now, you are mixing old and new Android libraries and that's not a good thing. Simply do the following when declaring (pay attention to the import statements):

your activity:

import androidx.appcompat.app.AppCompatActivity;

public class VspTestActivity extends AppCompatActivity {

and your fragment:

import androidx.fragment.app.Fragment

public class StageCollectionFragment extends Fragment ...

Even easier is to let Android Studio do it all for you. Just select Refactor > Migrate to AndroidX (on the Mac)